The respiratory system (also respiratory apparatus, ventilatory system) is a
biological system
A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures depending on what the system is. Examples of biological syst ...
consisting of specific
organs
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...
and structures used for
gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
in
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s and
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In
land animals the respiratory surface is internalized as linings of the
lungs.
Gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called
alveoli, and in birds they are known as
atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.
These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
, which branches in the middle of the chest into the two main
bronchi
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...
. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the
bronchiole
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start ...
s. In
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s the bronchioles are termed
parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic
alveoli in mammals and
atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of
breathing which involves the
muscles of respiration
The muscles of respiration are the muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during ...
.
In most
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
, and a number of other
aquatic animal
An aquatic animal is any animal, whether invertebrate or vertebrate, that lives in water for most or all of its lifetime. Many insects such as mosquitoes, mayflies, dragonflies and caddisflies have aquatic larvae, with winged adults. Aquatic ani ...
s (both
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s and
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s) the respiratory system consists of
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
s, which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in the gills which consist of thin or very flat
filaments and
lammelae which expose a very large surface area of highly
vascularized
Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splitt ...
tissue to the water.
Other animals, such as
insects
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...
, have respiratory systems with very simple anatomical features, and in
amphibians even the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different de ...
plays a vital role in gas exchange.
Plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants includes anatomical features such as
stomata, that are found in various parts of the plant.
Mammals
Anatomy
![illu quiz lung05](https://upload.wikimedia.org/wikipedia/commons/d/df/Illu_quiz_lung05.jpg)
In
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s and other
mammals, the anatomy of a typical respiratory system is the
respiratory tract. The tract is divided into an
upper and a
lower respiratory tract
The respiratory tract is the subdivision of the respiratory system involved with the process of Respiration (physiology), respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed ...
. The upper tract includes the
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes ...
,
nasal cavities
The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...
,
sinuses
Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphenoid ...
,
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
and the part of the
larynx above the
vocal folds
In humans, vocal cords, also known as vocal folds or voice reeds, are folds of throat tissues that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speec ...
. The lower tract (Fig. 2.) includes the lower part of the larynx, the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
,
bronchi
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...
,
bronchiole
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start ...
s and the
alveoli.
The branching airways of the lower tract are often described as the
respiratory tree
The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed in through the nose to t ...
or tracheobronchial tree (Fig. 2).
The intervals between successive branch points along the various branches of "tree" are often referred to as branching "generations", of which there are, in the adult human about 23. The earlier generations (approximately generations 0–16), consisting of the trachea and the bronchi, as well as the larger bronchioles which simply act as
air conduits, bringing air to the respiratory bronchioles, alveolar ducts and alveoli (approximately generations 17–23), where
gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
takes place.
Bronchiole
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start ...
s are defined as the small airways lacking any cartilaginous support.
The first bronchi to branch from the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
are the right and left main bronchi. Second, only in diameter to the trachea (1.8 cm), these bronchi (1 -1.4 cm in diameter)
enter the
lungs at each
hilum, where they branch into narrower secondary bronchi known as lobar bronchi, and these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi (1 to 6 mm in diameter)
are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi.
Compared to the 23 number (on average) of branchings of the respiratory tree in the adult human, the
mouse has only about 13 such branchings.
The alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates
dead space
''Dead Space'' is a science fiction/ horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a linear format; ...
, a volume of air (about 150 ml in the adult human) that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them.
At the end of inhalation the airways are filled with environmental air, which is exhaled without coming in contact with the gas exchanger.
Ventilatory volumes
![Lungvolumes Updated](https://upload.wikimedia.org/wikipedia/commons/1/16/Lungvolumes_Updated.png)
The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances (the resting
tidal volume
Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass.
Mechanical vent ...
of about 500 ml), and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by
spirometry.
A typical adult human spirogram with the names given to the various excursions in volume the lungs can undergo is illustrated below (Fig. 3):
Not all the air in the lungs can be expelled during maximally forced exhalation(
ERV). This is the
residual volume(volume of air remaining even after a forced exhalation) of about 1.0-1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume (i.e.
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
of about 2.5-3.0 liters, and
total lung capacity
Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle.
The average total lung capacity of an adult human male is about 6 litres of air.
Tidal breathing is normal, resting breathi ...
of about 6 liters) can therefore also not be measured by spirometry. Their measurement requires special techniques.
The rates at which air is breathed in or out, either through the mouth or nose or into or out of the alveoli are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the
respiratory rate
The respiratory rate is the rate at which breathing occurs; it is set and controlled by the respiratory center of the brain. A person's respiratory rate is usually measured in breaths per minute.
Measurement
The respiratory rate in humans is me ...
. An average healthy human breathes 12-16 times a minute.
Mechanics of breathing
In
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
, inhalation at rest is primarily due to the contraction of the
diaphragm. This is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity. When it contracts the sheet flattens, (i.e. moves downwards as shown in Fig. 7) increasing the volume of the thoracic cavity in the antero-posterior axis. The contracting diaphragm pushes the abdominal organs downwards. But because the pelvic floor prevents the lowermost abdominal organs from moving in that direction, the pliable abdominal contents cause the belly to bulge outwards to the front and sides, because the relaxed abdominal muscles do not resist this movement (Fig. 7). This entirely passive bulging (and shrinking during exhalation) of the abdomen during normal breathing is sometimes referred to as "abdominal breathing", although it is, in fact, "diaphragmatic breathing", which is not visible on the outside of the body. Mammals only use their abdominal muscles during forceful exhalation (see Fig. 8, and discussion below). Never during any form of inhalation.
As the diaphragm contracts, the
rib cage
The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels.
The sternum, together known as the thoracic cage, is a sem ...
is simultaneously enlarged by the ribs being pulled upwards by the
intercostal muscles
Intercostal muscles are many different groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping expand and shrink the size of ...
as shown in Fig. 4. All the ribs slant downwards from the rear to the front (as shown in Fig. 4); but the lowermost ribs ''also'' slant downwards from the midline outwards (Fig. 5). Thus the rib cage's transverse diameter can be increased in the same way as the antero-posterior diameter is increased by the so-called
pump handle movement
Pump-handle is a movement of the ribs that results in a change in the anteroposterior diameter of the thorax.
Definition
One of the most important functions of the ribs and diaphragm is the change in volume of the thorax that helps inspiration
...
shown in Fig. 4.
The enlargement of the thoracic cavity's vertical dimension by the contraction of the diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the ribs, causes the intrathoracic pressure to fall. The lungs interiors are open to the outside air and being elastic, therefore expand to fill the increased space,
pleura fluid between double-layered pleura covering of lungs helps in reducing friction while lungs expansion and contraction. The inflow of air into the lungs occurs via the
respiratory airways
The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed in through the nose to t ...
(Fig. 2). In a healthy person, these airways
begin with the nose.
