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For other uses, see Lung (disambiguation). Lung_sentence_0

This article uses anatomical terminology. Lung_sentence_1


SystemLung_header_cell_0_2_0 Respiratory systemLung_cell_0_2_1
LatinLung_header_cell_0_4_0 pulmoLung_cell_0_4_1
GreekLung_header_cell_0_5_0 πνεύμων (pneumon)Lung_cell_0_5_1
MeSHLung_header_cell_0_6_0 Lung_cell_0_6_1
TA98Lung_header_cell_0_7_0 Lung_cell_0_7_1
TA2Lung_header_cell_0_8_0 Lung_cell_0_8_1
FMALung_header_cell_0_9_0 Lung_cell_0_9_1

The lungs are the primary organs of the respiratory system in humans and many other animals including a few fish and some snails. Lung_sentence_2

In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. Lung_sentence_3

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 of gas exchange. Lung_sentence_4

Respiration is driven by different muscular systems in different species. Lung_sentence_5

Mammals, reptiles and birds use their different muscles to support and foster breathing. Lung_sentence_6

In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. Lung_sentence_7

In humans, the main muscle of respiration that drives breathing is the diaphragm. Lung_sentence_8

The lungs also provide airflow that makes vocal sounds including human speech possible. Lung_sentence_9

Humans have two lungs, a right lung, and a left lung. Lung_sentence_10

They are situated within the thoracic cavity of the chest. Lung_sentence_11

The right lung is bigger than the left, which shares space in the chest with the heart. Lung_sentence_12

The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier. Lung_sentence_13

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. Lung_sentence_14

The conducting zone ends at the terminal bronchioles. Lung_sentence_15

These divide into the respiratory bronchioles of the respiratory zone which divide into alveolar ducts that give rise to the alveolar sacs that contain the alveoli, where gas exchange takes place. Lung_sentence_16

Alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. Lung_sentence_17

Together, the lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli. Lung_sentence_18

Each lung is enclosed within a pleural sac that contains pleural fluid, which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. Lung_sentence_19

This sac also divides each lung into sections called lobes. Lung_sentence_20

The right lung has three lobes and the left has two. Lung_sentence_21

The lobes are further divided into bronchopulmonary segments and pulmonary lobules. Lung_sentence_22

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. Lung_sentence_23

The tissue of the lungs can be affected by a number of respiratory diseases, including pneumonia and lung cancer. Lung_sentence_24

Chronic obstructive pulmonary disease includes chronic bronchitis and emphysema, and can be related to smoking or exposure to harmful substances. Lung_sentence_25

A number of occupational lung diseases can be caused by substances such as coal dust, asbestos fibres, and crystalline silica dust. Lung_sentence_26

Diseases such as bronchitis can also affect the respiratory tract. Lung_sentence_27

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. Lung_sentence_28

In embryonic development, the lungs begin to develop as an outpouching of the foregut, a tube which goes on to form the upper part of the digestive system. Lung_sentence_29

When the lungs are formed the fetus is held in the fluid-filled amniotic sac and so they do not function to breathe. Lung_sentence_30

Blood is also diverted from the lungs through the ductus arteriosus. Lung_sentence_31

At birth however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire. Lung_sentence_32

The lungs only fully develop in early childhood. Lung_sentence_33

Structure Lung_section_0

Anatomy Lung_section_1

The lungs are located in the chest on either side of the heart in the rib cage. Lung_sentence_34

They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the diaphragm. Lung_sentence_35

The apex of the lung extends into the root of the neck, reaching shortly above the level of the sternal end of the first rib. Lung_sentence_36

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. Lung_sentence_37

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. Lung_sentence_38

The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces. Lung_sentence_39

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. Lung_sentence_40

The cardiac impression is an indentation formed on the surfaces of the lungs where they rest against the heart. Lung_sentence_41

Both lungs have a central recession called the hilum at the root of the lung, where the blood vessels and airways pass into the lungs. Lung_sentence_42

There are also bronchopulmonary lymph nodes on the hilum. Lung_sentence_43

The lungs are surrounded by the pulmonary pleurae. Lung_sentence_44

The pleurae are two serous membranes; the outer parietal pleura lines the inner wall of the rib cage and the inner visceral pleura directly lines the surface of the lungs. Lung_sentence_45

