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

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 vertebrates, two lungs are located near the backbone on either side of the heart. Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide 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. Respiration 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 birds 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 tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, 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

is the

diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...

. 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 of the chest. 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 that begins at the trachea and branches into the bronchi and bronchioles, and which receive air breathed in via the conducting zone. The conducting zone ends at the terminal bronchioles. These divide into the respiratory bronchioles of the respiratory zone which divide into

alveolar duct 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 give rise to the alveolar sacs 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 membranes are separated by a film of pleural fluid, 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 segments and pulmonary lobules. The lungs have a unique blood supply, receiving deoxygenated blood from the heart in the pulmonary circulation 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 tissue of the lungs can be affected by a number of respiratory diseases, including pneumonia and lung cancer. Chronic obstructive pulmonary disease includes chronic bronchitis 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 diseases can be caused by substances such as coal dust, asbestos fibres, and crystalline silica dust. Diseases such as bronchitis 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 ''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 πνεύμων "lung") as in pneumonia. 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 The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. The foregut arises from the endoderm, deve ...

, 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 is held in the fluid-filled amniotic sac and so they do not function to breathe. Blood is also diverted from the lungs through the ductus arteriosus. 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

The lungs are located in the chest on either side of the heart in the rib cage. 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

. 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 lungs stretch from close to the backbone in the rib cage to the front of the chest 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, and the carina 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 vessels and airways pass into the lungs making up the root of the lung. 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 membranes; the outer parietal pleura lines the inner wall of the rib cage 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 bet ...

directly lines the surface of the lungs. Between the pleurae is a potential space called the pleural cavity containing a thin layer of lubricating pleural fluid.

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 individuals ...

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 alve ...

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 segments. 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

end of the fourth costal cartilage; on the

mediastinal The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esophagu ...

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, and above this is a wide groove for the superior vena cava and right brachiocephalic vein; behind this, and close to the top of the lung is a groove for the brachiocephalic artery. There is a groove for the esophagus behind the hilum and the pulmonary ligament, and near the lower part of the esophageal groove is a deeper groove for the inferior vena cava before it enters the heart.Alt URL
/ref> The weight of the right lung varies between individuals, with a standard reference range 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 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 Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...

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 segments 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, and a groove below it for the descending aorta. The

left subclavian artery In human anatomy, the subclavian arteries are paired major arteries of the upper thorax, below the clavicle. They receive blood from the aortic arch. The left subclavian artery supplies blood to the left arm and the right subclavian artery supplie ...

, 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 esophagus may sit in a wider shallow impression at the base of the lung. The weight of the left lung, by standard reference range, 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 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, and accommodate the bronchial airways when they branch from the trachea. The bronchial airways terminate in

which make up the functional tissue (

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word π ...

) of the lung, and veins, arteries, nerves, and lymphatic vessels. The trachea and bronchi have plexuses of 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 of

, which has an underlying layer of loose connective tissue attached to the substance of the lung.

Connective tissue

STD 190219 SWITCH Tissue 2 93x CMLE 20SNR 50IT Elastin

The connective tissue of the lungs is made up of

elastic Elastic is a word often used to describe or identify certain types of elastomer, elastic used in garments or stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rubber used to hold objects togeth ...

and

collagen fibres Type I collagen is the most abundant collagen of the human body. It forms large, eosinophilic fibers known as collagen fibers. It is present in scar tissue, the end product when tissue heals by repair, as well as tendons, ligaments, the endom ...

that are interspersed between the capillaries and the alveolar walls.

Elastin Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bod ...

is the key protein of the extracellular matrix and is the main component of the

elastic fibres Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix composed of bundles of proteins (elastin) which are produced by a number of different cell types including fibroblasts, endothelial, smooth muscle, and air ...

. Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known as

lung compliance Lung compliance, or pulmonary compliance, is a measure of the lung's ability to stretch and expand (distensibility of elastic tissue). In clinical practice it is separated into two different measurements, static compliance and dynamic compliance. ...

. It is also responsible for the

elastic recoil Elastic recoil means the rebound of the lungs after having been stretched by inhalation,Sherwood, L. (2007). ''Human Physiology: From Cells to Systems'', 6th ed, Thomson Brooks/Cole, Belmont, CA. or rather, the ease with which the lung rebounds. Wi ...