(It is possible to begin with the mouth, which is the backup breathing system. However, chronic
mouth breathing
Mouth breathing, medically known as chronic oral ventilation, is long-term breathing through the mouth. It often is caused by an obstruction to breathing through the nose, the innate breathing organ in the human body. Chronic mouth breathing ma ...
leads to, or is a sign of, illness.
) It ends in the microscopic dead-end sacs called
alveoli, which are always open, though the diameters of the various sections can be changed by the
sympathetic and
parasympathetic nervous systems. The alveolar air pressure is therefore always close to atmospheric air pressure (about 100
kPa
KPA may refer to:
* Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA)
* Kensington (Olympia) station, London, England, National Rail station code
* Kenya Ports Authority
* ''Kiln phosphoric acid'', a ...
at sea level) at rest, with the pressure gradients because of lungs contraction and expansion cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa.
During exhalation, the diaphragm and intercostal muscles relax. This returns the chest and abdomen to a position determined by their anatomical elasticity. This is the "resting mid-position" of the thorax and abdomen (Fig. 7) when the lungs contain their
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
of air (the light blue area in the right hand illustration of Fig. 7), which in the adult human has a volume of about 2.5–3.0 liters (Fig. 3).
Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes passively more gently than it contracts actively during inhalation.
![Alveolar air](https://upload.wikimedia.org/wikipedia/commons/b/bd/Alveolar_air.png)
The volume of air that moves in ''or'' out (at the nose or mouth) during a single breathing cycle is called the
tidal volume
Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass.
Mechanical vent ...
. In a resting adult human it is about 500 ml per breath. At the end of exhalation, the airways contain about 150 ml of alveolar air which is the first air that is breathed back into the alveoli during inhalation.
This volume air that is breathed out of the alveoli and back in again is known as
dead space
''Dead Space'' is a science fiction/ horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a linear format; ...
ventilation, which has the consequence that of the 500 ml breathed into the alveoli with each breath only 350 ml (500 ml - 150 ml = 350 ml) is fresh warm and moistened air.
Since this 350 ml of fresh air is thoroughly mixed and diluted by the air that remains in the alveoli after a normal exhalation (i.e. the
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
of about 2.5–3.0 liters), it is clear that the composition of the alveolar air changes very little during the breathing cycle (see Fig. 9). The oxygen
tension
Tension may refer to:
Science
* Psychological stress
* Tension (physics), a force related to the stretching of an object (the opposite of compression)
* Tension (geology), a stress which stretches rocks in two opposite directions
* Voltage or el ...
(or partial pressure) remains close to 13-14 kPa (about 100 mm Hg), and that of carbon dioxide very close to 5.3 kPa (or 40 mm Hg). This contrasts with composition of the dry outside air at sea level, where the partial pressure of oxygen is 21 kPa (or 160 mm Hg) and that of carbon dioxide 0.04 kPa (or 0.3 mmHg).
During heavy breathing (
hyperpnea
Hyperpnea, or hyperpnoea (forced respiration) is increased volume of air taken during breathing. It can occur with or without an increase in respiration rate. It is characterized by deep breathing. It may be physiologic—as when required by oxyge ...
), as, for instance, during exercise, inhalation is brought about by a more powerful and greater excursion of the contracting diaphragm than at rest (Fig. 8). In addition, the "
accessory muscles of inhalation" exaggerate the actions of the intercostal muscles (Fig. 8). These accessory muscles of inhalation are muscles that extend from the
cervical vertebrae and base of the skull to the upper ribs and
sternum
The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...
, sometimes through an intermediary attachment to the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the r ...
s.
When they contract the rib cage's internal volume is increased to a far greater extent than can be achieved by contraction of the intercostal muscles alone. Seen from outside the body the lifting of the clavicles during strenuous or labored inhalation is sometimes called
clavicular breathing, seen especially during
asthma
Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
attacks and in people with
chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease (COPD) is a type of progressive lung disease characterized by long-term respiratory symptoms and airflow limitation. The main symptoms include shortness of breath and a cough, which may or may not produce ...
.
During heavy breathing, exhalation is caused by relaxation of all the muscles of inhalation. But now, the abdominal muscles, instead of remaining relaxed (as they do at rest), contract forcibly pulling the lower edges of the
rib cage
The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels.
The sternum, together known as the thoracic cage, is a sem ...
downwards (front and sides) (Fig. 8). This not only drastically decreases the size of the rib cage, but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax (Fig. 8). The end-exhalatory lung volume is now well below the resting mid-position and contains far less air than the resting "functional residual capacity". However, in a normal mammal, the lungs cannot be emptied completely. In an adult human, there is always still at least 1 liter of residual air left in the lungs after maximum exhalation.
The automatic rhythmical breathing in and out, can be interrupted by coughing, sneezing (forms of very forceful exhalation), by the expression of a wide range of emotions (laughing, sighing, crying out in pain, exasperated intakes of breath) and by such voluntary acts as speech, singing, whistling and the playing of wind instruments. All of these actions rely on the muscles described above, and their effects on the movement of air in and out of the lungs.
Although not a form of breathing, the
Valsalva maneuver
The Valsalva maneuver is performed by a forceful attempt of exhalation against a closed airway, usually done by closing one's mouth and pinching one's nose shut while expelling air out as if blowing up a balloon. Variations of the maneuver can ...
involves the respiratory muscles. It is, in fact, a very forceful exhalatory effort against a tightly closed
glottis, so that no air can escape from the lungs.
Instead abdominal contents are evacuated in the opposite direction, through orifices in the pelvic floor. The abdominal muscles contract very powerfully, causing the pressure inside the abdomen and thorax to rise to extremely high levels. The Valsalva maneuver can be carried out voluntarily but is more generally a reflex elicited when attempting to empty the abdomen during, for instance, difficult defecation, or during childbirth. Breathing ceases during this maneuver.
Gas exchange
![Alveolar Wall](https://upload.wikimedia.org/wikipedia/commons/8/8a/Alveolar_Wall.svg)
The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
, across a very thin membrane (known as the
blood–air barrier
The blood–air barrier or air–blood barrier, (alveolar–capillary barrier or membrane) exists in the gas exchanging region of the lungs. It exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. It is ...
), which forms the walls of the
pulmonary alveoli
A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. A ...
(Fig. 10). It consists of the
alveolar epithelial cells, their
basement membranes and the
endothelial cells
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
of the alveolar capillaries (Fig. 10).
This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called
alveoli (each between 75 and 300 µm in diameter) branching off from the respiratory
bronchiole
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start ...
s in the
lungs, thus providing an extremely large surface area (approximately 145 m
2) for gas exchange to occur.
The air contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a
partial pressure of oxygen of 13-14 kPa (100 mmHg), and a
partial pressure of carbon dioxide of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli).
As mentioned in
the section above, the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively.
This marked difference between the composition of the alveolar air and that of the ambient air can be maintained because the
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
is contained in dead-end sacs connected to the outside air by fairly narrow and relatively long tubes (the airways:
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes ...