Between the pleurae is a potential space called the pleural cavity containing a thin layer of lubricating pleural fluid. Lung_sentence_46

Lobes and segments Lung_section_2


Lobes and bronchopulmonary segmentsLung_table_caption_1
Right lungLung_header_cell_1_0_0 Left lungLung_header_cell_1_0_1
UpperLung_cell_1_1_0 Upper



Each lung is divided into lobes by the infoldings of the pleura as fissures. Lung_sentence_47

The fissures are double folds of pleura that section the lungs and help in their expansion. Lung_sentence_48

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. Lung_sentence_49

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. Lung_sentence_50

Each bronchopulmonary segment has its own (segmental) bronchus and arterial supply. Lung_sentence_51

Segments for the left and right lung are shown in the table. Lung_sentence_52

The segmental anatomy is useful clinically for localising disease processes in the lungs. Lung_sentence_53

A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue. Lung_sentence_54

Right lung Lung_section_3

The right lung has both more lobes and segments than the left. Lung_sentence_55

It is divided into three lobes, an upper, middle, and a lower lobe by two fissures, one oblique and one horizontal. Lung_sentence_56

The upper, horizontal fissure, separates the upper from the middle lobe. Lung_sentence_57

It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the sternal end of the fourth costal cartilage; on the mediastinal surface it may be traced back to the hilum. Lung_sentence_58

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. Lung_sentence_59

The mediastinal surface of the right lung is indented by a number of nearby structures. Lung_sentence_60

The heart sits in an impression called the cardiac impression. Lung_sentence_61

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. Lung_sentence_62

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. Lung_sentence_63

The weight of the right lung varies between individuals, with a standard reference range in men of 155–720 g (0.342–1.587 lb) and in women of 100–590 g (0.22–1.30 lb). Lung_sentence_64

Left lung Lung_section_4

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. Lung_sentence_65

The left lung, unlike the right, does not have a middle lobe, though it does have a homologous feature, a projection of the upper lobe termed the lingula. Lung_sentence_66

Its name means "little tongue". Lung_sentence_67

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. Lung_sentence_68

There are two bronchopulmonary segments of the lingula: superior and inferior. Lung_sentence_69

The mediastinal surface of the left lung has a large cardiac impression where the heart sits. Lung_sentence_70

This is deeper and larger than that on the right lung, at which level the heart projects to the left. Lung_sentence_71

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. Lung_sentence_72

The left subclavian artery, a branch off the aortic arch, sits in a groove from the arch to near the apex of the lung. Lung_sentence_73

A shallower groove in front of the artery and near the edge of the lung, lodges the left brachiocephalic vein. Lung_sentence_74

The esophagus may sit in a wider shallow impression at the base of the lung. Lung_sentence_75

The weight of the left lung, by standard reference range, in men is 110–675 g (0.243–1.488 lb) in women 105–515 g (0.231–1.135 lb). Lung_sentence_76

Microanatomy Lung_section_5

The lungs are part of the lower respiratory tract, and accommodate the bronchial airways when they branch from the trachea. Lung_sentence_77

The bronchial airways terminate in alveoli, the lung parenchyma (the tissue in between), and veins, arteries, nerves, and lymphatic vessels. Lung_sentence_78

The trachea and bronchi have plexuses of lymph capillaries in their mucosa and submucosa. Lung_sentence_79

The smaller bronchi have a single layer of lymph capillaries, and they are absent in the alveoli. Lung_sentence_80

Each lung is surrounded by a serous membrane of visceral pleura, which has an underlying layer of loose connective tissue attached to the substance of the lung. Lung_sentence_81

Connective tissue Lung_section_6

The connective tissue of the lungs is made up of elastic and collagen fibres that are interspersed between the capillaries and the alveolar walls. Lung_sentence_82

Elastin is the key protein of the extracellular matrix and is the main component of the elastic fibres. Lung_sentence_83

Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known as lung compliance. Lung_sentence_84

It is also responsible for the elastic recoil needed. Lung_sentence_85

Elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions. Lung_sentence_86

The connective tissue links all the alveoli to form the lung parenchyma which has a sponge-like appearance. Lung_sentence_87

The alveoli have interconnecting air passages in their walls known as the pores of Kohn. Lung_sentence_88