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 The pores of Kohn (also known as interalveolar connections or alveolar pores) are discrete holes in walls of adjacent alveoli. Cuboidal type II alveolar cells, which produce surfactant, usually form part of aperture. Etymology The pores of Kohn tak ...

Respiratory epithelium

All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with respiratory epithelium. This is a ciliated epithelium interspersed with

goblet cell Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secre ...

s which produce mucin the main component of mucus, ciliated cells,

basal cells The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The ''stratum basale'' is a single layer of columnar or ...

, and in the terminal bronchioles–

club cells Club cells, also known as bronchiolar exocrine cells, and formerly known as Clara cells, are low columnar/cuboidal cells with short microvilli, found in the small airways (bronchioles) of the lungs. Club cells are found in the ciliated simple epit ...

with actions similar to basal cells, and

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s. The epithelial cells, and the

submucosal gland Submucosal glands can refer to various racemose exocrine glands of the mucous type. These glands secrete mucus to facilitate the movement of particles along the body's various tubes, such as the throat and intestines. The mucosa is the lining of the ...

s throughout the respiratory tract secrete

airway surface liquid 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 is ...

(ASL), the composition of which is tightly regulated and determines how well mucociliary clearance works. 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 tracheal rings of

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck an ...

and smaller plates of cartilage that keep them open. Bronchioles are too narrow to support cartilage and their walls are of

smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

, 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''. Secondary-pulmonary-lobule-illustration

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 sacs, 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. The alveoli have interconnecting small air passages in their walls known as the

Alveoli

3D Medical Animation Bronchial Airways terminating ends

Alveoli consist of two types of alveolar cell and an alveolar macrophage. The two types of cell are known as

type I Type 1 or Type I or ''variant'', may refer to: Health *Diabetes mellitus type 1 (also known as "Type 1 Diabetes"), insulin-dependent diabetes * Type I female genital mutilation * Type 1 personality *Type I hypersensitivity (or immediate hypersensit ...

and

type II cells 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 ...

(also known as pneumocytes). Types I and II make up the walls and alveolar septa. Type I cells provide 95% of the surface area of each alveoli and are flat (" 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 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 membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...

s. Type I cells are not able to divide, and consequently rely on differentiation from Type II cells. Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and

lung surfactant Pulmonary surfactant is a surface-active complex of phospholipids and proteins formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorbing to the air-water inte ...

. 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 The lung microbiota, is the pulmonary microbial community consisting of a complex variety of microorganisms found in the lower respiratory tract particularly on the mucous layer and the epithelial surfaces. These microorganisms include bacteria, ...

that interacts with the airway epithelial cells; an interaction of probable importance in maintaining homeostasis. The

microbiota Microbiota are the range of microorganisms that may be commensal, symbiotic, or pathogenic found in and on all multicellular organisms, including plants. Microbiota include bacteria, archaea, protists, fungi, and viruses, and have been found t ...

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. Fungal

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

that are commonly found as mycobiota in the microbiota include '' Candida'', ''

Malassezia ''Malassezia'' (formerly known as ''Pityrosporum'') is a genus of fungi. It is the sole genus in family Malasseziaceae, which is the only family in order Malasseziales, itself the single member of class Malasseziomycetes. ''Malassezia'' species ...

'', ''

Saccharomyces ''Saccharomyces'' is a genus of fungi that includes many species of yeasts. ''Saccharomyces'' is from Greek σάκχαρον (sugar) and μύκης (fungus) and means ''sugar fungus''. Many members of this genus are considered very important in f ...

'', and '' Aspergillus''.

Respiratory tract

The lower respiratory tract is part of the respiratory system, and consists of the trachea 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) 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

s into the

, where the exchange of gases take place. Oxygen breathed in, diffuses through the walls of the alveoli into the enveloping capillaries and into the

circulation Circulation may refer to: Science and technology * Atmospheric circulation, the large-scale movement of air * Circulation (physics), the path integral of the fluid velocity around a closed curve in a fluid flow field * Circulatory system, a bio ...

