Thelungs are the primaryorgans of therespiratory system in many animals, including humans. Inmammals and most othertetrapods, two lungs are located near thebackbone on either side of theheart. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process ofgas exchange. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal systems to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles viabuccal pumping, a mechanism still seen in amphibians. In humans, the primary muscle that drives breathing is thediaphragm. The lungs also provide airflow that makesvocalisation includingspeech possible.
Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of thechest. The right lung is bigger than the left, and the left lung shares space in the chest with the heart. The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier. The lungs are part of thelower respiratory tract that begins at thetrachea and branches into thebronchi andbronchioles, which receive airbreathed in via theconducting zone. These divide until air reaches microscopicalveoli, where gas exchange takes place. Together, the lungs contain approximately 2,400 kilometers (1,500 mi) of airways and 300 to 500 million alveoli. Each lung is enclosed within apleural sac of twopleurae which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. The innervisceral pleura divides each lung asfissures into sections called lobes. The right lung has three lobes and the left has two. The lobes are further divided intobronchopulmonary segments and lobules. The lungs have a unique blood supply, receiving deoxygenated blood sent from the heart to receive oxygen (thepulmonary circulation) and a separate supply of oxygenated blood (thebronchial circulation).
The tissue of the lungs can be affected by severalrespiratory diseases includingpneumonia andlung cancer. Chronic diseases such aschronic obstructive pulmonary disease andemphysema can be related to smoking or exposure to harmful substances. Diseases such as bronchitis can also affect the respiratory tract. Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius (of the lungs) as in pulmonology, or with pneumo- (from Greek πνεύμων "lung") as in pneumonia.
Inembryonic development, the lungs begin to develop as an outpouching of theforegut, a tube which goes on to form the upper part of thedigestive system. When the lungs are formed thefetus is held in the fluid-filledamniotic sac and so they do not function to breathe. Blood is also diverted from the lungs through theductus 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.
In humans the lungs are located in thechest on either side of theheart in therib cage. They are conical in shape with a narrow roundedapex at the top, and a broad concavebase that rests on the convex surface of thediaphragm.[1] The apex of the lung extends into the root of the neck, reaching shortly above the level of thesternal end of thefirst rib. The lungs stretch from close to thebackbone in the rib cage to the front of thechest and downwards from the lower part of the trachea to the diaphragm.[1]
The left lung shares space with the heart, and has an indentation in its border called thecardiac notch of the left lung to accommodate this.[2][3] 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 thecarina where the trachea divides into the two main bronchi.[3] Thecardiac impression is an indentation formed on the surfaces of the lungs where they rest against the heart.
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.
The fissures are formed in earlyprenatal development by invaginations of the visceral pleura that divide the lobar bronchi, and section the lungs into lobes that helps in their expansion.[6][7] The right lung is divided into three lobes by ahorizontal fissure, and anoblique 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.[1][7]
The primary bronchi enter the lungs at the hilum and initially branch intosecondary bronchi also known as lobar bronchi that supply air to each lobe of the lung. The lobar bronchi branch intotertiary bronchi also known as segmental bronchi and these supply air to the further divisions of the lobes known asbronchopulmonary segments. Each bronchopulmonary segment has its own (segmental) bronchus andarterial supply.[8] Segments for the left and right lung are shown in the table.[5] The segmental anatomy is useful clinically for localising disease processes in the lungs.[5] A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue.[9]
The left lung (left) and right lung (right). The lobes of the lungs can be seen, and the centralroot of the lung is also present.