Respiratory epithelium Lung_section_7

Main article: Respiratory epithelium Lung_sentence_89

All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with respiratory epithelium. Lung_sentence_90

This is a ciliated epithelium interspersed with goblet cells which produce mucin the main component of mucus, ciliated cells, basal cells, and in the terminal bronchiolesclub cells with actions similar to basal cells, and macrophages. Lung_sentence_91

The epithelial cells, and the submucosal glands throughout the respiratory tract secrete airway surface liquid (ASL), the composition of which is tightly regulated and determines how well mucociliary clearance works. Lung_sentence_92

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. Lung_sentence_93

PNECs are innervated airway epithelial cells that are particularly focused at airway junction points. Lung_sentence_94

These cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. Lung_sentence_95

Cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas. Lung_sentence_96

Bronchial airways Lung_section_8

In the bronchi there are incomplete tracheal rings of cartilage and smaller plates of cartilage that keep them open. Lung_sentence_97

Bronchioles are too narrow to support cartilage and their walls are of smooth muscle, and this is largely absent in the narrower respiratory bronchioles which are mainly just of epithelium. Lung_sentence_98

The absence of cartilage in the terminal bronchioles gives them an alternative name of membranous bronchioles. Lung_sentence_99

Respiratory zone Lung_section_9

The conducting zone of the respiratory tract ends at the terminal bronchioles when they branch into the respiratory bronchioles. Lung_sentence_100

This marks the beginning of an acinus which includes the respiratory bronchioles, the alveolar ducts, alveolar sacs, and alveoli. Lung_sentence_101

This is also called the terminal respiratory unit. Lung_sentence_102

An acinus measures up to 10 mm in diameter. Lung_sentence_103

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. Lung_sentence_104

The unit described as the secondary pulmonary lobule is the lobule most referred to as the pulmonary lobule or respiratory lobule. Lung_sentence_105

This lobule is a discrete unit that is the smallest component of the lung that can be seen without aid. Lung_sentence_106

The secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules. Lung_sentence_107

The lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. Lung_sentence_108

The respiratory bronchioles supply the alveoli in each acinus and is accompanied by a pulmonary artery branch. Lung_sentence_109

Each lobule is enclosed by an interlobular septa. Lung_sentence_110

Each acinus is incompletely separated by an interlobular septa. Lung_sentence_111

The respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli. Lung_sentence_112

The walls of the alveoli are extremely thin allowing a fast rate of diffusion. Lung_sentence_113

The alveoli have interconnecting small air passages in their walls known as the pores of Kohn. Lung_sentence_114

Alveoli Lung_section_10

Main article: Pulmonary alveolus Lung_sentence_115

Alveoli consist of two types of alveolar cell and an alveolar macrophage. Lung_sentence_116

The two types of cell are known as type I and type II cells (also known as pneumocytes). Lung_sentence_117

Types I and II make up the walls and alveolar septa. Lung_sentence_118

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. Lung_sentence_119

Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4. Lung_sentence_120

Type I are squamous epithelial cells that make up the alveolar wall structure. Lung_sentence_121

They have extremely thin walls that enable an easy gas exchange. Lung_sentence_122

These type I cells also make up the alveolar septa which separate each alveolus. Lung_sentence_123

The septa consist of an epithelial lining and associated basement membranes. Lung_sentence_124

Type I cells are not able to divide, and consequently rely on differentiation from Type II cells. Lung_sentence_125

Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and lung surfactant. Lung_sentence_126

Type II cells are able to divide and differentiate to Type I cells. Lung_sentence_127

The alveolar macrophages have an important immunological role. Lung_sentence_128

They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels. Lung_sentence_129

Microbiome Lung_section_11

Main article: Lung microbiota Lung_sentence_130

There is a large presence of microorganisms in the lungs known as the lung microbiome or microbiota. Lung_sentence_131

The lung microbiome interacts with the airway epithelial cells. Lung_sentence_132

The microbiome is complex in healthy people, and altered in diseases such as asthma and COPD. Lung_sentence_133

The lung microbiome is dynamic and significant changes can take place in COPD following infection with rhinovirus. Lung_sentence_134

The interaction between the microbiome and the epithelial cells is of probable importance in the maintenance of stable homeostasis. Lung_sentence_135