, and carbon dioxide diffuses from the blood into the lungs to be 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 singles court. The bronchi in the conducting zone are reinforced with

hyaline cartilage Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has ...

in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by

. Air is warmed to , humidified and cleansed by the conducting zone. Particles from the air being removed by the

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

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 In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

known as the

Hering–Breuer reflex The Hering–Breuer inflation reflex, named for Josef Breuer and Ewald Hering, is a reflex triggered to prevent the over-inflation of the lung. Pulmonary stretch receptors present on the wall of bronchi and bronchioles of the airways respond to exce ...

that prevents the lungs from over-inflation, during forceful inspiration.

Blood supply

The lungs have a dual blood supply provided by a

bronchial 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 a pulmonary circulation. The bronchial circulation supplies oxygenated blood to the airways of the lungs, through the 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 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 The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...

. Input from the

parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...

occurs via the vagus nerve. When stimulated by

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...

, 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 adrenoceptors in the respiratory tract, which causes

bronchodilation A bronchodilator or broncholytic (although the latter occasionally includes secretory inhibition as well) is a substance that dilates the bronchi and bronchioles, decreasing resistance in the respiratory airway and increasing airflow to the lungs ...

. The action of breathing takes place because of nerve signals sent by the respiratory center in the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

, along the

phrenic nerve The phrenic nerve is a mixed motor/sensory nerve which originates from the C3-C5 spinal nerves in the neck. The nerve is important for breathing because it provides exclusive motor control of the diaphragm, the primary muscle of respiration. In ...

from the

cervical plexus The cervical plexus is a plexus of the anterior rami of the first four cervical spinal nerves which arise from C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medi ...

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 The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

, trachea, bronchi and lungs that make up the respiratory tract, begin to form during the fourth week of

embryogenesis 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 ...

from the lung bud which appears ventrally to the caudal portion of the

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 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 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 intercellular signalling protein –

sonic hedgehog Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog''. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organogenesis and ...

(SHH), fibroblast growth factors

FGF10 Fibroblast growth factor 10 is a protein that in humans is encoded by the ''FGF10'' gene. Function The protein encoded by this gene is a member of the fibroblast growth factor (FGF) family. FGF family members possess broad mitogenic and cell s ...

and FGFR2b, and

bone morphogenetic protein Bone morphogenetic proteins (BMPs) are a group of growth factors also known as cytokines and as metabologens. Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of piv ...

BMP4. FGF10 is seen to have the most prominent role. FGF10 is a

paracrine signalling Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over ...

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 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

. 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 type I alveolar cells where gas exchange will take place, together with the type II alveolar cells that secrete pulmonary surfactant, appear. The surfactant reduces the

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...

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 Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Nyctalopia (ni ...

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 birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. 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

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies 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 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 between the lungs and the blood. The

alveolar 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 * ...

and 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. ribcage during inhalation

The lungs are not capable of expanding to 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

, and the intercostal muscles which pull the rib cage upwards as shown in the diagram. During

breathing out 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 ...

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

heavy breathing Heavy Breathing is an American rock band from Washington D.C. formed in 2010 by guitarist Erick Jackson, drummer Jeff Schmid, and keyboardist Amanda Kleinman, who were formerly in the band The Apes. They have released three albums, "Body Problem ...

as in exertion, a large number of 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. 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

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 Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secre ...

secrete mucus which also contains several antimicrobial compounds such as

defensin Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, ...

antiprotease In biology and biochemistry, protease inhibitors, or antiproteases, are molecules that inhibit the function of proteases (enzymes that aid the breakdown of proteins). Many naturally occurring protease inhibitors are proteins. In medicine, ''prot ...

s, and

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...

s. 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

s, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis; and

dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. ...

s which present antigens to activate components of the adaptive immune system such as T cells and

B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

s. The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the

mouth 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 ...

and behind the mouth in the 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 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 ...

as part of the renin–angiotensin system. The inner lining of the blood vessels secretes

angiotensin-converting enzyme Angiotensin-converting enzyme (), or ACE, is a central component of the renin–angiotensin system (RAS), which controls blood pressure by regulating the volume of fluids in the body. It converts the hormone angiotensin I to the active vasoconstr ...

(ACE) an enzyme that catalyses the conversion of angiotensin I to

angiotensin II 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 adre ...

. The lungs are involved in the blood's acid–base homeostasis by expelling carbon dioxide when breathing. The lungs also serve a protective role. Several blood-borne substances, such as a few types of

prostaglandin The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derive ...

s, leukotrienes,

serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vas ...