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.[10] 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 thesternal end of the fourthcostal cartilage; on themediastinal surface it may be traced back to thehilum.[1] 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.[1][7]
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 theazygos vein, and above this is a wide groove for thesuperior vena cava and rightbrachiocephalic vein; behind this, and close to the top of the lung is a groove for thebrachiocephalic artery. There is a groove for theoesophagus behind the hilum and thepulmonary ligament, and near the lower part of the oesophageal groove is a deeper groove for theinferior vena cava before it enters the heart.[3]
The weight of the right lung varies between individuals, with a standardreference range in men of 155–720 g (0.342–1.587 lb)[11] and in women of 100–590 g (0.22–1.30 lb).[12]
The left lung is divided into two lobes, an upper and a lower lobe, by the oblique fissure, which extends from thecostal to the mediastinal surface of the lung both above and below thehilum.[1] The left lung, unlike the right, does not have a middle lobe, though it does have ahomologous feature, a projection of the upper lobe termed thelingula. 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.[13][14] There are twobronchopulmonary segments of the lingula: superior and inferior.[1]
The mediastinal surface of the left lung has a largecardiac 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.[3]
On the same surface, immediately above the hilum, is a well-marked curved groove for theaortic arch, and a groove below it for thedescending aorta. Theleft 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 leftbrachiocephalic vein. The oesophagus may sit in a wider shallow impression at the base of the lung.[3]
By standardreference range, the weight of the left lung is 110–675 g (0.243–1.488 lb)[11] in men and 105–515 g (0.231–1.135 lb) in women.[12]
The lungs are part of thelower respiratory tract, and accommodate the bronchial airways when they branch from the trachea. The bronchial airways terminate inalveoli which make up the functional tissue (parenchyma) of the lung, and veins, arteries, nerves, andlymphatic vessels.[3][15] The trachea and bronchi have plexuses oflymph capillaries in their mucosa and submucosa. The smaller bronchi have a single layer of lymph capillaries, and they are absent in the alveoli.[16] The lungs are supplied with the largest lymphatic drainage system of any other organ in the body.[17] Each lung is surrounded by aserous membrane ofvisceral pleura, which has an underlying layer ofloose connective tissue attached to the substance of the lung.[18]
The connective tissue of the lungs is made up ofelastic andcollagen fibres that are interspersed between the capillaries and the alveolar walls.Elastin is the keyprotein of theextracellular matrix and is the main component of theelastic fibres.[19] Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known aslung compliance. It is also responsible for theelastic recoil needed. Elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions.[19] 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 thepores of Kohn.[20]
Pulmonary neuroendocrine cells are found throughout the respiratory epithelium including the alveolar epithelium,[22] though they only account for around 0.5 percent of the total epithelial population.[23] PNECs are innervated airway epithelial cells that are particularly focused at airway junction points.[23] 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.[24]
In the bronchi there are incompletetracheal rings ofcartilage and smaller plates of cartilage that keep them open.[25]: 472 Bronchioles are too narrow to support cartilage and their walls are ofsmooth muscle, and this is largely absent in the narrowerrespiratory bronchioles which are mainly just of epithelium.[25]: 472 The absence of cartilage in the terminal bronchioles gives them an alternative name ofmembranous bronchioles.[26]
A lobule of the lung enclosed in septa and supplied by a terminal bronchiole that branches into the respiratory bronchioles. Each respiratory bronchiole supplies the alveoli held in each acinus accompanied by a pulmonary artery branch.
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 theacinus which includes the respiratory bronchioles, the alveolar ducts,alveolar sacs, and alveoli.[27] An acinus measures up to 10 mm in diameter.[28] Aprimary pulmonary lobule is the part of the lung distal to the respiratory bronchiole.[29] Thus, it includes the alveolar ducts, sacs, and alveoli but not the respiratory bronchioles.[30]
The unit described as thesecondary pulmonary lobule is the lobule most referred to as thepulmonary lobule orrespiratory lobule.[25]: 489 [31] This lobule is a discrete unit that is the smallest component of the lung that can be seen without aid.[29] The secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules.[30] 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 apulmonary artery branch. Each lobule is enclosed by an interlobular septum. Each acinus is incompletely separated by an intralobular septum.[28]
The respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli.[20] The walls of the alveoli are extremely thin allowing a fast rate ofdiffusion. The alveoli have interconnecting small air passages in their walls known as thepores of Kohn.[20]
Alveoli and their capillary networks3D medical illustration showing different terminating ends of bronchioles
Alveoli consist of two types ofalveolar cell and analveolar macrophage. The two types of cell are known astype I andtype II cells[32] (also known as pneumocytes).[3] Types I and II make up the walls andalveolar 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.[33] Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.[32][33]
Type I aresquamous epithelial cells that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange.[32] These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associatedbasement membranes.[33] Type I cells are not able to divide, and consequently rely ondifferentiation from Type II cells.[33]
Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, andlung surfactant.[34][32] Type II cells are able to divide and differentiate to Type I cells.[33]
Thealveolar macrophages have an important role in theimmune system. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.[33]
Thelower respiratory tract is part of therespiratory system, and consists of thetrachea and the structures below this including the lungs.[32] The trachea receives air from thepharynx and travels down to a place where it splits (thecarina) into a right and left primarybronchus. 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 therespiratory bronchioles. These in turn supply air throughalveolar ducts into thealveoli, where theexchange of gases take place.[32] Oxygenbreathed in,diffuses through the walls of the alveoli into the envelopingcapillaries and into thecirculation,[20] and carbon dioxide diffuses from the blood into the lungs to bebreathed out.