Fungal genera that are commonly found in the lung microbiota, known as the lung mycobiome include Candida, Malassezia, Saccharomyces, and Aspergillus. Lung_sentence_136

Respiratory tract Lung_section_12

Main article: Respiratory tract Lung_sentence_137

The lower respiratory tract is part of the respiratory system, and consists of the trachea and the structures below this including the lungs. Lung_sentence_138

The trachea receives air from the pharynx and travels down to a place where it splits (the carina) into a right and left bronchus. Lung_sentence_139

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. Lung_sentence_140

These in turn supply air through alveolar ducts into the alveoli, where the exchange of gases take place. Lung_sentence_141

Oxygen breathed in, diffuses through the walls of the alveoli into the enveloping capillaries and into the circulation, and carbon dioxide diffuses from the blood into the lungs to be breathed out. Lung_sentence_142

Estimates of the total surface area of lungs vary from 50 to 75 square metres (540 to 810 sq ft); 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. Lung_sentence_143

The bronchi in the conducting zone are reinforced with hyaline cartilage in order to hold open the airways. Lung_sentence_144

The bronchioles have no cartilage and are surrounded instead by smooth muscle. Lung_sentence_145

Air is warmed to 37 °C (99 °F), humidified and cleansed by the conducting zone. Lung_sentence_146

Particles from the air being removed by the cilia on the respiratory epithelium lining the passageways, in a process called mucociliary clearance. Lung_sentence_147

Pulmonary stretch receptors in the smooth muscle of the airways initiate a reflex known as the Hering–Breuer reflex that prevents the lungs from over-inflation, during forceful inspiration. Lung_sentence_148

Blood supply Lung_section_13

Main article: Pulmonary circulation Lung_sentence_149

The lungs have a dual blood supply provided by a bronchial and a pulmonary circulation. Lung_sentence_150

The bronchial circulation supplies oxygenated blood to the airways of the lungs, through the bronchial arteries that leave the aorta. Lung_sentence_151

There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles. Lung_sentence_152

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. Lung_sentence_153

The blood volume of the lungs is about 450 millilitres on average, about 9% of the total blood volume of the entire circulatory system. Lung_sentence_154

This quantity can easily fluctuate from between one-half and twice the normal volume. Lung_sentence_155

Also, in the event of blood loss through hemorrhage, blood from the lungs can partially compensate by automatically transferring to the systemic circulation. Lung_sentence_156

Nerve supply Lung_section_14

The lungs are supplied by nerves of the autonomic nervous system. Lung_sentence_157

Input from the parasympathetic nervous system occurs via the vagus nerve. Lung_sentence_158

When stimulated by acetylcholine, this causes constriction of the smooth muscle lining the bronchus and bronchioles, and increases the secretions from glands. Lung_sentence_159

The lungs also have a sympathetic tone from norepinephrine acting on the beta 2 adrenoceptors in the respiratory tract, which causes bronchodilation. Lung_sentence_160

The action of breathing takes place because of nerve signals sent by the respiratory center in the brainstem, along the phrenic nerve from the cervical plexus to the diaphragm. Lung_sentence_161

Variation Lung_section_15

The lobes of the lung are subject to anatomical variations. Lung_sentence_162

A horizontal interlobar fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. Lung_sentence_163

An accessory fissure was also found in 14% and 22% of left and right lungs, respectively. Lung_sentence_164

An oblique fissure was found to be incomplete in 21% to 47% of left lungs. Lung_sentence_165

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. Lung_sentence_166

A variation in the airway branching structure has been found specifically in the central airway branching. Lung_sentence_167

This variation is associated with the development of COPD in adulthood. Lung_sentence_168

Development Lung_section_16

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. Lung_sentence_169

The larynx, trachea, bronchi and lungs that make up the respiratory tract, begin to form during the fourth week of embryogenesis from the lung bud which appears ventrally to the caudal portion of the foregut. Lung_sentence_170

The respiratory tract has a branching structure, and is also known as the respiratory tree. Lung_sentence_171

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. Lung_sentence_172

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. Lung_sentence_173

Each bronchus branches into bronchioles. Lung_sentence_174

The branching is a result of the tip of each tube bifurcating. Lung_sentence_175

The branching process forms the bronchi, bronchioles, and ultimately the alveoli. Lung_sentence_176

