The lungs are the primary
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 ...
of the
respiratory system
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 ...
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, culture, ...
s and most other
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, including some
snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
s and a small number of
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 li ...
. In
mammal
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 or ...
s and most other
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, ...
s, two lungs are located near the
backbone
The backbone is the vertebral column of a vertebrate.
Arts, entertainment, and media Film
* ''Backbone'' (1923 film), a 1923 lost silent film starring Alfred Lunt
* ''Backbone'' (1975 film), a 1975 Yugoslavian drama directed by Vlatko Gilić
...
on either side of the
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
. Their function in the respiratory system is to extract
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 wel ...
from the
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
and transfer it 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 ...
, and to release
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 transpar ...
from the bloodstream into the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, in a process of
gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, 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 liqui ...
.
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 ...
is driven by different
muscular systems in different species. Mammals,
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s and
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 lightweigh ...
s use their different
muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s to support and foster
breathing
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cellular ...
. In earlier
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s, air was driven into the lungs by the
pharyngeal muscles
The pharyngeal muscles are a group of muscles that form the pharynx, which is posterior to the oral cavity, determining the shape of its lumen, and affecting its sound properties as the primary resonating cavity.
The pharyngeal muscles (involunta ...
via
buccal pumping
Buccal pumping is "breathing with one's cheeks": a method of ventilation used in respiration in which the animal moves the floor of its mouth in a rhythmic manner that is externally apparent.Brainerd, E. L. (1999). New perspectives on the evolutio ...
, a mechanism still seen in
amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s. In humans, the main
muscle of respiration that drives
breathing
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cellular ...
is the
diaphragm. The lungs also provide airflow that makes vocal sounds including human
speech
Speech is a human vocal communication using language. Each language uses Phonetics, phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if ...
possible.
Humans have two lungs, one on the left and one on the right. They are situated within the
thoracic cavity
The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. There ...
of the
chest
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
. The right lung is bigger and heavier than the left, which shares space in the chest with the heart. The lungs together weigh approximately . The lungs are part of the
lower respiratory tract
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 th ...
that begins at the
trachea
The trachea, also known as the windpipe, is a Cartilage, 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 ...
and branches into 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
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 which receive
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
breathed in via the
conducting zone
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 ...
. The conducting zone ends at the
terminal bronchioles
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 o ...
. These divide into the
respiratory bronchioles
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 o ...
of the
respiratory zone
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 ...
which divide into
alveolar ducts that give rise to the
alveolar sac
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. Al ...
s that contain the
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
, where gas exchange takes place. Alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. Together, the lungs contain approximately of airways and 300 to 500 million alveoli. Each lung is enclosed within a
pleural sac
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 ...
of two
membranes
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
called
pleurae
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 be ...
; the membranes are separated by a film of
pleural fluid
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 be ...
, which allows the inner and outer membranes to slide over each other whilst breathing takes place, without much friction. The inner pleura also divides each lung into sections called
lobes. The right lung has three lobes and the left has two. The lobes are further divided into
bronchopulmonary segment
A bronchopulmonary segment is a portion of lung supplied by a specific segmental bronchus and its vessels. These arteries branch from the pulmonary and bronchial arteries, and run together through the center of the segment. Veins and lymphatic vess ...
s and
pulmonary lobules. The lungs have a unique blood supply, receiving deoxygenated blood from the heart in the
pulmonary circulation
The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs ...
for the purposes of receiving oxygen and releasing carbon dioxide, and a separate supply of oxygenated blood to the tissue of the lungs, in the
bronchial circulation
The bronchial circulation is the part of the circulatory system that supplies nutrients and oxygen to the cells that constitute the lungs, as well as carrying waste products away from them. It is complementary to the pulmonary circulation that brin ...
.
The
tissue of the lungs can be affected by a number of
respiratory disease
Respiratory diseases, or lung diseases, are pathology, pathological conditions affecting the organs and tissues that make gas exchange difficult in Breathing, air-breathing animals. They include conditions of the respiratory tract including the t ...
s, including
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 severity ...
and
lung cancer
Lung cancer, also known as lung carcinoma (since about 98–99% of all lung cancers are carcinomas), is a malignant lung tumor characterized by uncontrolled cell growth in tissue (biology), tissues of the lung. Lung carcinomas derive from tran ...
.
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 ...
includes
chronic 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. ...
and
emphysema
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...
, and can be related to
smoking
Smoking is a practice in which a substance is burned and the resulting smoke is typically breathed in to be tasted and absorbed into the bloodstream. Most commonly, the substance used is the dried leaves of the tobacco plant, which have bee ...
or exposure to
harmful substances. A number of
occupational lung disease
Occupational lung diseases are work-related, lung conditions that have been caused or made worse by the materials a person is exposed to within the workplace. It includes a broad group of diseases, including occupational asthma, industrial bronch ...
s can be caused by substances such as
coal dust Coal dust is a fine powdered form of which is created by the crushing, grinding, or pulverizing of coal. Because of the brittle nature of coal, coal dust can be created during mining, transportation, or by mechanically handling coal. It is a form ...
,
asbestos fibres, and
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
dust. Diseases such as
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. ...
can also affect the
respiratory tract
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 th ...
. Medical terms related to the lung often begin with ''pulmo-'', from the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''pulmonarius'' (of the lungs) as in
pulmonology
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. ...
, or with ''pneumo-'' (from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
πνεύμων "lung") as in
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 severity ...
.
In
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
, the lungs begin to develop as an outpouching of the
foregut, a tube which goes on to form the upper part of the
digestive system
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller compone ...