Estimates of the total surface area of lungs vary from 50 to 75 square metres (540 to 810 sq ft);[32][33] although this is often quoted in textbooks and the media being "the size of a tennis court",[33][38][39] it is actually less than half the size of asingles court.[40]
The lungs have a dual blood supply provided by abronchial and apulmonary circulation.[4] Thebronchial circulation supplies oxygenated blood to the airways of the lungs, through thebronchial arteries that leave theaorta. There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles.[32] Thepulmonary 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.[32]
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.[42]
The lobes of the lung are subject toanatomical variations.[46] 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.[47] An oblique fissure was found to be incomplete in 21% to 47% of left lungs.[48] 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.[46]
A variation in the airway branching structure has been found specifically in the central airwaybranching. This variation is associated with the development ofCOPD in adulthood.[49]
The development of the human lungs arise from thelaryngotracheal groove and develop to maturity over several weeks in the foetus and for several years following birth.[50]
Lungs during development, showing the early branching of the primitive bronchial buds
The respiratory tract has a branching structure, and is also known as the respiratory tree.[53] In the embryo this structure is developed in the process ofbranching morphogenesis,[54] 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 abronchial bud grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles.[55] The branching is a result of the tip of each tube bifurcating.[53] The branching process forms the bronchi, bronchioles, and ultimately the alveoli.[53] The four genes mostly associated with branching morphogenesis in the lung are theintercellular signalling protein –sonic hedgehog (SHH),fibroblast growth factorsFGF10 and FGFR2b, andbone morphogenetic proteinBMP4. FGF10 is seen to have the most prominent role. FGF10 is aparacrine signalling molecule needed for epithelial branching, and SHH inhibits FGF10.[53][55] 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 leftprimary bronchial buds on each side of the trachea.[56][57] 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.[57] From the sixth week to the sixteenth week, the major elements of the lungs appear except thealveoli.[58] 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.[59] During the period covering the 26th week until birth the importantblood–air barrier is established. Specialisedtype I alveolar cells wheregas exchange will take place, together with thetype II alveolar cells that secretepulmonary surfactant, appear. The surfactant reduces thesurface 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,[60]and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.[50]
The developing lung is particularly vulnerable to changes in the levels ofvitamin 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.[61]
Atbirth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated.After birth the infant'scentral nervous system reacts to the sudden change in temperature and environment. This triggers the first breath, within about ten seconds after delivery.[62] Before birth, the lungs are filled with fetal lung fluid.[63] After the first breath, the fluid is quickly absorbed into the body or exhaled. Theresistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompaniesother changes which result in an increased amount of blood entering the lung tissues.[62]
At birth, the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present.[50] The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung.[64][65] Alveolar septa have a doublecapillary 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.[66]
The major function of the lungs isgas exchange between the lungs and the blood.[67] Thealveolar andpulmonary capillary gases equilibrate across the thinblood–air barrier.[34][68][69] 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.[68][70]
The lungs are not capable of expanding tobreathe on their own, and will only do so when there is an increase in the volume of the thoracic cavity.[71] This is achieved by themuscles of respiration, through the contraction of thediaphragm, and theintercostal muscles which pull therib cage upwards as shown in the diagram.[72] Duringbreathing out the muscles relax, returning the lungs to their resting position.[73] At this point the lungs contain thefunctional residual capacity (FRC) of air, which, in the adult human, has a volume of about 2.5–3.0 litres.[73]
Duringheavy breathing as inexertion, a large number ofaccessory muscles in the neck and abdomen are recruited, that during exhalation pull the ribcage down, decreasing the volume of the thoracic cavity.[73] The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left.[73]Lung function testing is carried out to evaluatelung volumes and capacities.