The four genes mostly associated with branching morphogenesis in the lung are the intercellular signalling proteinsonic hedgehog (SHH), fibroblast growth factors FGF10 and FGFR2b, and bone morphogenetic protein BMP4. Lung_sentence_177

FGF10 is seen to have the most prominent role. Lung_sentence_178

FGF10 is a paracrine signalling molecule needed for epithelial branching, and SHH inhibits FGF10. Lung_sentence_179

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. Lung_sentence_180

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. Lung_sentence_181

During the fifth week the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. Lung_sentence_182

These give rise to the lobes of the lungs, three on the right and two on the left. Lung_sentence_183

Over the following week, the secondary buds branch into tertiary buds, about ten on each side. Lung_sentence_184

From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli. Lung_sentence_185

From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Lung_sentence_186

Bronchioles and alveolar ducts also develop. Lung_sentence_187

By week 26 the terminal bronchioles have formed which branch into two respiratory bronchioles. Lung_sentence_188

During the period covering the 26th week until birth the important blood–air barrier is established. Lung_sentence_189

Specialised type I alveolar cells where gas exchange will take place, together with the type II alveolar cells that secrete pulmonary surfactant, appear. Lung_sentence_190

The surfactant reduces the surface tension at the air-alveolar surface which allows expansion of the alveolar sacs. Lung_sentence_191

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. Lung_sentence_192

Vitamin A deficiency Lung_section_17

Main article: Vitamin A deficiency Lung_sentence_193

The developing lung is particularly vulnerable to changes in the levels of vitamin A. Lung_sentence_194

Vitamin A deficiency has been linked to changes in the epithelial lining of the lung and in the lung parenchyma. Lung_sentence_195

This can disrupt the normal physiology of the lung and predispose to respiratory diseases. Lung_sentence_196

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. Lung_sentence_197

The extracellular matrix maintains lung elasticity; the basement membrane is associated with alveolar epithelium and is important in the blood-air barrier. Lung_sentence_198

The deficiency is associated with functional defects and disease states. Lung_sentence_199

Vitamin A is crucial in the development of the alveoli which continues for several years after birth. Lung_sentence_200

After birth Lung_section_18

At birth, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. Lung_sentence_201

After birth the infant's central nervous system reacts to the sudden change in temperature and environment. Lung_sentence_202

This triggers the first breath, within about 10 seconds after delivery. Lung_sentence_203

Before birth, the lungs are filled with fetal lung fluid. Lung_sentence_204

After the first breath, the fluid is quickly absorbed into the body or exhaled. Lung_sentence_205

The resistance in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. Lung_sentence_206

This accompanies other changes which result in an increased amount of blood entering the lung tissues. Lung_sentence_207

At birth the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present. Lung_sentence_208

The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung. Lung_sentence_209

Alveolar septa have a double capillary network instead of the single network of the developed lung. Lung_sentence_210

Only after the maturation of the capillary network can the lung enter a normal phase of growth. Lung_sentence_211

Following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged. Lung_sentence_212

Function Lung_section_19

Main articles: Respiratory system, Breathing, and Gas exchange Lung_sentence_213

Gas exchange Lung_section_20

The major function of the lungs is gas exchange between the lungs and the blood. Lung_sentence_214

The alveolar and pulmonary capillary gases equilibrate across the thin blood–air barrier. Lung_sentence_215

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 m) for gas exchange to occur. Lung_sentence_216

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. Lung_sentence_217

This is achieved by the muscles of respiration, through the contraction of the diaphragm, and the intercostal muscles which pull the rib cage upwards as shown in the diagram. Lung_sentence_218

During breathing out the muscles relax, returning the lungs to their resting position. Lung_sentence_219

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. Lung_sentence_220

During heavy breathing 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. Lung_sentence_221

The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left. Lung_sentence_222

Lung function testing is carried out to evaluate lung volumes and capacities. Lung_sentence_223

Protection Lung_section_21

The lungs possess several characteristics which protect against infection. Lung_sentence_224

The respiratory tract is lined by respiratory epithelium or respiratory mucosa, with hair-like projections called cilia that beat rhythmically and carry mucus. Lung_sentence_225

This mucociliary clearance is an important defence system against air-borne infection. Lung_sentence_226