,
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
,
larynx,
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
,
bronchi
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...
and their branches down to the
bronchioles), through which the air has to be breathed both in and out (i.e. there is no unidirectional through-flow as there is in the
bird lung). This typical mammalian anatomy combined with the fact that the lungs are not emptied and re-inflated with each breath (leaving a substantial volume of air, of about 2.5-3.0 liters, in the alveoli after exhalation), ensures that the composition of the alveolar air is only minimally disturbed when the 350 ml of fresh air is mixed into it with each inhalation. Thus the animal is provided with a very special "portable atmosphere", whose composition differs significantly from the
present-day ambient air. It is this portable atmosphere (the
functional residual capacity
Functional residual capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm ...
) to which the blood and therefore the body tissues are exposed – not to the outside air.
The resulting arterial partial pressures of oxygen and carbon dioxide are
homeostatically controlled. A rise in the arterial partial pressure of CO
2 and, to a lesser extent, a fall in the arterial partial pressure of O
2, will reflexly cause deeper and faster breathing until the
blood gas tension
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monox ...
s in the lungs, and therefore the arterial blood, return to normal. The converse happens when the carbon dioxide tension falls, or, again to a lesser extent, the oxygen tension rises: the rate and depth of breathing are reduced until blood gas normality is restored.
Since the blood arriving in the alveolar capillaries has a partial pressure of O
2 of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13-14 kPa (100 mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has a partial pressure of CO
2 of also about 6 kPa (45 mmHg), whereas that of the alveolar air is 5.3 kPa (40 mmHg), there is a net movement of carbon dioxide out of the capillaries into the alveoli. The changes brought about by these net flows of individual gases into and out of the alveolar air necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds or so. This is very tightly controlled by the monitoring of the arterial blood gases (which accurately reflect composition of the alveolar air) by the
aortic and
carotid bodies, as well as by the
blood gas and pH sensor on the anterior surface of the
medulla oblongata in the brain. There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the
bronchioles and
pulmonary capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
, and are therefore responsible for directing the flow of air and blood to different parts of the lungs.
It is only as a result of accurately maintaining the composition of the 3 liters of alveolar air that with each breath some carbon dioxide is discharged into the atmosphere and some oxygen is taken up from the outside air. If more carbon dioxide than usual has been lost by a short period of
hyperventilation, respiration will be slowed down or halted until the alveolar partial pressure of carbon dioxide has returned to 5.3 kPa (40 mmHg). It is therefore strictly speaking untrue that the primary function of the respiratory system is to rid the body of carbon dioxide “waste”. The carbon dioxide that is breathed out with each breath could probably be more correctly be seen as a byproduct of the body's extracellular fluid
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
and
pH homeostats
If these homeostats are compromised, then a
respiratory acidosis
Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis).
Carbon dioxide is produced continu ...
, or a
respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
will occur. In the long run these can be compensated by renal adjustments to the
H+ and HCO3− concentrations in the plasma; but since this takes time, the
hyperventilation syndrome can, for instance, occur when agitation or anxiety cause a person to breathe fast and deeply thus causing a distressing
respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
through the blowing off of too much CO
2 from the blood into the outside air.
Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
. The oxygen is held on the hemoglobin by four
ferrous iron
In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+.
The adjective ferrous or the prefix ferro- is often used to ...
-containing
heme
Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.
In biochemical terms, heme is a coordination complex "consis ...
groups per hemoglobin molecule. When all the heme groups carry one O
2 molecule each the blood is said to be “saturated” with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase the oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as bicarbonate ions (HCO
3−) in the plasma. However the conversion of dissolved CO
2 into HCO
3− (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and through alveolar capillaries on the other. The reaction is therefore catalyzed by
carbonic anhydrase, an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
inside the
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s.
The reaction can go in both directions depending on the prevailing partial pressure of CO
2.
A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as
carbamino groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO
2, and carbamino groups) in arterial blood (i.e. after it has equilibrated with the alveolar air) is about 26 mM (or 58 ml/100 ml),
compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml/100 ml blood).
Control of ventilation
Ventilation of the lungs in mammals occurs via the
respiratory center
The respiratory center is located in the medulla oblongata and pons, in the brainstem. The respiratory center is made up of three major respiratory groups of neurons, two in the medulla and one in the pons. In the medulla they are the dorsal ...
s in the
medulla oblongata and the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
of the
brainstem.
These areas form a series of
neural pathway
In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to an ...
s which receive information about the
partial pressures of oxygen and carbon dioxide in the
arterial blood
Arterial blood is the oxygenated blood in the circulatory system found in the pulmonary vein, the left chambers of the heart, and in the artery, arteries. It is bright red in color, while venous blood is dark red in color (but looks purple through ...
. This information determines the average rate of ventilation of the
alveoli of the
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side ...
, to keep these
pressures constant. The respiratory center does so via
motor nerves which activate the
diaphragm and other
muscles of respiration
The muscles of respiration are the muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during ...
.
The breathing rate increases when the
partial pressure of carbon dioxide in the blood increases. This is detected by
central blood gas chemoreceptors on the anterior surface of the
medulla oblongata.
The
aortic and
carotid bodies, are the
peripheral blood gas chemoreceptors which are particularly sensitive to the arterial
partial pressure of O2 though they also respond, but less strongly, to the partial pressure of
CO2.
At sea level, under normal circumstances, the breathing rate and depth, is determined primarily by the arterial partial pressure of carbon dioxide rather than by the arterial
partial pressure of oxygen
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monox ...
, which is allowed to vary within a fairly wide range before the respiratory centers in the medulla oblongata and pons respond to it to change the rate and depth of breathing.
Exercise increases the breathing rate due to the extra carbon dioxide produced by the enhanced metabolism of the exercising muscles.
In addition passive movements of the limbs also reflexively produce an increase in the breathing rate.
Information received from
stretch receptor
Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the Medulla oblongata, medulla in the brain stem via Afferent nerve fiber, afferent nerve fibers. Examples include stre ...
s in the lungs limits
tidal volume
Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass.
Mechanical vent ...
(the depth of inhalation and exhalation).
Responses to low atmospheric pressures
The
alveoli are open (via the airways) to the atmosphere, with the result that alveolar air pressure is exactly the same as the ambient air pressure at sea level, at altitude, or in any artificial atmosphere (e.g. a diving chamber, or decompression chamber) in which the individual is breathing freely. With
expansion of the lungs the alveolar air occupies a larger volume, and its
pressure falls proportionally, causing air to flow in through the airways, until the pressure in the alveoli is again at the ambient air pressure. The reverse happens during exhalation. This ''process'' (of inhalation and exhalation) is exactly the same at sea level, as on top of
Mt. Everest
Mount Everest (; Tibetic languages, Tibetan: ''Chomolungma'' ; ) is List of highest mountains on Earth, Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border ru ...
, or in a
diving chamber
A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of ...
or
decompression chamber
A diving chamber is a vessel for human occupation, which may have an entrance that can be sealed to hold an internal pressure significantly higher than ambient pressure, a pressurised gas system to control the internal pressure, and a supply of ...