. When the lungs are formed the
fetus
A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal deve ...
is held in the
fluid-filled amniotic sac
The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until s ...
and so they do not function to breathe. Blood is also diverted from the lungs through the
ductus arteriosus
The ''ductus arteriosus'', also called the ''ductus Botalli'', named after the Italian physiologist Leonardo Botallo, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta. It a ...
.
At birth, however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire. The lungs only fully develop in early childhood.
Structure
Anatomy
![Illu bronchi lungs](https://upload.wikimedia.org/wikipedia/commons/d/db/Illu_bronchi_lungs.jpg)
The lungs are located in the
chest
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
on either side of the
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
in 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 semi- ...
. They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the
diaphragm.
The apex of the lung extends into the root of the neck, reaching shortly above the level of the
sternal
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. Sh ...
end of the
first rib
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 semi ...
. The lungs stretch from close to the
backbone
The backbone is the vertebral column of a vertebrate.
Arts, entertainment, and media Film
* ''Backbone'' (1923 film), a 1923 lost silent film starring Alfred Lunt
* ''Backbone'' (1975 film), a 1975 Yugoslavian drama directed by Vlatko Gilić
...
in the rib cage to the front of the
chest
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
and downwards from the lower part of the trachea to the diaphragm.
The left lung shares space with the heart, and has an indentation in its border called the cardiac notch of the left lung to accommodate this.
The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces. The medial surfaces of the lungs face towards the centre of the chest, and lie against the heart,
great vessels
Great vessels are the large vessels that bring blood to and from the heart. These are:
*Superior vena cava
*Inferior vena cava
*Pulmonary arteries
* Pulmonary veins
*Aorta
Transposition of the great vessels is a group of congenital heart defec ...
, and the
carina
Carina may refer to:
Places
Australia
* Carina, Queensland, a suburb in Brisbane
* Carina Heights, Queensland, a suburb in Brisbane
* Carina, Victoria, a locality in Mildura
Serbia
* Carina, Osečina, a village in the Kolubara District
...
where the trachea divides into the two main bronchi.
The cardiac impression is an indentation formed on the surfaces of the lungs where they rest against the heart.
Both lungs have a central recession called the
hilum, where the
blood vessel
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...
s and
airways pass into the lungs making up the
root of the lung
The root of the lung is a group of structures that emerge at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression of the lung. It is nearer to the back (posterior border) than the front (ant ...
.
There are also
bronchopulmonary lymph nodes on the hilum.
The lungs are surrounded by the
pulmonary pleurae
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 be ...
. The pleurae are two
serous membrane
The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...
s; the outer
parietal 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 be ...
lines the inner wall 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 semi- ...
and the inner
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 ...
directly lines the surface of the lungs. Between the pleurae is a
potential space
In anatomy, a potential space is a space between two adjacent structures that are normally pressed together (directly apposed). Many anatomic spaces are potential spaces, which means that they are potential rather than realized (with their realiz ...
called the
pleural cavity
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 ...
containing a thin layer of lubricating
pleural fluid
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 be ...
.
Lobes
Each lung is divided into sections called lobes by the infoldings of the visceral pleura as fissures. Lobes are divided into segments, and segments have further divisions as lobules. There are three lobes in the right lung and two lobes in the left lung.
Fissures
The fissures are formed in early
prenatal development
Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal devel ...
by invaginations of the visceral pleura that divide the lobar bronchi, and section the lungs into lobes that helps in their expansion.
The right lung is divided into three lobes by a horizontal fissure, and an oblique fissure. The left lung is divided into two lobes by an oblique fissure which is closely aligned with the oblique fissure in the right lung. In the right lung the upper horizontal fissure, separates the upper (superior) lobe from the middle lobe. The lower, oblique fissure separates the lower lobe from the middle and upper lobes.
Variations
Variation or Variations may refer to:
Science and mathematics
* Variation (astronomy), any perturbation of the mean motion or orbit of a planet or satellite, particularly of the moon
* Genetic variation, the difference in DNA among individua ...
in the fissures are fairly common being either incompletely formed
or present as an extra fissure as in the
azygos fissure, or absent. Incomplete fissures are responsible for
interlobar collateral ventilation, airflow between lobes which is unwanted in some
lung volume reduction
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alv ...
procedures.
Segments
The main or primary bronchi enter the lungs at the hilum and initially branch into
secondary bronchi also known as lobar bronchi that supply air to each lobe of the lung. The lobar bronchi branch into
tertiary bronchi also known as segmental bronchi and these supply air to the further divisions of the lobes known as
bronchopulmonary segment
A bronchopulmonary segment is a portion of lung supplied by a specific segmental bronchus and its vessels. These arteries branch from the pulmonary and bronchial arteries, and run together through the center of the segment. Veins and lymphatic vess ...
s. Each bronchopulmonary segment has its own (segmental) bronchus and
arterial supply. Segments for the left and right lung are shown in the table.
The segmental anatomy is useful clinically for localising disease processes in the lungs.
A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue.
Right lung
The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower lobe by two fissures, one oblique and one horizontal.
The upper, horizontal fissure, separates the upper from the middle lobe. It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the
sternal
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. Sh ...
end of the fourth
costal cartilage
The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension.
...
; on the
mediastinal surface it may be traced back to the
hilum.
The lower, oblique fissure, separates the lower from the middle and upper lobes and is closely aligned with the oblique fissure in the left lung.
The mediastinal surface of the right lung is indented by a number of nearby structures. The heart sits in an impression called the cardiac impression. Above the hilum of the lung is an arched groove for the
azygos vein
The azygos vein is a vein running up the right side of the thoracic vertebral column draining itself towards the superior vena cava. It connects the systems of superior vena cava and inferior vena cava and can provide an alternative path for blood ...