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. Lung_sentence_227

The lining of the lung also secretes immunoglobulin A which protects against respiratory infections; goblet cells secrete mucus which also contains several antimicrobial compounds such as defensins, antiproteases, and antioxidants. Lung_sentence_228

A rare type of specialised cell called a pulmonary ionocyte that is suggested may regulate mucus viscosity has been described. Lung_sentence_229

In addition, the lining of the lung also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis; and dendritic cells which present antigens to activate components of the adaptive immune system such as T-cells and B-cells. Lung_sentence_230

The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Lung_sentence_231

Smaller particles deposit in the mouth and behind the mouth in the oropharynx, and larger particles are trapped in nasal hair after inhalation. Lung_sentence_232

Other Lung_section_22

In addition to their function in respiration, the lungs have a number of other functions. Lung_sentence_233

They are involved in maintaining homeostasis, helping in the regulation of blood pressure as part of the renin–angiotensin system. Lung_sentence_234

The inner lining of the blood vessels secretes angiotensin-converting enzyme (ACE) an enzyme that catalyses the conversion of angiotensin I to angiotensin II. Lung_sentence_235

The lungs are involved in the blood's acid-base homeostasis by expelling carbon dioxide when breathing. Lung_sentence_236

The lungs also serve a protective role. Lung_sentence_237

Several blood-borne substances, such as a few types of prostaglandins, leukotrienes, serotonin and bradykinin, are excreted through the lungs. Lung_sentence_238

Drugs and other substances can be absorbed, modified or excreted in the lungs. Lung_sentence_239

The lungs filter out small blood clots from veins and prevent them from entering arteries and causing strokes. Lung_sentence_240

The lungs also play a pivotal role in speech by providing air and airflow for the creation of vocal sounds, and other paralanguage communications such as sighs and gasps. Lung_sentence_241

New research suggests a role of the lungs in the production of blood platelets. Lung_sentence_242

Gene and protein expression Lung_section_23

Further information: Bioinformatics § Gene and protein expression Lung_sentence_243

About 20,000 protein coding genes are expressed in human cells and almost 75% of these genes are expressed in the normal lung. Lung_sentence_244

A little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. Lung_sentence_245

The highest expression of lung specific proteins are different surfactant proteins, such as SFTPA1, SFTPB and SFTPC, and napsin, expressed in type II pneumocytes. Lung_sentence_246

Other proteins with elevated expression in the lung are the dynein protein DNAH5 in ciliated cells, and the secreted SCGB1A1 protein in mucus-secreting goblet cells of the airway mucosa. Lung_sentence_247

Clinical significance Lung_section_24

Main articles: Respiratory disease and Pulmonology Lung_sentence_248

Lungs can be affected by a variety of diseases. Lung_sentence_249

Pulmonology is the medical speciality that deals with diseases involving the respiratory tract, and cardiothoracic surgery is the surgical field that deals with surgery of the lungs. Lung_sentence_250

Inflammation and infection Lung_section_25

Inflammatory conditions of the lung tissue are pneumonia, of the respiratory tract are bronchitis and bronchiolitis, and of the pleurae surrounding the lungs pleurisy. Lung_sentence_251

Inflammation is usually caused by infections due to bacteria or viruses. Lung_sentence_252

When the lung tissue is inflamed due to other causes it is called pneumonitis. Lung_sentence_253

One major cause of bacterial pneumonia is tuberculosis. Lung_sentence_254

Chronic infections often occur in those with immunodeficiency and can include a fungal infection by Aspergillus fumigatus that can lead to an aspergilloma forming in the lung. Lung_sentence_255

Blood-supply changes Lung_section_26

A pulmonary embolism is a blood clot that becomes lodged in the pulmonary arteries. Lung_sentence_256

The majority of emboli arise because of deep vein thrombosis in the legs. Lung_sentence_257

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. Lung_sentence_258

Pulmonary hypertension describes an increased pressure at the beginning of the pulmonary artery that has a large number of differing causes. Lung_sentence_259

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. Lung_sentence_260

A lung contusion is a bruise caused by chest trauma. Lung_sentence_261

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. Lung_sentence_262

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. Lung_sentence_263

These may be investigated using a chest X-ray or CT scan, and may require the insertion of a surgical drain until the underlying cause is identified and treated. Lung_sentence_264