The lungs possess several characteristics which protect against infection. The respiratory tract is lined byrespiratory epithelium or respiratory mucosa, with hair-like projections calledcilia that beat rhythmically and carrymucus. Thismucociliary clearance is an important defence system against air-borne infection.[34] 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.[33][74]: 661–730 The lining of the lung also secretesimmunoglobulin A which protects against respiratory infections;[74]goblet cells secrete mucus[33] which also contains several antimicrobial compounds such asdefensins,antiproteases, andantioxidants.[74] A rare type of specialised cell called apulmonary ionocyte that is suggested may regulate mucus viscosity has been described.[75][76][77] In addition, the lining of the lung also containsmacrophages, immune cells which engulf and destroy debris and microbes that enter the lung in a process known asphagocytosis; anddendritic cells which present antigens to activate components of theadaptive immune system such asT cells andB cells.[74]
The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in themouth and behind the mouth in theoropharynx, and larger particles are trapped innasal hair after inhalation.[74]
The lungs also serve a protective role. Several blood-borne substances, such as a few types ofprostaglandins,leukotrienes,serotonin andbradykinin, are excreted through the lungs.[78] Drugs and other substances can be absorbed, modified or excreted in the lungs.[71][80] The lungs filter out smallblood clots fromveins and prevent them from enteringarteries and causingstrokes.[79]
About 20,000protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal lung.[83][84] 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 differentsurfactant proteins,[34] such asSFTPA1,SFTPB andSFTPC, andnapsin, expressed in type II pneumocytes. Other proteins with elevated expression in the lung are thedynein proteinDNAH5 in ciliated cells, and the secretedSCGB1A1 protein in mucus-secretinggoblet cells of the airway mucosa.[85]
Alcohol affects the lungs and can cause inflammatoryalcoholic lung disease. Acute exposure to alcohol stimulates the beating ofcilia in the respiratory epithelium. However, chronic exposure has the effect of desensitising the ciliary response which reducesmucociliary clearance (MCC). MCC is an innate defense system protecting against pollutants and pathogens, and when this is disrupted the numbers ofalveolar macrophages are decreased. A subsequent inflammatory response is the release ofcytokines. Another consequence is the susceptibility to infection.[90][91]
Alung 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 cavitypleural effusion, or other substances such as air (pneumothorax), blood (hemothorax), or rarer causes. These may be investigated using achest X-ray orCT scan, and may require the insertion of asurgical drain until the underlying cause is identified and treated.[74]
Many obstructive lung diseases are managed by avoiding triggers (such asdust mites orsmoking), with symptom control such asbronchodilators, and with suppression of inflammation (such as throughcorticosteroids) in severe cases. A common cause of chronic bronchitis, and emphysema, is smoking; and common causes ofbronchiectasis include severe infections andcystic fibrosis. The definitive cause ofasthma is not yet known, but it has been linked to other atopic diseases.[74][92]
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 theelastin in the lung's connective tissue that can also result in emphysema. Elastase is inhibited by theacute-phase protein,alpha-1 antitrypsin, and when there is adeficiency in this, emphysema can develop. With persistent stress from smoking, theairway 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 ofCOPD, 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.[93] Basal cells become further deranged in a smoker's transition to clinically defined COPD.[93]
Lung cancer can either arise directly from lung tissue or as a result ofmetastasis from another part of the body. There are two main types of primary tumour described as eithersmall-cell ornon-small-cell lung carcinomas. The major risk factor for cancer issmoking. Once a cancer is identified it isstaged using scans such as aCT scan and a sample of tissue from abiopsy is taken. Cancers may be treated surgically by removing the tumour, the use ofradiotherapy,chemotherapy or a combination, or with the aim ofsymptom control.[74]Lung cancer screening is being recommended in the United States for high-risk populations.[94]
Apneumothorax (collapsed lung) is an abnormal collection of air in thepleural space that causes an uncoupling of the lung from thechest wall.[97] 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) to burst and leak high pressure air into the pleural space.
As part of aphysical examination in response to respiratory symptoms ofshortness of breath, andcough, alung examination may be carried out. This exam includespalpation andauscultation.[98] The areas of the lungs that can belistened to using a stethoscope are called thelung 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 theaxillae, the left axilla for the lingual, the right axilla for the middle right lobe. The anterior fields can also be auscultated from the front.[99] An area known as thetriangle of auscultation is an area of thinner musculature on the back which allows improved listening.[100] Abnormalbreathing sounds heard during a lung exam can indicate the presence of a lung condition;wheezing for example is commonly associated withasthma andCOPD.
Lung function testing is carried out by evaluating a person's capacity to inhale and exhale in different circumstances.[101] The volume of air inhaled and exhaled by a person at rest is thetidal volume (normally 500–750 mL); theinspiratory reserve volume andexpiratory 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'svital capacity. Not all air is expelled from the lungs even after a forced breath out; the remainder of the air is called theresidual volume. Together these terms are referred to aslung volumes.[101]
Pulmonaryplethysmographs are used to measurefunctional residual capacity.[102] 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.[103] The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between four and six litres.[101] 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.[103][104]
Other lung function tests includespirometry, 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 thevital capacity. In particular, how much a person is able to exhale in one second (calledforced 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 isrestrictive orobstructive.[74][101] Another test is that of the lung'sdiffusing capacity – this is a measure of the transfer of gas from air to the blood in the lung capillaries.
Öpke-hésip, a Uyghur dish made withlamb lung andrice sausage
Mammal lung is one of the main types ofoffal, or pluck, alongside theheart andtrachea, and is consumed as a foodstuff around the world in dishes such as Scottishhaggis. The United StatesFood and Drug Administration legally prohibits the sale of animal lungs due to concerns such asfungal spores or cross-contamination with other organs, although this has been criticised as unfounded.[105]
On inhalation, air travels to air sacs near the back of a bird. The air then passes through the lungs to air sacs near the front of the bird, from where the air is exhaled.The cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from left to right in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).[106][107] Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.