, and above this is a wide groove for the
superior vena cava
The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart. It is a large-diameter (24 mm) short length vein th ...
and right
brachiocephalic vein
The left and right brachiocephalic veins (previously called innominate veins) are major veins in the upper chest, formed by the union of each corresponding internal jugular vein and subclavian vein. This is at the level of the sternoclavicular j ...
; behind this, and close to the top of the lung is a groove for the
brachiocephalic artery
The brachiocephalic artery (or brachiocephalic trunk or innominate artery) is an artery of the mediastinum that supplies blood to the right arm and the head and neck.
It is the first branch of the aortic arch. Soon after it emerges, the brachioce ...
. There is a groove for the esophagus behind the hilum and the
pulmonary ligament
The root of the lung is a group of structures that emerge at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression of the lung. It is nearer to the back (posterior border) than the front (ant ...
, and near the lower part of the esophageal groove is a deeper groove for the
inferior vena cava
The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of the ...
before it enters the heart.
[Alt URL]
/ref>
The weight of the right lung varies between individuals, with a standard reference range
In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, o ...
in men of and in women of .
Left lung
The left lung is divided into two lobes, an upper and a lower lobe, by the oblique fissure, which extends from the costal
Costal may refer to:
* an adjective related to the rib () in anatomy
** Costal cartilage, a type of cartilage forming bars which serve to prolong the ribs forward
** Costal margin, the medial margin formed by the false ribs
** Costal surface (disa ...
to the mediastinal surface of the lung both above and below the hilum. The left lung, unlike the right, does not have a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the lingula. Its name means "little tongue". The lingula on the left lung serves as an anatomic parallel to the middle lobe on the right lung, with both areas being predisposed to similar infections and anatomic complications. There are two bronchopulmonary segment
A bronchopulmonary segment is a portion of lung supplied by a specific segmental bronchus and its vessels. These arteries branch from the pulmonary and bronchial arteries, and run together through the center of the segment. Veins and lymphatic vess ...
s of the lingula: superior and inferior.
The mediastinal surface of the left lung has a large ''cardiac impression'' where the heart sits. This is deeper and larger than that on the right lung, at which level the heart projects to the left.
On the same surface, immediately above the hilum, is a well-marked curved groove for the aortic arch
The aortic arch, arch of the aorta, or transverse aortic arch () is the part of the aorta between the ascending and descending aorta. The arch travels backward, so that it ultimately runs to the left of the trachea.
Structure
The aorta begins a ...
, and a groove below it for the descending aorta
In human anatomy, the descending aorta is part of the aorta, the largest artery in the body. The descending aorta begins at the aortic arch and runs down through the chest and abdomen. The descending aorta anatomically consists of two portions o ...
. The left subclavian artery, a branch off the aortic arch, sits in a groove from the arch to near the apex of the lung. A shallower groove in front of the artery and near the edge of the lung, lodges the left brachiocephalic vein
The left and right brachiocephalic veins (previously called innominate veins) are major veins in the upper chest, formed by the union of each corresponding internal jugular vein and subclavian vein. This is at the level of the sternoclavicular j ...
. The esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
may sit in a wider shallow impression at the base of the lung.
The weight of the left lung, by standard reference range
In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, o ...
, in men is in women .
Illustrations
File:CT-Thorax-5.0-B70f-Lungs.jpg, Chest CT (axial lung window)
File:COR-2-STND-CHEST-LUNGS.jpg, Chest CT (coronal lung window)
File:CT-Thorax-5.0-B70f.ogg, Chest CT (axial lung window)
File:COR-2-STND-CHEST.ogg, Chest CT (coronal lung window)
File:Meet the lungs.webm, "Meet the lungs" from Khan Academy
Khan Academy is an American non-profit educational organization created in 2008 by Sal Khan. Its goal is creating a set of online tools that help educate students. The organization produces short lessons in the form of videos. Its website also in ...
File:MP1 Pulmonology.webm, Pulmonology Video
File:Lobes of the Lung ogg.mov (1).ogg, 3D anatomy of the lung lobes and fissures.
Microanatomy
The lungs are part of the lower respiratory tract
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 th ...
, and accommodate the bronchial airways when they branch from the trachea. The bronchial airways terminate in pulmonary alveolus, alveoli which make up the functional tissue (lung parenchyma, parenchyma) of the lung, and veins, arteries, nerves, and lymphatic vessels. The trachea and bronchi have plexuses of lymph capillary, lymph capillaries in their mucosa and submucosa. The smaller bronchi have a single layer of lymph capillaries, and they are absent in the alveoli. The lungs are supplied with the largest lymphatic drainage system of any other organ in the body. Each lung is surrounded by a serous membrane
The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...
of pulmonary pleurae, visceral pleura, which has an underlying layer of loose connective tissue attached to the substance of the lung.
Connective tissue
The connective tissue of the lungs is made up of Elastic fiber, elastic and collagen fibres that are interspersed between the capillaries and the alveolar walls. Elastin is the key protein of the extracellular matrix and is the main component of the elastic fibres. Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known as lung compliance. It is also responsible for the elastic recoil needed. Elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions. The connective tissue links all the alveoli to form the lung parenchyma which has a sponge-like appearance. The alveoli have interconnecting air passages in their walls known as the pores of Kohn.[
]
Respiratory epithelium
All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with respiratory epithelium. This is a cilium, ciliated epithelium interspersed with goblet cells which produce mucin the main component of mucus, ciliated cells, airway basal cell, basal cells, and in the terminal bronchioles–club cells with actions similar to basal cells, and macrophages. The epithelial cells, and the submucosal glands throughout the respiratory tract secrete Mucus#Respiratory system, airway surface liquid (ASL), the composition of which is tightly regulated and determines how well mucociliary clearance works.