Obstructive lung diseases Lung_section_27

Asthma, chronic bronchitis, bronchiectasis and chronic obstructive pulmonary disease (COPD) are all obstructive lung diseases characterised by airway obstruction. Lung_sentence_265

This limits the amount of air that is able to enter alveoli because of constriction of the bronchial tree, due to inflammation. Lung_sentence_266

Obstructive lung diseases are often identified because of symptoms and diagnosed with pulmonary function tests such as spirometry. Lung_sentence_267

Many obstructive lung diseases are managed by avoiding triggers (such as dust mites or smoking), with symptom control such as bronchodilators, and with suppression of inflammation (such as through corticosteroids) in severe cases. Lung_sentence_268

A common cause of chronic bronchitis, and emphysema, is smoking; and common causes of bronchiectasis include severe infections and cystic fibrosis. Lung_sentence_269

The definitive cause of asthma is not yet known. Lung_sentence_270

The breakdown of alveolar tissue, often as a result of tobacco-smoking leads to emphysema, which can become severe enough to develop into COPD. Lung_sentence_271

Elastase breaks down the elastin in the lung's connective tissue that can also result in emphysema. Lung_sentence_272

Elastase is inhibited by the acute-phase protein, alpha-1 antitrypsin, and when there is a deficiency in this, emphysema can develop. Lung_sentence_273

With persistent stress from smoking, the airway basal cells become disarranged and lose their regenerative ability needed to repair the epithelial barrier. Lung_sentence_274

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. Lung_sentence_275

Studies have shown that the initial development of emphysema is centred on the early changes in the airway epithelium of the small airways. Lung_sentence_276

Basal cells become further deranged in a smoker's transition to clinically defined COPD. Lung_sentence_277

Restrictive lung diseases Lung_section_28

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. Lung_sentence_278

These include pulmonary fibrosis which can occur when the lung is inflamed for a long period of time. Lung_sentence_279

Fibrosis in the lung replaces functioning lung tissue with fibrous connective tissue. Lung_sentence_280

This can be due to a large variety of occupational lung diseases such as Coalworker's pneumoconiosis, autoimmune diseases or more rarely to a reaction to medication. Lung_sentence_281

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. Lung_sentence_282

Cancers Lung_section_29

Lung cancer can either arise directly from lung tissue or as a result of metastasis from another part of the body. Lung_sentence_283

There are two main types of primary tumour described as either small-cell or non-small-cell lung carcinomas. Lung_sentence_284

The major risk factor for cancer is smoking. Lung_sentence_285

Once a cancer is identified it is staged using scans such as a CT scan and a sample of tissue (a ) is taken. Lung_sentence_286

Cancers may be treated by surgically removing the tumour, radiotherapy, chemotherapy or combinations thereof, or with the aim of symptom control. Lung_sentence_287

Lung cancer screening is being recommended in the United States for high-risk populations. Lung_sentence_288

Congenital disorders Lung_section_30

Congenital disorders include cystic fibrosis, pulmonary hypoplasia (an incomplete development of the lungs)congenital diaphragmatic hernia, and infant respiratory distress syndrome caused by a deficiency in lung surfactant. Lung_sentence_289

An azygos lobe is a congenital anatomical variation which though usually without effect can cause problems in thoracoscopic procedures. Lung_sentence_290

Others Lung_section_31

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. Lung_sentence_291

The lung cannot expand against the air pressure inside the pleural space. Lung_sentence_292

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. Lung_sentence_293

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. Lung_sentence_294

Lung examination Lung_section_32

Main articles: Respiratory examination and Respiratory sounds Lung_sentence_295

As part of a physical examination in response to respiratory symptoms of shortness of breath, and cough, a lung examination may be carried out. Lung_sentence_296

This exam includes palpation and auscultation. Lung_sentence_297

The areas of the lungs that can be listened to using a stethoscope are called the lung fields, and these are the posterior, lateral, and anterior lung fields. Lung_sentence_298

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. Lung_sentence_299

The anterior fields can also be auscultated from the front. Lung_sentence_300

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. Lung_sentence_301

Lung function testing Lung_section_33

Main articles: Pulmonary function testing and Lung volumes Lung_sentence_302

Lung function testing is carried out by evaluating a person's capacity to inhale and exhale in different circumstances. Lung_sentence_303