The lungs of birds are relatively small, but are connected to eight or nineair 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.[106][108]
The lungs of birds contain millions of tiny parallel passages calledparabronchi. Small sacs calledatria radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site ofgas exchange by simple diffusion.[108] The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).[106][107]
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.[109]
Parabronchi in which the air flow is unidirectional are calledpaleopulmonic 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 termedneopulmonic parabronchi.[108]
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. Intuataras,snakes, and somelizards, the lungs are simpler in structure, similar to that of typical amphibians.[109]
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.[109]
Bothcrocodilians andmonitor lizards have lungs similar to those of birds, providing a unidirectional airflow and even possessing air sacs.[110] The now extinctpterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case oflonchodectids,Tupuxuara, andazhdarchoids, the hindlimbs.[111]
Reptilian lungs typically receive air via expansion and contraction of the ribs driven byaxial muscles and buccal pumping.Crocodilians also rely on thehepatic piston method, in which the liver is pulled back by a muscle anchored to thepubic bone (part of the pelvis) called the diaphragmaticus,[112] which in turn creates negative pressure in the crocodile's thoracic cavity, allowing air to be moved into the lungs byBoyle's law.Turtles, which are unable to move their ribs, instead use their forelimbs andpectoral girdle to force air in and out of the lungs.[109]
Theaxolotl (Ambystoma mexicanum) retains its larval form with gills into adulthood.
The lungs of mostfrogs and otheramphibians 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 apositive pressure system to get air to their lungs, forcing air down into the lungs bybuccal pumping. This is distinct from most higher vertebrates, who use a breathing system driven bynegative pressure where the lungs are inflated by expanding the rib cage.[113] 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 theskull, forcing the air into the lungs.[114]
Due to the possibility of respiration across the skin combined with small size, all known lunglesstetrapods are amphibians. The majority of salamander species arelungless 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.[115] Other known lungless tetrapods are theBornean flat-headed frog[116] andAtretochoana eiselti, acaecilian.[117]
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.[109]
Lungs are found in three groups of fish; thecoelacanths, thebichirs and thelungfish. Like in tetrapods, but unlike fish with swim bladder, the opening is at the ventral side of the oesophagus. The coelacanth has a nonfunctional and unpaired vestigial lung surrounded by a fatty organ.[118] Bichirs, the only group ofray-finned fish with lungs, have a pair which are hollow unchambered sacs, where the gas-exchange occurs on very flat folds that increase their inner surface area. The lungs oflungfish show more resemblance to tetrapod lungs. There is an elaborate network of parenchymal septa, dividing them into numerous respiration chambers.[119][120] In theAustralian lungfish, there is only a single lung, albeit divided into two lobes. Other lungfish, however, have traditionally been considered having two lungs, but newer research defines paired lungs as bilateral lung buds that arise simultaneously and are both connected directly to the foregut, which is only seen in tetrapods.[121] In all lungfish, including the Australian, the lungs are located in the upper dorsal part of the body, with the connecting duct curving around and above the oesophagus. The blood supply also twists around the oesophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.[109]
A number ofinvertebrates have lung-like structures that serve a similar respiratory purpose to true vertebrate lungs, but are not evolutionarily related and only arise out ofconvergent evolution. Somearachnids, such asspiders andscorpions, have structures calledbook lungs used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs.[122] Scorpions havespiracles on their body for the entrance of air to the book lungs.[123]
Thecoconut crab is terrestrial and uses structures calledbranchiostegal lungs to breathe air.[124] Juveniles are released into the ocean, however adults cannot swim and possess an only rudimentary set of gills. The adult crabs can breathe on land and hold their breath underwater.[125] The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.[126]
The lungs of today's terrestrialvertebrates and thegas bladders of today'sfish are believed to have evolved from simple sacs, as outpocketings of theoesophagus, that allowed early fish to gulp air under oxygen-poor conditions.[129] These outpocketings first arose in thebony fish. In most of theray-finned fish, the sacs evolved into closed off gas bladders, while a number ofcarp,trout,herring,catfish, andeels have retained thephysostome condition with the sac being open to the oesophagus. In more basal bony fish, such as thegar,bichir,bowfin and thelobe-finned fish, the bladders have evolved to primarily function as lungs.[129] The lobe-finned fish gave rise to the land-basedtetrapods. Thus, the lungs of vertebrates arehomologous to the gas bladders of fish (but not to theirgills).[130]
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