Neuroendocrine cell#Pulmonary neuroendocrine cells, Pulmonary neuroendocrine cells are found throughout the respiratory epithelium including the alveolar epithelium, though they only account for around 0.5 per cent of the total epithelial population. PNECs are innervated airway epithelial cells that are particularly focused at airway junction points. These cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. Cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas.
Bronchial airways
In the bronchi there are incomplete Trachea#Structure, tracheal rings of cartilage and smaller plates of cartilage that keep them open. Bronchioles are too narrow to support cartilage and their walls are of smooth muscle, and this is largely absent in the narrower respiratory bronchioles which are mainly just of epithelium. The absence of cartilage in the terminal bronchioles gives them an alternative name of ''membranous bronchioles''.
Respiratory zone
The conducting zone of the respiratory tract ends at the terminal bronchioles when they branch into the respiratory bronchioles. This marks the beginning of the terminal respiratory unit called the acinus which includes the respiratory bronchioles, the alveolar ducts, alveolar sac
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. Al ...
s, and alveoli. An acinus measures up to 10 mm in diameter.[ A primary pulmonary lobule is that part of the acinus that includes the alveolar ducts, sacs, and alveoli but does not include the respiratory bronchioles.] The unit described as the secondary pulmonary lobule is the lobule most referred to as the pulmonary lobule or respiratory lobule. This lobule is a discrete unit that is the smallest component of the lung that can be seen without aid. The secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules.[ The lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. The respiratory bronchioles supply the alveoli in each acinus and is accompanied by a pulmonary artery branch. Each lobule is enclosed by an interlobular septa. Each acinus is incompletely separated by an interlobular septa.]
The respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli.[ The walls of the alveoli are extremely thin allowing a fast rate of Diffusion#Diffusion vs. bulk flow, diffusion. The alveoli have interconnecting small air passages in their walls known as the pores of Kohn.][
]
Alveoli
Alveoli consist of two types of alveolar cell and an alveolar macrophage. The two types of cell are known as Pulmonary alveolus#Type I cells, type I and Pulmonary alveolus#Type II cells, type II cells (also known as pneumocytes). Types I and II make up the walls and alveolar septum, alveolar septa. Type I cells provide 95% of the surface area of each alveoli and are flat ("squamous epithelial cell, squamous"), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape. Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.
Type I are epithelium, squamous epithelial cells that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange. These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated basement membranes. Type I cells are not able to divide, and consequently rely on cellular differentiation, differentiation from Type II cells.
Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and pulmonary surfactant, lung surfactant. Type II cells are able to divide and differentiate to Type I cells.
The alveolar macrophages have an important role in the immune system. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.
Microbiota
There is a large presence of microorganisms in the lungs known as the lung microbiota that interacts with the airway epithelial cells; an interaction of probable importance in maintaining homeostasis. The Microbiota#Humans, microbiota is complex and dynamic in healthy people, and altered in diseases such as asthma and COPD. For example significant changes can take place in COPD following infection with rhinovirus. Fungus, Fungal genera that are commonly found as mycobiota in the microbiota include ''Candida (fungus), Candida'', ''Malassezia'', ''Saccharomyces'', and ''Aspergillus''.
Respiratory tract
The Respiratory tract#Lower respiratory tract, lower respiratory tract is part of the respiratory system
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 ...
, and consists of the trachea
The trachea, also known as the windpipe, is a Cartilage, 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 ...
and the structures below this including the lungs. The trachea receives air from the pharynx and travels down to a place where it splits (the carina
Carina may refer to:
Places
Australia
* Carina, Queensland, a suburb in Brisbane
* Carina Heights, Queensland, a suburb in Brisbane
* Carina, Victoria, a locality in Mildura
Serbia
* Carina, Osečina, a village in the Kolubara District
...
) into a right and left primary bronchus. These supply air to the right and left lungs, splitting progressively into the secondary and tertiary bronchi for the lobes of the lungs, and into smaller and smaller bronchioles until they become the respiratory bronchioles. These in turn supply air through alveolar ducts into the alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
, where the gas exchange, exchange of gases take place. Oxygen breathed in, Molecular diffusion#Biology, diffuses through the walls of the alveoli into the enveloping capillaries and into the circulatory system, circulation, and carbon dioxide diffuses from the blood into the lungs to be exhalation, breathed out.
Estimates of the total surface area of lungs vary from ; although this is often quoted in textbooks and the media being "the size of a tennis court", it is actually less than half the size of a Tennis court, singles court.
The bronchi in the conducting zone
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 ...
are reinforced with hyaline cartilage in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by Smooth muscle tissue, smooth muscle. Air is warmed to , humidity, humidified and cleansed by the conducting zone. Particulates, Particles from the air being removed by the cilia on the respiratory epithelium lining the passageways, in a process called mucociliary clearance.
Pulmonary stretch receptors in the smooth muscle of the airways initiate a reflex known as the Hering–Breuer reflex that prevents the lungs from over-inflation, during forceful inspiration.
Blood supply
The lungs have a dual blood supply provided by a bronchial circulation, bronchial and a pulmonary circulation
The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs ...
. The bronchial circulation
The bronchial circulation is the part of the circulatory system that supplies nutrients and oxygen to the cells that constitute the lungs, as well as carrying waste products away from them. It is complementary to the pulmonary circulation that brin ...
supplies oxygenated blood to the airways of the lungs, through the bronchial artery, bronchial arteries that leave the aorta. There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles. The pulmonary circulation
The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs ...
carries deoxygenated blood from the heart to the lungs and returns the oxygenated blood to the heart to supply the rest of the body.