.
![Altitude and air pressure & Everest](https://upload.wikimedia.org/wikipedia/commons/9/9d/Altitude_and_air_pressure_%26_Everest.jpg)
However, as one rises above sea level the
density of the air decreases exponentially (see Fig. 14), halving approximately
with every 5500 m rise in altitude.
Since the composition of the atmospheric air is almost constant below 80 km, as a result of the continuous mixing effect of the weather, the concentration of oxygen in the air (mmols O
2 per liter of ambient air) decreases at the same rate as the fall in air pressure with altitude. Therefore, in order to breathe in the same amount of oxygen per minute, the person has to inhale a proportionately greater volume of air per minute at altitude than at sea level. This is achieved by breathing deeper and faster (i.e.
hyperpnea
Hyperpnea, or hyperpnoea (forced respiration) is increased volume of air taken during breathing. It can occur with or without an increase in respiration rate. It is characterized by deep breathing. It may be physiologic—as when required by oxyge ...
) than at sea level (see below).
![Mount_Everest_as_seen_from_Drukair2_PLW_edit](https://upload.wikimedia.org/wikipedia/commons/d/d1/Mount_Everest_as_seen_from_Drukair2_PLW_edit.jpg)
There is, however, a complication that increases the volume of air that needs to be inhaled per minute (
respiratory minute volume
Minute ventilation (or respiratory minute volume or minute volume) is the volume of gas inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs per minute. It is an important parameter in respiratory medicine due ...
) to provide the same amount of oxygen to the lungs at altitude as at sea level. During inhalation the air is warmed and saturated with water vapor during its passage through the
nose passages and
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
.
Saturated water vapor pressure is dependent only on temperature. At a body core temperature of 37 °C it is 6.3
kPa
KPA may refer to:
* Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA)
* Kensington (Olympia) station, London, England, National Rail station code
* Kenya Ports Authority
* ''Kiln phosphoric acid'', a ...
(47.0 mmHg), irrespective of any other influences, including altitude. Thus at sea level, where the ambient atmospheric pressure is about 100 kPa, the moistened air that flows into the lungs from the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
consists of water vapor (6.3 kPa), nitrogen (74.0 kPa), oxygen (19.7 kPa) and trace amounts of carbon dioxide and other gases (a total of 100 kPa). In dry air the
partial pressure of O
2 at sea level is 21.0 kPa (i.e. 21% of 100 kPa), compared to the 19.7 kPa of oxygen entering the alveolar air. (The tracheal partial pressure of oxygen is 21% of
00 kPa – 6.3 kPa= 19.7 kPa). At the summit of
Mt. Everest
Mount Everest (; Tibetic languages, Tibetan: ''Chomolungma'' ; ) is List of highest mountains on Earth, Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The China–Nepal border ru ...
(at an altitude of 8,848 m or 29,029 ft) the total
atmospheric pressure is 33.7 kPa, of which 7.1 kPa (or 21%) is oxygen.
The air entering the lungs also has a total pressure of 33.7 kPa, of which 6.3 kPa is, unavoidably, water vapor (as it is at sea level). This reduces the partial pressure of oxygen entering the alveoli to 5.8 kPa (or 21% of
3.7 kPa – 6.3 kPa= 5.8 kPa). The reduction in the partial pressure of oxygen in the inhaled air is therefore substantially greater than the reduction of the total atmospheric pressure at altitude would suggest (on Mt Everest: 5.8 kPa ''vs.'' 7.1 kPa).
A further minor complication exists at altitude. If the volume of the lungs were to be instantaneously doubled at the beginning of inhalation, the air pressure inside the lungs would be halved. This happens regardless of altitude. Thus, halving of the sea level air pressure (100 kPa) results in an intrapulmonary air pressure of 50 kPa. Doing the same at 5500 m, where the atmospheric pressure is only 50 kPa, the intrapulmonary air pressure falls to 25 kPa. Therefore, the same change in lung volume at sea level results in a 50 kPa difference in pressure between the ambient air and the intrapulmonary air, whereas it result in a difference of only 25 kPa at 5500 m. The driving pressure forcing air into the lungs during inhalation is therefore halved at this altitude. The ''rate'' of inflow of air into the lungs during inhalation at sea level is therefore twice that which occurs at 5500 m. However, in reality, inhalation and exhalation occur far more gently and less abruptly than in the example given. The differences between the atmospheric and intrapulmonary pressures, driving air in and out of the lungs during the breathing cycle, are in the region of only 2–3 kPa.
A doubling or more of these small pressure differences could be achieved only by very major changes in the breathing effort at high altitudes.
All of the above influences of low atmospheric pressures on breathing are accommodated primarily by breathing deeper and faster (
hyperpnea
Hyperpnea, or hyperpnoea (forced respiration) is increased volume of air taken during breathing. It can occur with or without an increase in respiration rate. It is characterized by deep breathing. It may be physiologic—as when required by oxyge ...
). The exact degree of hyperpnea is determined by the
blood gas homeostat, which regulates the
partial pressures of oxygen and carbon dioxide in the arterial blood. This
homeostat
The Homeostat is one of the first devices capable of adapting itself to the environment; it exhibited behaviours such as habituation, reinforcement and learning through its ability to maintain homeostasis in a changing environment. It was built b ...
prioritizes the regulation of the arterial
partial pressure of carbon dioxide over that of oxygen at sea level.
That is to say, at sea level the arterial partial pressure of CO
2 is maintained at very close to 5.3 kPa (or 40 mmHg) under a wide range of circumstances, at the expense of the arterial partial pressure of O
2, which is allowed to vary within a very wide range of values, before eliciting a corrective ventilatory response. However, when the atmospheric pressure (and therefore the partial pressure of O
2 in the ambient air) falls to below 50-75% of its value at sea level, oxygen
homeostasis
In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
is given priority over carbon dioxide homeostasis.
This switch-over occurs at an elevation of about 2500 m (or about 8000 ft). If this switch occurs relatively abruptly, the hyperpnea at high altitude will cause a severe fall in the arterial partial pressure of carbon dioxide, with a
consequent rise in the pH of the arterial plasma. This is one contributor to
high altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, then
hypoxia may complicate the clinical picture with potentially fatal results.
There are oxygen sensors in the smaller
bronchi
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus. ...
and
bronchiole
The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start ...
s. In response to low partial pressures of oxygen in the inhaled air these sensors reflexively cause the pulmonary arterioles to constrict. (This is the exact opposite of the corresponding reflex in the tissues, where low arterial partial pressures of O
2 cause arteriolar vasodilation.) At altitude this causes the
pulmonary arterial pressure to rise resulting in a much more even distribution of blood flow to the lungs than occurs at sea level. At sea level the pulmonary arterial pressure is very low, with the result that
the tops of the lungs receive far less blood than the bases, which are relatively over-perfused with blood. It is only in the middle of the lungs that the
blood and air flow to the alveoli are ideally matched. At altitude this variation in the
ventilation/perfusion ratio In respiratory physiology, the ventilation/perfusion ratio (V/Q ratio) is a ratio used to assess the efficiency and adequacy of the matching of two variables:
* V – ventilation – the air that reaches the alveoli
* Q – perfusion – the blo ...
of alveoli from the tops of the lungs to the bottoms is eliminated, with all the alveoli perfused and ventilated in more or less the physiologically ideal manner. This is a further important contributor to the
acclimatatization to high altitudes and low oxygen pressures.