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

blood clots A thrombus (plural thrombi), colloquially called a blood clot, is the final product of the blood coagulation step in hemostasis. There are two components to a thrombus: aggregated platelets and red blood cells that form a plug, and a mesh of cr ...

from veins and prevent them from entering arteries and causing

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

s. The lungs also play a pivotal role in

by providing air and airflow for the creation of vocal sounds, and other paralanguage communications such as sighs and

gasp Paralanguage, also known as vocalics, is a component of meta-communication that may modify meaning, give nuanced meaning, or convey emotion, by using techniques such as prosody, pitch, volume, intonation, etc. It is sometimes defined as relati ...

s. Research suggests a role of the lungs in the production of blood platelets.

Gene and protein expression

About 20,000 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 Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming ...

proteins, such as

SFTPA1 Surfactant protein A1 (SP-A1), also known as Pulmonary surfactant-associated protein A1 (PSP-A) is a protein that in humans is encoded by the ''SFTPA1'' gene. Summary SP-A1 is primarily synthesised in type II alveolar cells in the lung, as par ...

SFTPB Surfactant protein B is an essential lipid-associated protein found in pulmonary surfactant. Without it, the lung would not be able to inflate after a deep breath out. It rearranges lipid molecules in the fluid lining the lung so that tiny air sa ...

and

SFTPC Surfactant protein C (SP-C), is one of the pulmonary surfactant proteins. In humans this is encoded by the ''SFTPC'' gene. It is a membrane protein. Structure SFTPC is a 197-residue protein made up of two halves: a unique N-terminal propept ...

, and napsin, expressed in type II pneumocytes. Other proteins with elevated expression in the lung are the

dynein Dyneins are a family of cytoskeletal motor proteins that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargos, provides forces and displacements importa ...

protein DNAH5 in ciliated cells, and the secreted SCGB1A1 protein in mucus-secreting

s of the airway mucosa.

Clinical significance

Lungs can be affected by a number of diseases and disorders.

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. ...

is the

medical speciality A medical specialty is a branch of medical practice that is focused on a defined group of patients, diseases, skills, or philosophy. Examples include those branches of medicine that deal exclusively with children (paediatrics), cancer (oncology), ...

that deals with respiratory diseases involving the lungs and respiratory system. Cardiothoracic surgery deals with

surgery Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...

of the lungs including lung volume reduction surgery, lobectomy,

pneumectomy A pneumonectomy (or pneumectomy) is a surgical procedure to remove a lung first successfully done in 1933 by Dr. Evarts Graham. This is not to be confused with a lobectomy or segmentectomy, which only removes one part of the lung. There are two ...

and

lung transplantation Lung transplantation, or pulmonary transplantation, is a surgical procedure in which one or both lungs are replaced by lungs from a donor. Donor lungs can be retrieved from a living or deceased donor. A living donor can only donate one lung lobe. ...

Inflammation and infection

Inflammatory conditions of the lung tissue are pneumonia, of the respiratory tract are bronchitis and bronchiolitis, and of the pleurae 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 infections often occur in those with

immunodeficiency Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

and can include a

fungal infection Fungal infection, also known as mycosis, is disease caused by fungi. Different types are traditionally divided according to the part of the body affected; superficial, subcutaneous, and systemic. Superficial fungal infections include common ti ...

by ''

Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...

'' that can lead to an aspergilloma forming in the lung. Alcohol affects the lungs and can cause inflammatory

alcoholic lung disease Alcoholic lung disease is disease of the lungs caused by excessive alcohol. The term 'alcoholic lung disease' is not a generally accepted medical diagnosis, and "the association between alcohol abuse and acute lung injury remains largely unreco ...

. Acute exposure to alcohol stimulates the beating of

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, 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

lung contusion 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 of t ...

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 A chest radiograph, called a chest X-ray (CXR), or chest film, is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures. Chest radiographs are the most common film taken in med ...

CT scan A computed tomography scan (CT scan; formerly called computed axial tomography scan or CAT scan) is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers ...

, and may require the insertion of a

surgical drain A surgical drain is a tube used to remove pus, blood or other fluids from a wound, body cavity, or organ. They are commonly placed by surgeons or interventional radiologists after procedures or some types of injuries, but they can also be used ...

until the underlying cause is identified and treated.