The blood volume of the lungs is about 450 millilitres on average, about 9% of the total blood volume of the entire circulatory system. This quantity can easily fluctuate from between one-half and twice the normal volume. Also, in the event of blood loss through hemorrhage, blood from the lungs can partially compensate by automatically transferring to the systemic circulation.
Nerve supply
The lungs are supplied by nerves of the autonomic nervous system. Input from the parasympathetic nervous system occurs via the vagus nerve. When stimulated by acetylcholine, this causes constriction of the smooth muscle lining the bronchus and bronchioles, and increases the secretions from glands. The lungs also have a sympathetic tone from norepinephrine acting on the Beta-2 adrenergic receptor, beta 2 adrenoceptors in the respiratory tract, which causes bronchodilation.
The action of breathing takes place because of nerve signals sent by the respiratory center in the brainstem, along the phrenic nerve from the cervical plexus to the diaphragm.
Variation
The lobes of the lung are subject to anatomical variations. A horizontal interlobar fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. An accessory fissure was also found in 14% and 22% of left and right lungs, respectively. An oblique fissure was found to be incomplete in 21% to 47% of left lungs. In some cases a fissure is absent, or extra, resulting in a right lung with only two lobes, or a left lung with three lobes.
A variation in the airway branching structure has been found specifically in the central airway
branching. This variation is associated with the development of COPD in adulthood.
Development
The development of the human lungs arise from the laryngotracheal groove and develop to maturity over several weeks in the foetus and for several years following birth.
The larynx, trachea
The trachea, also known as the windpipe, is a Cartilage, 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 ...
, bronchus, bronchi and lungs that make up the respiratory tract, begin to form during the fourth week of human embryogenesis, embryogenesis from the lung bud which appears ventrally to the caudal portion of the foregut.
The respiratory tract has a branching structure, and is also known as the respiratory tree. In the embryo this structure is developed in the process of Morphogenesis#Branching morphogenesis, branching morphogenesis, and is generated by the repeated splitting of the tip of the branch. In the development of the lungs (as in some other organs) the epithelium forms branching tubes. The lung has a left-right symmetry and each bud known as a lung bud, bronchial bud grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles. The branching is a result of the tip of each tube bifurcating. The branching process forms the bronchi, bronchioles, and ultimately the alveoli. The four genes mostly associated with branching morphogenesis in the lung are the Hedgehog signaling pathway, intercellular signalling protein – sonic hedgehog (SHH), fibroblast growth factors FGF10 and FGFR2b, and bone morphogenetic protein bone morphogenetic protein 4, BMP4. FGF10 is seen to have the most prominent role. FGF10 is a paracrine signalling molecule needed for epithelial branching, and SHH inhibits FGF10. The development of the alveoli is influenced by a different mechanism whereby continued bifurcation is stopped and the distal tips become dilated to form the alveoli.
At the end of the fourth week the lung bud divides into two, the right and left lung bud, primary bronchial buds on each side of the trachea. During the fifth week the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. These give rise to the lobes of the lungs, three on the right and two on the left. Over the following week, the secondary buds branch into tertiary buds, about ten on each side. From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
. From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. By week 26 the terminal bronchioles have formed which branch into two respiratory bronchioles. During the period covering the 26th week until birth the important blood–air barrier is established. Specialised alveolar cells, type I alveolar cells where gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, 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 liqui ...
will take place, together with the alveolar cells, type II alveolar cells that secrete pulmonary surfactant, appear. The surfactant reduces the surface tension at the air-alveolar surface which allows expansion of the alveolar sacs. The alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts,
and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.[
]
Vitamin A deficiency
The developing lung is particularly vulnerable to changes in the levels of vitamin A. Vitamin A deficiency has been linked to changes in the epithelial lining of the lung and in the lung parenchyma. This can disrupt the normal physiology of the lung and predispose to respiratory diseases. Severe nutritional deficiency in vitamin A results in a reduction in the formation of the alveolar walls (septa) and to notable changes in the respiratory epithelium; alterations are noted in the extracellular matrix and in the protein content of the basement membrane. The extracellular matrix maintains lung elasticity; the basement membrane is associated with alveolar epithelium and is important in the blood-air barrier. The deficiency is associated with functional defects and disease states. Vitamin A is crucial in the development of the alveoli which continues for several years after birth.
After birth
At Childbirth, birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. Adaptation to extrauterine life#Breathing and circulation, After birth the infant's central nervous system reacts to the sudden change in temperature and environment. This triggers the first breath, within about 10 seconds after delivery. Before birth, the lungs are filled with fetal lung fluid. After the first breath, the fluid is quickly absorbed into the body or exhaled. The vascular resistance, resistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies Adaptation to extrauterine life, other changes which result in an increased amount of blood entering the lung tissues.
At birth the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present. The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung. Alveolar septa have a double Capillary#Structure, capillary network instead of the single network of the developed lung. Only after the maturation of the capillary network can the lung enter a normal phase of growth. Following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged.
Function
Gas exchange
The major function of the lungs is gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, 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 liqui ...
between the lungs and the blood. The pulmonary alveolus, alveolar and Pulmonary circulation, pulmonary capillary gases equilibrate across the thin blood–air barrier. This thin membrane (about 0.5 –2 μm thick) is folded into about 300 million alveoli, providing an extremely large surface area (estimates varying between 70 and 145 m2) for gas exchange to occur.