The kidneys measure the oxygen ''content'' (mmol O
2/liter blood, rather than the partial pressure of O
2) of the arterial blood. When the oxygen content of the blood is chronically low, as at high altitude, the oxygen-sensitive kidney cells secrete
erythropoietin (EPO) into the blood.
This hormone stimulates the
red bone marrow to increase its rate of red cell production, which leads to an increase in the
hematocrit
The hematocrit () (Ht or HCT), also known by several other names, is the volume percentage (vol%) of red blood cells (RBCs) in blood, measured as part of a blood test. The measurement depends on the number and size of red blood cells. It is norm ...
of the blood, and a consequent increase in its oxygen carrying capacity (due to the now high
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
content of the blood). In other words, at the same arterial partial pressure of O
2, a person with a high hematocrit carries more oxygen per liter of blood than a person with a lower hematocrit does. High altitude dwellers therefore have higher hematocrits than sea-level residents.
Other functions of the lungs
Local defenses
Irritation of nerve endings within the
nasal passages or
airway
The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed in through the nose to ...
s, can induce a
cough reflex The cough reflex occurs when stimulation of cough receptors in the respiratory tract by dust or other foreign particles produces a cough, which causes rapidly moving air which usually remove the foreign material before it reaches the lungs. This typ ...
and
sneezing
A sneeze (also known as sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and ...
. These responses cause air to be expelled forcefully from the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
or
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes ...
, respectively. In this manner, irritants caught in the
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
which lines the respiratory tract are expelled or moved to the
mouth where they can be
swallowed
Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing ...
.
During coughing, contraction of the smooth muscle in the airway walls narrows the trachea by pulling the ends of the cartilage plates together and by pushing soft tissue into the lumen. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus.
Respiratory epithelium
Respiratory epithelium, or airway epithelium, is a type of ciliated columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways. It is not present in the vocal cords o ...
can secrete a variety of molecules that aid in the defense of the lungs. These include secretory
immunoglobulins (IgA),
collectin Collectins (collagen-containing C-type lectins) are a part of the innate immune system. They form a family of collagenous Ca2+-dependent defense lectins, which are found in animals. Collectins are soluble pattern recognition receptors (PRRs). Their ...
s,
defensins and other peptides and
proteases
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...
,
reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
, and
reactive nitrogen species
Reactive nitrogen species (RNS) are a family of antimicrobial molecules derived from nitric oxide (•NO) and superoxide (O2•−) produced via the enzymatic activity of inducible nitric oxide synthase 2 ( NOS2) and NADPH oxidase respectivel ...
. These secretions can act directly as antimicrobials to help keep the airway free of infection. A variety of
chemokine
Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In additio ...
s and
cytokine
Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...
s are also secreted that recruit the traditional immune cells and others to the site of infections.
Surfactant immune function is primarily attributed to two proteins: SP-A and SP-D. These proteins can bind to sugars on the surface of pathogens and thereby
opsonize
Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. ...
them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection.
Most of the respiratory system is lined with mucous membranes that contain
mucosa-associated lymphoid tissue
The mucosa-associated lymphoid tissue (MALT), also called mucosa-associated lymphatic tissue, is a diffuse system of small concentrations of lymphoid tissue found in various submucosal membrane sites of the body, such as the gastrointestinal tr ...
, which produces
white blood cells such as
lymphocyte
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic ad ...
s.
Prevention of alveolar collapse
The lungs make a
surfactant, a surface-active
lipoprotein
A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, su ...
complex (phospholipoprotein) formed by
type II alveolar cells. It floats on the surface of the thin watery layer which lines the insides of the alveoli, reducing the water's surface tension.
The surface tension of a watery surface (the water-air interface) tends to make that surface shrink.
When that surface is curved as it is in the alveoli of the lungs, the shrinkage of the surface decreases the diameter of the alveoli. The more acute the curvature of the water-air interface
the greater the tendency for the alveolus to collapse.
This has three effects. Firstly the surface tension inside the alveoli resists expansion of the alveoli during inhalation (i.e. it makes the lung stiff, or non-compliant). Surfactant reduces the surface tension and therefore makes the lungs more
compliant, or less stiff, than if it were not there. Secondly, the diameters of the alveoli increase and decrease during the breathing cycle. This means that the alveoli have a
greater tendency to collapse (i.e. cause
atelectasis
Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct ...
) at the end of exhalation that at the end of inhalation. Since surfactant floats on the watery surface, its molecules are more tightly packed together when the alveoli shrink during exhalation.
This causes them to have a greater surface tension-lowering effect when the alveoli are small than when they are large (as at the end of inhalation, when the surfactant molecules are more widely spaced). The tendency for the alveoli to collapse is therefore almost the same at the end of exhalation as at the end of inhalation. Thirdly, the surface tension of the curved watery layer lining the alveoli tends to draw water from the lung tissues into the alveoli. Surfactant reduces this danger to negligible levels, and keeps the alveoli dry.
Pre-term babies who are unable to manufacture surfactant have lungs that tend to collapse each time they breathe out. Unless treated, this condition, called
respiratory distress syndrome, is fatal. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using
steroids as a means of furthering the development of type II alveolar cells. In fact, once a
premature birth
Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is between 2 ...
is threatened, every effort is made to delay the birth, and a series of
steroid injections is frequently administered to the mother during this delay in an effort to promote lung maturation.
Contributions to whole body functions
The lung vessels contain a
fibrinolytic system that dissolves
clots that may have arrived in the pulmonary circulation by
embolism
An embolism is the lodging of an embolus, a blockage-causing piece of material, inside a blood vessel. The embolus may be a blood clot (thrombus), a fat globule (fat embolism), a bubble of air or other gas ( gas embolism), amniotic fluid (am ...
, often from the deep veins in the legs. They also release a variety of substances that enter the systemic arterial blood, and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Some
prostaglandins are removed from the circulation, while others are synthesized in the lungs and released into the blood when lung tissue is stretched.
The lungs activate one hormone. The physiologically inactive decapeptide
angiotensin I
Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the a ...
is converted to the
aldosterone-releasing octapeptide,
angiotensin II, in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Angiotensin II also has a direct effect on
arteriolar walls, causing arteriolar
vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vess ...
, and consequently a rise in
arterial blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
. Large amounts of the
angiotensin-converting enzyme responsible for this activation are located on the surfaces of the
endothelial cells
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
of the alveolar capillaries. The converting enzyme also inactivates
bradykinin
Bradykinin (BK) (Greek brady-, slow; -kinin, kīn(eîn) to move) is a peptide that promotes inflammation. It causes arterioles to dilate (enlarge) via the release of prostacyclin, nitric oxide, and endothelium-derived hyperpolarizing factor and ...