Obstructive lung diseases

Asthma,

bronchiectasis Bronchiectasis is a disease in which there is permanent enlargement of parts of the bronchi, airways of the lung. Symptoms typically include a chronic cough with sputum, mucus production. Other symptoms include shortness of breath, hemoptysis, co ...

, and chronic obstructive pulmonary disease (COPD) that includes chronic bronchitis, and

, 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 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 ...

such as

spirometry Spirometry (meaning ''the measuring of breath'') is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is he ...

. Many obstructive lung diseases are managed by avoiding triggers (such as

dust mite House dust mites (HDM, or simply dust mites) are various species of acariform mites belonging to the family Pyroglyphidae that are found in association with dust in dwellings. They are known for causing allergies. Biology Species The curren ...

s or

), with symptom control such as bronchodilators, and with suppression of inflammation (such as through

corticosteroid Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, glucocorticoids and mineralocorticoids, are involv ...

s) in severe cases. A common cause of chronic bronchitis, and emphysema, is smoking; and common causes of

include severe infections and

cystic fibrosis Cystic fibrosis (CF) is a rare genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. O ...

. 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 Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bod ...

in the lung's connective tissue that can also result in emphysema. Elastase is inhibited by the

acute-phase protein Acute-phase proteins (APPs) are a class of proteins whose concentrations in blood plasma either increase (positive acute-phase proteins) or decrease (negative acute-phase proteins) in response to inflammation. This response is called the ''acute-p ...

, alpha-1 antitrypsin, and when there is a

deficiency A deficiency is generally a lack of something. It may also refer to: *A deficient number, in mathematics, a number ''n'' for which ''σ''(''n'') < 2''n'' * 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 diseases such as

Coalworker's pneumoconiosis Coal workers' pneumoconiosis (CWP), also known as black lung disease or black lung, is an occupational type of pneumoconiosis caused by long-term exposure to coal dust. It is common in coal miners and others who work with coal. It is similar to b ...

autoimmune disease An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. At least 80 types of autoimmune diseases have been identified, with some evidence suggesting that there may be more than 100 types. Nearly a ...

s 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 or non-small-cell lung carcinomas. The major risk factor for cancer is

. Once a cancer is identified it is

staged ''Staged'' is a British television comedy series, set during the COVID-19 pandemic in the United Kingdom and primarily filmed using video-conferencing technology. The first series premiered on 10 June 2020 on BBC One, and the second series prem ...

using scans such as a

and a sample of tissue from a biopsy is taken. Cancers may be treated surgically by removing the tumour, the use of

radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...

, chemotherapy or a combination, or with the aim of symptom control.

Lung cancer screening Lung cancer screening refers to cancer screening strategies used to identify early lung cancers before they cause symptoms, at a point where they are more likely to be curable. Lung cancer screening is critically important because of the inciden ...

is being recommended in the United States for high-risk populations.

Congenital disorders

Congenital disorders include

pulmonary hypoplasia Pulmonary hypoplasia is incomplete development of the lungs, resulting in an abnormally low number or size of bronchopulmonary segments or alveoli. A congenital malformation, it most often occurs secondary to other fetal abnormalities that inter ...

(an incomplete development of the lungs) congenital diaphragmatic hernia, and

infant respiratory distress syndrome Infantile respiratory distress syndrome (IRDS), also called respiratory distress syndrome of newborn, or increasingly surfactant deficiency disorder (SDD), and previously called hyaline membrane disease (HMD), is a syndrome in premature infants c ...

caused by a deficiency in lung surfactant. An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in 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 thoracic wall or chest wall is the boundary of the thoracic cavity. Structure The bone, bony human skeleton, skeletal part of the thoracic wall is the rib cage, and the rest is made up of muscle, skin, and fasciae. The chest wall has 10 lay ...

. 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 diver Scuba diving is a mode of underwater diving whereby divers use breathing equipment that is completely independent of a surface air supply. The name "scuba", an acronym for "Self-Contained Underwater Breathing Apparatus", was coined by Chris ...

s ascending while holding their breath with their lungs fully inflated can cause air sacs (

) to burst and leak high pressure air into the pleural space.