The lungs are not capable of expanding to breathing, breathe on their own, and will only do so when there is an increase in the volume of the thoracic cavity. This is achieved by the muscles of respiration, through the contraction of the Thoracic diaphragm, diaphragm, and the intercostal muscles which pull 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 semi- ...
upwards as shown in the diagram. During exhalation, breathing out the muscles relax, returning the lungs to their resting position. At this point the lungs contain the functional residual capacity (FRC) of air, which, in the adult human, has a volume of about 2.5–3.0 litres.
During Hyperpnoea, heavy breathing as in exertion, a large number of Muscles of respiration#Accessory muscles, accessory muscles in the neck and abdomen are recruited, that during exhalation pull the ribcage down, decreasing the volume of the thoracic cavity. The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left. #Function testing, Lung function testing is carried out to evaluate lung volumes and capacities.
Protection
The lungs possess several characteristics which protect against infection. The respiratory tract is lined by respiratory epithelium or respiratory mucosa, with hair-like projections called cilia that beat rhythmically and carry mucus. This mucociliary clearance is an important defence system against air-borne infection.[ The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia.] The lining of the lung also secretes immunoglobulin A which protects against respiratory infections; goblet cells secrete mucus which also contains several antimicrobial compounds such as defensins, antiproteases, and antioxidants. A rare type of specialised cell called a pulmonary ionocyte that is suggested may regulate mucus viscosity has been described. In addition, the lining of the lung also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis; and dendritic cells which present antigens to activate components of the adaptive immune system such as T cells and B cells.
The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the human mouth, mouth and behind the mouth in the Pharynx#Oropharynx, oropharynx, and larger particles are trapped in nasal hair after inhalation.
Other
In addition to their function in respiration, the lungs have a number of other functions. They are involved in maintaining homeostasis, helping in the regulation of blood pressure as part of the renin–angiotensin system. The endothelium, inner lining of the blood vessels secretes angiotensin-converting enzyme (ACE) an enzyme that catalysis, catalyses the conversion of angiotensin I to angiotensin II. The lungs are involved in the blood's acid–base homeostasis by expelling 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 transpar ...
when breathing.
The lungs also serve a protective role. Several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin, are excreted through the lungs. Drugs and other substances can be absorbed, modified or excreted in the lungs. The lungs filter out small thrombus, blood clots from veins and prevent them from entering artery, arteries and causing strokes.
The lungs also play a pivotal role in speech
Speech is a human vocal communication using language. Each language uses Phonetics, phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if ...
by providing air and airflow for the creation of vocal sounds, and other paralanguage Paralanguage#Specific forms of paralinguistic respiration, communications such as sighing, sighs and gasps.
Research suggests a role of the lungs in the production of blood platelets.
Gene and protein expression
About 20,000 Human genome#Coding sequences (protein-coding genes), protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal lung. A little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. The highest expression of lung specific proteins are different surfactant proteins,[ such as Surfactant protein A1, SFTPA1, Surfactant protein B, SFTPB and Surfactant protein C, SFTPC, and NAPSA, napsin, expressed in type II pneumocytes. Other proteins with elevated expression in the lung are the dynein protein DNAH5 in ciliated cells, and the secreted Uteroglobin, SCGB1A1 protein in mucus-secreting goblet cells of the airway mucosa.
]
Clinical significance
Lungs can be affected by a number of diseases and disorders. Pulmonology is the specialty (medicine), medical speciality that deals with respiratory disease
Respiratory diseases, or lung diseases, are pathology, pathological conditions affecting the organs and tissues that make gas exchange difficult in Breathing, air-breathing animals. They include conditions of the respiratory tract including the t ...
s involving the lungs and respiratory system
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 ...
. Cardiothoracic surgery deals with surgery of the lungs including lung volume reduction surgery, lobectomy, pneumectomy and lung transplantation.
Inflammation and infection
Inflammation, Inflammatory conditions of the lung tissue are 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 severity ...
, of the respiratory tract are 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. ...
and bronchiolitis, and of the pleurae
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 be ...
surrounding the lungs pleurisy. Inflammation is usually caused by infections due to bacteria or viruses. When the lung tissue is inflamed due to other causes it is called pneumonitis. One major cause of bacterial pneumonia is tuberculosis. Chronic condition, Chronic infections often occur in those with immunodeficiency and can include a fungal infection by ''Aspergillus fumigatus'' that can lead to an aspergilloma forming in the lung.
Alcohol affects the lungs and can cause inflammatory alcoholic lung disease. Acute exposure to alcohol stimulates the beating of cilia in the respiratory epithelium. However, chronic exposure has the effect of desensitising the ciliary response which reduces mucociliary clearance (MCC). MCC is an innate defense system protecting against pollutants and pathogens, and when this is disrupted the numbers of alveolar macrophages are decreased. A subsequent inflammatory response is the release of cytokines. Another consequence is the susceptibility to infection.
Blood-supply changes
A pulmonary embolism is a blood clot that becomes lodged in the pulmonary arteries. The majority of emboli arise because of deep vein thrombosis in the legs. Pulmonary emboli may be investigated using a ventilation/perfusion scan, CT pulmonary angiography, a CT scan of the arteries of the lung, or blood tests such as the D-dimer. Pulmonary hypertension describes an increased pressure at the beginning of the pulmonary artery that has a large number of differing causes. Other rarer conditions may also affect the blood supply of the lung, such as granulomatosis with polyangiitis, which causes inflammation of the small blood vessels of the lungs and kidneys.
A pulmonary contusion, lung contusion is a bruise caused by chest trauma. It results in hemorrhage of the alveoli causing a build-up of fluid which can impair breathing, and this can be either mild or severe.