. Circulation time through the alveolar capillaries is less than one second, yet 70% of the angiotensin I reaching the lungs is converted to angiotensin II in a single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells.
Vocalization
The movement of gas through the
larynx,
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
and
mouth allows humans to
speak, or ''
phonate
The term phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, ''phonation'' is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the defin ...
''. Vocalization, or singing, in birds occurs via the
syrinx, an organ located at the base of the trachea. The vibration of air flowing across the larynx (
vocal cords
In humans, vocal cords, also known as vocal folds or voice reeds, are folds of throat tissues that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speec ...
), in humans, and the syrinx, in birds, results in sound. Because of this, gas movement is vital for
communication
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inqui ...
purposes.
Temperature control
Panting in dogs, cats, birds and some other animals provides a means of reducing body temperature, by evaporating saliva in the mouth (instead of evaporating sweat on the skin).
Clinical significance
Disorders of the respiratory system can be classified into several general groups:
* Airway obstructive conditions (e.g.,
emphysema,
bronchitis
Bronchitis is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...
,
asthma
Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...
)
* Pulmonary restrictive conditions (e.g.,
fibrosis
Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...
,
sarcoidosis
Sarcoidosis (also known as ''Besnier-Boeck-Schaumann disease'') is a disease involving abnormal collections of inflammatory cells that form lumps known as granulomata. The disease usually begins in the lungs, skin, or lymph nodes. Less commonly a ...
, alveolar damage,
pleural effusion
A pleural effusion is accumulation of excessive fluid in the pleural space, the potential space that surrounds each lung.
Under normal conditions, pleural fluid is secreted by the parietal pleural capillaries at a rate of 0.6 millilitre per k ...
)
* Vascular diseases (e.g.,
pulmonary edema
Pulmonary edema, also known as pulmonary congestion, is excessive liquid accumulation in the tissue and air spaces (usually alveoli) of the lungs. It leads to impaired gas exchange and may cause hypoxemia and respiratory failure. It is due t ...
,
pulmonary embolism
Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream ( embolism). Symptoms of a PE may include shortness of breath, chest pain particularly upon breathin ...
,
pulmonary hypertension)
* Infectious, environmental and other "diseases" (e.g.,
pneumonia
Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severi ...
,
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, i ...
,
asbestosis
Asbestosis is long-term inflammation and scarring of the lungs due to asbestos fibers. Symptoms may include shortness of breath, cough, wheezing, and chest tightness. Complications may include lung cancer, mesothelioma, and pulmonary heart d ...
,
particulate pollutants)
* Primary cancers (e.g.
bronchial carcinoma,
mesothelioma
Mesothelioma is a type of cancer that develops from the thin layer of tissue that covers many of the internal organs (known as the mesothelium). The most common area affected is the lining of the lungs and chest wall. Less commonly the lining ...
)
* Secondary cancers (e.g. cancers that originated elsewhere in the body, but have seeded themselves in the lungs)
* Insufficient surfactant (e.g.
respiratory distress syndrome in pre-term babies) .
Disorders of the respiratory system are usually treated by a
pulmonologist
Pulmonology (, , from Latin ''pulmō, -ōnis'' "lung" and the Greek suffix "study of"), pneumology (, built on Greek πνεύμων "lung") or pneumonology () is a medical specialty that deals with diseases involving the respiratory tract. and
respiratory therapist
A respiratory therapist is a specialized healthcare practitioner trained in critical care and cardio-pulmonary medicine in order to work therapeutically with people who have acute critical conditions, cardiac and pulmonary disease. Respirator ...
.
Where there is an inability to breathe or insufficiency in breathing a
medical ventilator
A ventilator is a piece of medical technology that provides mechanical ventilation by moving breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators ...
may be used.
Exceptional mammals
Horses
Horses are
obligate nasal breathers which means that they are different from many other mammals because they do not have the option of breathing through their mouths and must take in air through their noses.
Elephants
The
elephant
Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae ...
is the only mammal known to have no
pleural space
The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication bet ...
. Rather, the
parietal and
visceral pleura
The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls.
The inner pleura, called the visceral pleura, covers the surface of each lung and dips b ...
are both composed of dense
connective tissue and joined to each other via loose connective tissue. This lack of a pleural space, along with an unusually thick
diaphragm, are thought to be
evolutionary adaptations allowing the elephant to remain underwater for long periods of time while breathing through its
trunk which emerges as a snorkel.
In the elephant the lungs are attached to the diaphragm and breathing relies mainly on the diaphragm rather than the expansion of the ribcage.
Birds
![Bird's respiratory system](https://upload.wikimedia.org/wikipedia/commons/4/43/Bird%27s_respiratory_system.jpg)
The respiratory system of birds differs significantly from that found in mammals. Firstly, they have rigid lungs which do not expand and contract during the breathing cycle. Instead an extensive system of
air sacs
Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, birds possess the most air sacs (9–11), with their extinct dinosaurian relatives showing a great increase in the pneumatization (presence ...
(Fig. 15) distributed throughout their bodies act as the bellows drawing environmental air into the sacs, and expelling the spent air after it has passed through the lungs (Fig. 18).
Birds also do not have
diaphragms or
pleural cavities.
Bird lungs are smaller than those in mammals of comparable size, but the air sacs account for 15% of the total body volume, compared to the 7% devoted to the
alveoli which act as the bellows in mammals.
Inhalation and exhalation are brought about by alternately increasing and decreasing the volume of the entire thoraco-abdominal cavity (or
coelom) using both their abdominal and costal muscles.
During inhalation the muscles attached to the vertebral ribs (Fig. 17) contract angling them forwards and outwards. This pushes the sternal ribs, to which they are attached at almost right angles, downwards and forwards, taking the
sternum
The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...
(with its prominent
keel
The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...
) in the same direction (Fig. 17). This increases both the vertical and transverse diameters of thoracic portion of the trunk. The forward and downward movement of, particularly, the
posterior end of the sternum pulls the abdominal wall downwards, increasing the volume of that region of the trunk as well.
The increase in volume of the entire trunk cavity reduces the air pressure in all the thoraco-abdominal air sacs, causing them to fill with air as described below.
During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs
anteriorly, and to the pelvis (pubis and ilium in Fig. 17)
posteriorly
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
(forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.
![Cross-current exchanger](https://upload.wikimedia.org/wikipedia/commons/2/2d/Cross-current_exchanger.jpg)
During inhalation air enters the
trachea
The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...
via the nostrils and mouth, and continues to just beyond the
syrinx at which point the trachea branches into two
primary bronchi, going to the two lungs (Fig. 16). The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig. 16).
The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the
caudal end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or
parabronchi) where
gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
occurs (Fig. 16).
As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig. 18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.
From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,
the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.