Examination

As part of a physical examination in response to respiratory symptoms of shortness of breath, and cough, a lung examination may be carried out. This exam includes palpation and

auscultation Auscultation (based on the Latin verb ''auscultare'' "to listen") is listening to the internal sounds of the body, usually using a stethoscope. Auscultation is performed for the purposes of examining the circulatory and respiratory systems (hea ...

. The areas of the lungs that can be listened to using a stethoscope are called the

lung fields A respiratory examination, or lung examination, is performed as part of a physical examination, in response to respiratory symptoms such as shortness of breath, cough, or chest pain, and is often carried out with a cardiac examination. The four ...

, 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 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

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

residual volume In medicine, residual volume may refer to: * Residual volume, air remaining in the lungs after a maximal exhalation; see lung volumes * Residual volume, urine remaining in the bladder after voiding; see urinary retention * Gastric 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. 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

, 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

forced expiratory volume Spirometry (meaning ''the measuring of breath'') is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is h ...

(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 In semantics, a modifier is said to be restrictive (or ''defining'') if it restricts the reference of its head. For example, in "the red car is fancier than the blue one", ''red'' and ''blue'' are restrictive, because they restrict which cars ''c ...

or 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 Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capabl ...

. 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 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

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

s, 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.

Amphisbaenian Amphisbaenia (called amphisbaenians or worm lizards) is a group of usually legless squamates, comprising over 200 extant species. Amphisbaenians are characterized by their long bodies, the reduction or loss of the limbs, and rudimentary eyes. As ...

s, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung. Both crocodilians and monitor lizards have lungs similar to those of birds, providing a unidirectional airflow and even possessing air sacs. The now extinct

pterosaurs Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of lonchodectids,

tupuxuara ''Tupuxuara'' is a genus of large, crested, and toothless Pterodactyloidea, pterodactyloid pterosaur from the Early Cretaceous period (Albian stage) of what is now the Romualdo Formation of the Santana Group, Brazil, about 125 to 112 million ye ...

, and azhdarchoids, the hindlimbs. Reptilian lungs typically receive air via expansion and contraction of the ribs driven by axial muscles and buccal pumping.

Crocodilian Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...

s also rely on the hepatic piston method, in which the liver is pulled back by a muscle anchored to the 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

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. This is distinct from most higher vertebrates, who use a breathing system driven by 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 tetrapods 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 ''Atretochoana eiselti'' is a species of caecilian originally known only from two preserved specimens discovered by Sir Graham Hales in the Brazilian rainforest, while on an expedition with Sir Brian Doll in the late 1800s, but rediscovered in 2 ...

'', a caecilian. The lungs of amphibians typically have a few narrow internal walls ( 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 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 lung A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and conn ...

s used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs. Scorpions have

spiracle Spiracle or spiraculum may refer to: * Spiracle (arthropods), opening in the exoskeletons of some arthropods * Spiracle (vertebrates), openings on the surface of some vertebrates * Spiraculum, a genus of land snails in family Cyclophoridae Cycl ...

s 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 Pulmonata or pulmonates, is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group include ...

are mostly

land snail A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. ''Land snail'' is the common name for terrestrial gastropod mollusks that have shells (those without shells are known as ...

s and

slug Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a smal ...

s 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 vertebrates and the

gas bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth w ...

s of today's fish are believed to have evolved from simple sacs, as outpocketings of the esophagus, that allowed early fish to gulp air under oxygen-poor conditions. These outpocketings first arose in the

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

. In most of the ray-finned fish the sacs evolved into closed off gas bladders, while a number of

carp Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...

, trout,

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, i ...

, catfish, and eels have retained the

physostome Physostomes are fishes that have a pneumatic duct connecting the gas bladder to the alimentary canal. This allows the gas bladder to be filled or emptied via the mouth. This not only allows the fish to fill their bladder by gulping air, but also ...

condition with the sac being open to the esophagus. In more basal bony fish, such as the gar, bichir,

bowfin The bowfin (''Amia calva'') is a bony fish, native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being the sole surviving species of the Halecomorphi ...

and the lobe-finned fish, the bladders have evolved to primarily function as lungs. The lobe-finned fish gave rise to the land-based tetrapods. Thus, the lungs of vertebrates are

to the gas bladders of fish (but not to their gills).

References

External links

Lung at the Human Protein Atlas
{{DEFAULTSORT:Human Lung Human anatomy by organ Articles containing video clips