The function of the lungs can also be affected by compression from fluid in the pleural cavity pleural effusion, or other substances such as air (pneumothorax), blood (hemothorax), or rarer causes. These may be investigated using a chest X-ray or CT scan, and may require the insertion of a surgical drain until the underlying cause is identified and treated.
Obstructive lung diseases
Asthma, bronchiectasis, and chronic obstructive pulmonary disease (COPD) that includes chronic 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. ...
, and emphysema
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...
, are all obstructive lung diseases characterised by airway obstruction. This limits the amount of air that is able to enter alveoli because of constriction of the bronchial tree, due to inflammation. Obstructive lung diseases are often identified because of symptoms and diagnosed with pulmonary function tests such as spirometry. Many obstructive lung diseases are managed by avoiding triggers (such as dust mites or smoking
Smoking is a practice in which a substance is burned and the resulting smoke is typically breathed in to be tasted and absorbed into the bloodstream. Most commonly, the substance used is the dried leaves of the tobacco plant, which have bee ...
), with symptom control such as bronchodilators, and with suppression of inflammation (such as through corticosteroids) in severe cases. A common cause of chronic bronchitis, and emphysema, is smoking; and common causes of bronchiectasis include severe infections and cystic fibrosis. The definitive cause of asthma is not yet known.
The breakdown of alveolar tissue, often as a result of tobacco-smoking leads to emphysema, which can become severe enough to develop into COPD. Elastase breaks down the elastin in the lung's connective tissue that can also result in emphysema. Elastase is inhibited by the acute-phase protein, alpha-1 antitrypsin, and when there is a Alpha-1 antitrypsin deficiency, deficiency in this, emphysema can develop. With persistent stress from smoking, the airway basal cells become disarranged and lose their regenerative ability needed to repair the epithelial barrier. The disorganised basal cells are seen to be responsible for the major airway changes that are characteristic of COPD, and with continued stress can undergo a malignant transformation. Studies have shown that the initial development of emphysema is centred on the early changes in the airway epithelium of the small airways. Basal cells become further deranged in a smoker's transition to clinically defined COPD.
Restrictive lung diseases
Some types of chronic lung diseases are classified as restrictive lung disease, because of a restriction in the amount of lung tissue involved in respiration. These include pulmonary fibrosis which can occur when the lung is inflamed for a long period of time. Fibrosis in the lung replaces functioning lung tissue with fibrous connective tissue. This can be due to a large variety of occupational lung disease
Occupational lung diseases are work-related, lung conditions that have been caused or made worse by the materials a person is exposed to within the workplace. It includes a broad group of diseases, including occupational asthma, industrial bronch ...
s such as Coalworker's pneumoconiosis, autoimmune diseases or more rarely to a reaction to medication. Severe respiratory disorders, where spontaneous breathing is not enough to maintain life, may need the use of mechanical ventilation to ensure an adequate supply of air.
Cancers
Lung cancer can either arise directly from lung tissue or as a result of metastasis from another part of the body. There are two main types of primary tumour described as either small-cell lung carcinoma, small-cell or non-small-cell lung carcinomas. The major risk factor for cancer is tobacco smoking, smoking. Once a cancer is identified it is tumour staging, staged using scans such as a CT scan and a sample of tissue from a biopsy is taken. Cancers may be treated surgically by removing the tumour, the use of radiotherapy, chemotherapy or a combination, or with the aim of palliative care, symptom control. Lung cancer screening is being recommended in the United States for high-risk populations.
Congenital disorders
Congenital disorders include cystic fibrosis, pulmonary hypoplasia (an incomplete development of the lungs)congenital diaphragmatic hernia, and infant respiratory distress syndrome caused by a deficiency in lung surfactant. An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in thoracoscopy, thoracoscopic procedures.
Pleural space pressure
A pneumothorax (collapsed lung) is an abnormal collection of air in the pleural space that causes an uncoupling of the lung from the chest wall. The lung cannot expand against the air pressure inside the pleural space. An easy to understand example is a traumatic pneumothorax, where air enters the pleural space from outside the body, as occurs with puncture to the chest wall. Similarly, scuba divers ascending while holding their breath with their lungs fully inflated can cause air sacs (alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
) to burst and leak high pressure air into the pleural space.
Examination
As part of a physical examination in response to respiratory symptoms of dyspnea, shortness of breath, and cough, a respiratory examination, lung examination may be carried out. This exam includes palpation and auscultation. The areas of the lungs that can be auscultation, listened to using a stethoscope are called the lung fields, and these are the posterior, lateral, and anterior lung fields. The posterior fields can be listened to from the back and include: the lower lobes (taking up three quarters of the posterior fields); the anterior fields taking up the other quarter; and the lateral fields under the axillae, the left axilla for the lingual, the right axilla for the middle right lobe. The anterior fields can also be auscultated from the front. An area known as the triangle of auscultation is an area of thinner musculature on the back which allows improved listening. Abnormal respiratory sounds, breathing sounds heard during a lung exam can indicate the presence of a lung condition; wheezing for example is commonly associated with asthma and COPD.
Function testing
Pulmonary function testing, Lung function testing is carried out by evaluating a person's capacity to inhale and exhale in different circumstances. The volume of air inhaled and exhaled by a person at rest is the tidal volume (normally 500-750mL); the inspiratory reserve volume and expiratory reserve volume are the additional amounts a person is able to forcibly inhale and exhale respectively. The summed total of forced inspiration and expiration is a person's vital capacity. Not all air is expelled from the lungs even after a forced breath out; the remainder of the air is called the Lung volumes, residual volume. Together these terms are referred to as lung volumes.