![BirdRespiration](https://upload.wikimedia.org/wikipedia/commons/f/f2/BirdRespiration.svg)
During exhalation the pressure in the posterior air sacs (which were filled with fresh air during inhalation) increases due to the contraction of the oblique muscle described above. The aerodynamics of the interconnecting openings from the posterior air sacs to the dorsobronchi and intrapulmonary bronchi ensures that the air leaves these sacs in the direction of the lungs (via the dorsobronchi), rather than returning down the intrapulmonary bronchi (Fig. 18).
From the dorsobronchi the fresh air from the posterior air sacs flows through the parabronchi (in the same direction as occurred during inhalation) into ventrobronchi. The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi direct the "spent", oxygen poor air from these two organs to the trachea from where it escapes to the exterior.
Oxygenated air therefore flows constantly (during the entire breathing cycle) in a single direction through the parabronchi.
The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system (Fig. 19).
The
partial pressure of oxygen
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monox ...
in the parabronchi declines along their lengths as O
2 diffuses into the blood. The blood capillaries leaving the exchanger near the entrance of airflow take up more O
2 than do the capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air,
but is nevertheless less than half that of the inhaled air,
thus achieving roughly the same systemic arterial blood partial pressure of oxygen as
mammals do with their bellows-type lungs.
The trachea is an area of
dead space
''Dead Space'' is a science fiction/ horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a linear format; ...
: the oxygen-poor air it contains at the end of exhalation is the first air to re-enter the posterior air sacs and lungs. In comparison to the
mammalian respiratory tract, the dead space volume in a bird is, on average, 4.5 times greater than it is in mammals of the same size.
Birds with long necks will inevitably have long tracheae, and must therefore take deeper breaths than mammals do to make allowances for their greater dead space volumes. In some birds (e.g. the
whooper swan
The whooper swan ( /ˈhuːpə(ɹ) swɒn/) (''Cygnus cygnus''), also known as the common swan, pronounced ''hooper swan'', is a large northern hemisphere swan. It is the Eurasian counterpart of the North American trumpeter swan, and the type speci ...
, ''Cygnus cygnus'', the
white spoonbill
The Eurasian spoonbill (''Platalea leucorodia''), or common spoonbill, is a wading bird of the ibis and spoonbill family Threskiornithidae. The genus name ''Platalea'' is from Latin and means "broad", referring to the distinctive shape of the b ...
, ''Platalea leucorodia'', the
whooping crane
The whooping crane (''Grus americana'') is the tallest North American bird, named for its whooping sound. It is an endangered crane species. Along with the sandhill crane (''Antigone canadensis''), it is one of only two crane species native to ...
, ''Grus americana'', and the
helmeted curassow
The helmeted curassow (''Pauxi pauxi'') or northern helmeted curassow, is a large terrestrial bird in the family Cracidae found in the subtropical cloud-forest in steep, mountainous regions of western Venezuela and northern Colombia. There are t ...
, ''Pauxi pauxi'') the trachea, which some cranes can be 1.5 m long,
is coiled back and forth within the body, drastically increasing the dead space ventilation.
The purpose of this extraordinary feature is unknown.
Reptiles
The
anatomical structure of the
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side ...
is less complex in
reptiles than in
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
, with reptiles lacking the very extensive airway tree structure found in mammalian lungs.
Gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a ...
in reptiles still occurs in
alveoli however.
Reptiles do not possess a
diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of
intercostal muscles
Intercostal muscles are many different groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping expand and shrink the size of ...
in all reptiles except
turtles
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tur ...
. In turtles, contraction of specific pairs of flank muscles governs
inhalation and
exhalation
Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing.
This happens due to elastic properties of the lungs, ...
.
Amphibians
Both the lungs and the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different de ...
serve as respiratory organs in
amphibians. The ventilation of the lungs in amphibians relies on
positive pressure ventilation
Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is based on clinician familiarity and institutional pref ...
. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils into the
oral cavity
In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on t ...
. With the nostrils and mouth closed, the floor of the oral cavity is then pushed up, which forces air down the trachea into the lungs. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
from specialised cells, and is involved in
cutaneous respiration Cutaneous respiration, or cutaneous gas exchange (sometimes called, skin breathing), is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be ...
. While the lungs are of primary organs for gas exchange between the blood and the environmental air (when out of the water), the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.
Some amphibians have gills, either in the early stages of their development (e.g.
tadpole
A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...
s of
frogs
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...
), while others retain them into adulthood (e.g. some
salamanders
Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...
).
Fish
![breathing in fish](https://upload.wikimedia.org/wikipedia/commons/b/b2/Breathing_in_fish.jpg)
Oxygen is poorly soluble in water. Fully aerated
fresh water therefore contains only 8–10 ml O
2/liter compared to the O
2 concentration of 210 ml/liter in the air at sea level.
Furthermore, the
coefficient of diffusion (i.e. the rate at which a substances diffuses from a region of high concentration to one of low concentration, under standard conditions) of the respiratory gases is
typically 10,000 faster in air than in water.
Thus oxygen, for instance, has a diffusion coefficient of 17.6 mm
2/s in air, but only 0.0021 mm
2/s in water.
[CRC Press Online: CRC Handbook of Chemistry and Physics, Section 6, 91st Edition]
/ref>
/ref> The corresponding values for carbon dioxide are 16 mm2/s in air and 0.0016 mm2/s in water. This means that when oxygen is taken up from the water in contact with a gas exchanger, it is replaced considerably more slowly by the oxygen from the oxygen-rich regions small distances away from the exchanger than would have occurred in air. Fish have developed gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
deal with these problems. Gills are specialized organs containing filaments, which further divide into lamellae
Lamella (plural lamellae) means a small plate or flake in Latin, and in English may refer to:
Biology
* Lamella (mycology), a papery rib beneath a mushroom cap
* Lamella (botany)
* Lamella (surface anatomy), a plate-like structure in an animal
* ...
. The lamellae contain a dense thin walled capillary network that exposes a large gas exchange surface area to the very large volumes of water passing over them.
Gills use a countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...
system that increases the efficiency of oxygen-uptake from the water. Fresh oxygenated water taken in through the mouth is uninterruptedly "pumped" through the gills in one direction, while the blood in the lamellae flows in the opposite direction, creating the countercurrent blood and water flow (Fig. 22), on which the fish's survival depends.
Water is drawn in through the mouth by closing the operculum (gill cover), and enlarging the mouth cavity (Fig. 23). Simultaneously the gill chambers enlarge, producing a lower pressure there than in the mouth causing water to flow over the gills. The mouth cavity then contracts, inducing the closure of the passive oral valves, thereby preventing the back-flow of water from the mouth (Fig. 23). The water in the mouth is, instead, forced over the gills, while the gill chambers contract emptying the water they contain through the opercular openings (Fig. 23). Back-flow into the gill chamber during the inhalatory phase is prevented by a membrane along the ventroposterior border of the operculum (diagram on the left in Fig. 23). Thus the mouth cavity and gill chambers act alternately as suction pump and pressure pump to maintain a steady flow of water over the gills in one direction. Since the blood in the lamellar capillaries flows in the opposite direction to that of the water, the consequent countercurrent flow of blood and water maintains steep concentration gradients for oxygen and carbon dioxide along the entire length of each capillary (lower diagram in Fig. 22). Oxygen is, therefore, able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water. Although countercurrent exchange systems theoretically allow an almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood.
In certain active pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
sharks, water passes through the mouth and over the gills while they are moving, in a process known as "ram ventilation". While at rest, most sharks pump water over their gills, as most bony fish do, to ensure that oxygenated water continues to flow over their gills. But a small number of species have lost the ability to pump water through their gills and must swim without rest. These species are ''obligate ram ventilators'' and would presumably asphyxiate
Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that ca ...
if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species.
There are a few fish that can obtain oxygen for brief periods of time from air swallowed from above the surface of the water. Thus lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
possess one or two lungs, and the labyrinth fish have developed a special "labyrinth organ", which characterizes this suborder of fish. The labyrinth organ is a much-folded supra branchial accessory breathing organ
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
. It is formed by a vascularized
Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splitt ...
expansion of the epibranchial bone of the first gill arch, and is used for respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
in air.[Pinter, H. (1986). Labyrinth Fish. Barron's Educational Series, Inc., ] This organ allows labyrinth fish to take in oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
directly from the air, instead of taking it from the water in which they reside through the use of gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
. The labyrinth organ helps the oxygen in the inhaled air to be absorbed into the bloodstream
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
. As a result, labyrinth fish can survive for a short period of time out of water, as they can inhale the air around them, provided they stay moist. Labyrinth fish are not born with functional labyrinth organs. The development of the organ is gradual and most juvenile labyrinth fish breathe entirely with their gills and develop the labyrinth organs when they grow older.
Invertebrates
Arthropods
Some species of crab use a respiratory organ called a branchiostegal lung
A branchiostegal lung is a respiration organ used by some air-breathing arthropods. It is one of the most significant adaptations of some crabs and hermit crabs such as the coconut crab to their terrestrial habitats.
The branchiostegal (gill) tiss ...
. Its gill-like structure increases the surface area for gas exchange which is more suited to taking oxygen from the air than from water. Some of the smallest spider
Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...
s and mite
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear e ...
s can breathe simply by exchanging gas through the surface of the body. Larger spiders, scorpion
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always en ...
s and other arthropods use a primitive book lung.
Insects
Most insects breath passively through their spiracles (special openings in the exoskeleton
An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
) and the air reaches every part of the body by means of a series of smaller and smaller tubes called 'trachaea' when their diameters are relatively large, and 'tracheole
Tracheole (trā'kē-ōl') is a fine respiratory tube of the trachea of an insect or a spider, part of the respiratory system.
Tracheoles are about 1 µm in diameter, and they convey oxygen to cells while providing a means for carbon dioxide ...
s' when their diameters are very small. The tracheoles make contact with individual cells throughout the body. They are partially filled with fluid, which can be withdrawn from the individual tracheoles when the tissues, such as muscles, are active and have a high demand for oxygen, bringing the air closer to the active cells. This is probably brought about by the buildup of lactic acid in the active muscles causing an osmotic gradient, moving the water out of the tracheoles and into the active cells. Diffusion of gases is effective over small distances but not over larger ones, this is one of the reasons insects are all relatively small. Insects which do not have spiracles and trachaea, such as some Collembola, breathe directly through their skins, also by diffusion of gases.
The number of spiracles an insect has is variable between species, however, they always come in pairs, one on each side of the body, and usually one pair per segment. Some of the Diplura have eleven, with four pairs on the thorax, but in most of the ancient forms of insects, such as Dragonflies and Grasshoppers there are two thoracic and eight abdominal spiracles. However, in most of the remaining insects, there are fewer. It is at the level of the tracheoles that oxygen is delivered to the cells for respiration.
Insects were once believed to exchange gases with the environment continuously by the simple diffusion
Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) o ...
of gases into the tracheal system. More recently, however, large variation in insect ventilatory patterns has been documented and insect respiration appears to be highly variable. Some small insects do not demonstrate continuous respiratory movements and may lack muscular control of the spiracles. Others, however, utilize muscular contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
of the abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...
along with coordinated spiracle contraction and relaxation to generate cyclical gas exchange patterns and to reduce water loss into the atmosphere. The most extreme form of these patterns is termed discontinuous gas exchange Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of arthropod gas exchange that have been documented primarily in insects; they occur when the insect i ...
cycles.
Molluscs
Molluscs
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...
generally possess gills that allow gas exchange between the aqueous environment and their circulatory systems. These animals also possess a heart that pumps blood containing hemocyanin
Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2) ...
as its oxygen-capturing molecule. Hence, this respiratory system is similar to that of vertebrate fish. The respiratory system of gastropods The respiratory system of gastropods varies greatly in form. These variations were once used as a basis for dividing the group into subclasses. The majority of marine gastropods breathe through a single gill, supplied with oxygen by a current of wa ...
can include either gills or a lung.
Plants
Plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s use carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
gas in the process of photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
, and exhale oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
gas as waste. The chemical equation of photosynthesis is 6 CO2 (carbon dioxide) and 6 H2O (water), which in the presence of sunlight makes C6H12O6 (glucose) and 6 O2 (oxygen). Photosynthesis uses electrons on the carbon atoms as the repository for the energy obtained from sunlight. Respiration is the opposite of photosynthesis. It reclaims the energy to power chemical reactions in cells. In so doing the carbon atoms and their electrons are combined with oxygen forming CO2 which is easily removed from both the cells and the organism. Plants use both processes, photosynthesis to capture the energy and oxidative metabolism to use it.
Plant respiration is limited by the process of diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
. Plants take in carbon dioxide through holes, known as stomata, that can open and close on the undersides of their leaves and sometimes other parts of their anatomy. Most plants require some oxygen for catabolic
Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipid ...
processes (break-down reactions that release energy). But the quantity of O2 used per hour is small as they are not involved in activities that require high rates of aerobic
Aerobic means "requiring air," in which "air" usually means oxygen.
Aerobic may also refer to
* Aerobic exercise, prolonged exercise of moderate intensity
* Aerobics, a form of aerobic exercise
* Aerobic respiration, the aerobic process of cel ...
metabolism. Their requirement for air, however, is very high as they need CO2 for photosynthesis, which constitutes only 0.04% of the environmental air. Thus, to make 1 g of glucose requires the removal of all the CO2 from ''at least'' 18.7 liters of air at sea level. But inefficiencies in the photosynthetic process cause considerably greater volumes of air to be used.
See also
*
*
*
*Pulmonary function testing
Pulmonary function testing (PFT) is a complete evaluation of the respiratory system including patient history, physical examinations, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity ...
(PFT)
References
External links
A high school level description of the respiratory system
A simple guide for high school students
The Respiratory System
University level (Microsoft Word document)
by noted respiratory physiologist John B. West (also a
YouTube
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