Pulmonary plethysmographs are used to measure functional residual capacity. Functional residual capacity cannot be measured by tests that rely on breathing out, as a person is only able to breathe a maximum of 80% of their total functional capacity. The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between 4 and 6 litres. Females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. tobacco smoking, Smokers have a lower capacity than nonsmokers. Thinner persons tend to have a larger capacity. Lung capacity can be increased by physical training as much as 40% but the effect may be modified by exposure to air pollution.
Other lung function tests include spirometry, measuring the amount (volume) and flow of air that can be inhaled and exhaled. The maximum volume of breath that can be exhaled is called the vital capacity. In particular, how much a person is able to exhale in one second (called Spirometry#Parameters, forced expiratory volume (FEV1)) as a proportion of how much they are able to exhale in total (FEV). This ratio, the FEV1/FEV ratio, is important to distinguish whether a lung disease is restrictive lung disease, restrictive or obstructive lung disease, obstructive. Another test is that of the lung's diffusing capacity – this is a measure of the transfer of gas from air to the blood in the lung capillaries.
Other animals
Birds
The lungs of birds are relatively small, but are connected to 8 or 9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.
The lungs of birds contain millions of tiny parallel passages called parabronchi. Small sacs called ''atria'' radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site of gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, 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 liqui ...
by simple diffusion. The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).
The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. The air sacs expand and contract due to changes in the volume in the thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronised with movement of the flight muscles.
Parabronchi in which the air flow is unidirectional are called ''paleopulmonic parabronchi'' and are found in all birds. Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. These are termed ''neopulmonic parabronchi''.
Reptiles
The lungs of most reptiles have a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to alveoli in mammals, but much larger and fewer in number. These give the lung a sponge-like texture. In tuataras, snakes, and some lizards, the lungs are simpler in structure, similar to that of typical amphibians.
Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. Amphisbaenians, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung.
Both Crocodylia, crocodilians and Monitor Lizard, monitor lizards have lungs similar to those of birds, providing a unidirectional airflow and even possessing air sacs. The now extinct Pterosauria, pterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of Lonchodectidae, lonchodectids, tupuxuara, and Azhdarchoidea, azhdarchoids, the hindlimbs.
Reptile, Reptilian lungs typically receive air via expansion and contraction of the ribs driven by axial skeleton, axial muscles and buccal pumping. Crocodilians also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the pubis (bone), pubic bone (part of the pelvis) called the diaphragmaticus, which in turn creates negative pressure in the crocodile's thoracic cavity, allowing air to be moved into the lungs by Boyle's law. Turtles, which are unable to move their ribs, instead use their forelimbs and pectoral girdle to force air in and out of the lungs.
Amphibians
The lungs of most frogs and other amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping
Buccal pumping is "breathing with one's cheeks": a method of ventilation used in respiration in which the animal moves the floor of its mouth in a rhythmic manner that is externally apparent.Brainerd, E. L. (1999). New perspectives on the evolutio ...
. This is distinct from most higher vertebrates, who use a breathing system driven by negative room pressure, negative pressure where the lungs are inflated by expanding the rib cage. In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs.
Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s are amphibians. The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. This necessarily restricts their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume. Other known lungless tetrapods are the Bornean flat-headed frog and ''Atretochoana eiselti'', a caecilian.
The lungs of amphibians typically have a few narrow internal walls (:wikt:septum, septa) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honeycomb appearance. In some salamanders even these are lacking, and the lung has a smooth wall. In caecilians, as in snakes, only the right lung attains any size or development.
Lungfish
The lungs of lungfish are similar to those of amphibians, with few, if any, internal septa. In the Australian lungfish, there is only a single lung, albeit divided into two lobes. Other lungfish and ''Polypterus'', however, have two lungs, which are located in the upper part of the body, with the connecting duct curving around and above the esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
. The blood supply also twists around the esophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.
Invertebrates
Some invertebrates have lung-like structures that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids, such as spiders and scorpions, have structures called book lungs used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs. Scorpions have Spiracle (arthropods), spiracles on their body for the entrance of air to the book lungs.
The coconut crab is terrestrial and uses structures called branchiostegal lungs to breathe air. They cannot swim and would drown in water, yet they possess a rudimentary set of gills. They can breathe on land and hold their breath underwater. The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.
Pulmonates are mostly land snails and slugs that have developed a simple lung from the mantle cavity. An externally located opening called the pneumostome allows air to be taken into the mantle cavity lung.
Evolutionary origins
The lungs of today's terrestrial 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, ...
s and the gas bladders of today's 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 li ...
are believed to have evolved from simple sacs, as outpocketings of the esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
, that allowed early fish to gulp air under oxygen-poor conditions. These outpocketings first arose in the Osteichthyes, bony fish. In most of the Actinopterygii, ray-finned fish the sacs evolved into closed off gas bladders, while a number of carp, trout, herring, catfish, and eels have retained the physostome condition with the sac being open to the esophagus. In more basal bony fish, such as the gar, bichir, bowfin and the Sarcopterygii, lobe-finned fish, the bladders have evolved to primarily function as lungs. The lobe-finned fish gave rise to the land-based tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s. Thus, the lungs of vertebrates are homology (biology), homologous to the gas bladders of fish (but not to their gills).
See also
* Atelectasis
* Drowning
* Interstitial lung disease
* Liquid breathing
* Lung abscess
* Lung-on-a-chip
* Secarecytosis
* List of terms of lung size and activity
References
Further reading
Dr D.R. Johnson: Introductory anatomy, respiratory system
leeds.ac.uk
sln.fi.edu
people.eku.edu
External links
Lung at the Human Protein Atlas
{{DEFAULTSORT:Human Lung
Lung,
Human anatomy by organ
Articles containing video clips