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. Lung_sentence_304

The summed total of forced inspiration and expiration is a person's vital capacity. Lung_sentence_305

Not all air is expelled from the lungs even after a forced breath out; the remainder of the air is called the residual volume. Lung_sentence_306

Together these terms are referred to as lung volumes. Lung_sentence_307

Pulmonary plethysmographs are used to measure functional residual capacity. Lung_sentence_308

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. Lung_sentence_309

The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between 4 and 6 litres. Lung_sentence_310

Females tend to have a 20–25% lower capacity than males. Lung_sentence_311

Tall people tend to have a larger total lung capacity than shorter people. Lung_sentence_312

Smokers have a lower capacity than nonsmokers. Lung_sentence_313

Thinner persons tend to have a larger capacity. Lung_sentence_314

Lung capacity can be increased by physical training as much as 40% but the effect may be modified by exposure to air pollution. Lung_sentence_315

Other lung function tests include spirometry, measuring the amount (volume) and flow of air that can be inhaled and exhaled. Lung_sentence_316

The maximum volume of breath that can be exhaled is called the vital capacity. Lung_sentence_317

In particular, how much a person is able to exhale in one second (called forced expiratory volume (FEV1)) as a proportion of how much they are able to exhale in total (FEV). Lung_sentence_318

This ratio, the FEV1/FEV ratio, is important to distinguish whether a lung disease is restrictive or obstructive. Lung_sentence_319

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. Lung_sentence_320

Other animals Lung_section_34

Birds Lung_section_35

Main article: Bird anatomy § Respiratory system Lung_sentence_321

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. Lung_sentence_322

On inhalation, air travels through the trachea of a bird into the air sacs. Lung_sentence_323

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. Lung_sentence_324

From here, the air is exhaled. Lung_sentence_325

These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals. Lung_sentence_326

The lungs of birds contain millions of tiny parallel passages called parabronchi. Lung_sentence_327

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. Lung_sentence_328

The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right). Lung_sentence_329

The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. Lung_sentence_330

The air sacs expand and contract due to changes in the volume in the thorax and abdomen. Lung_sentence_331

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. Lung_sentence_332

Parabronchi in which the air flow is unidirectional are called paleopulmonic parabronchi and are found in all birds. Lung_sentence_333

Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. Lung_sentence_334

These are termed neopulmonic parabronchi. Lung_sentence_335

Reptiles Lung_section_36

Main article: Reptile anatomy § Respiratory system Lung_sentence_336

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. Lung_sentence_337

These pockets are similar to alveoli in mammals, but much larger and fewer in number. Lung_sentence_338

These give the lung a sponge-like texture. Lung_sentence_339

In tuataras, snakes, and some lizards, the lungs are simpler in structure, similar to that of typical amphibians. Lung_sentence_340

Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. Lung_sentence_341

Amphisbaenians, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung. Lung_sentence_342

Both crocodilians and monitor lizards have developed lungs similar to those of birds, providing a unidirectional airflow and even possessing air sacs. Lung_sentence_343

The now extinct pterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of lonchodectids, tupuxuara, and azhdarchoids, the hindlimbs. Lung_sentence_344

Reptilian lungs typically receive air via expansion and contraction of the ribs driven by axial muscles and buccal pumping. Lung_sentence_345

Crocodilians 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. Lung_sentence_346

Turtles, which are unable to move their ribs, instead use their forelimbs and pectoral girdle to force air in and out of the lungs. Lung_sentence_347

Amphibians Lung_section_37

The lungs of most frogs and other amphibians are simple and balloon-like, with gas exchange limited to the outer surface of the lung. Lung_sentence_348

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. Lung_sentence_349

Amphibians employ a positive pressure system to get air to their lungs, forcing air down into the lungs by buccal pumping. Lung_sentence_350

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. Lung_sentence_351

In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. Lung_sentence_352

The throat muscles then presses the throat against the underside of the skull, forcing the air into the lungs. Lung_sentence_353

Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapods are amphibians. Lung_sentence_354

The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. Lung_sentence_355

This necessarily restricts their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume. Lung_sentence_356

Other known lungless tetrapods are the Bornean flat-headed frog and Atretochoana eiselti, a caecilian. Lung_sentence_357

The lungs of amphibians typically have a few narrow internal walls () of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honey-comb appearance. Lung_sentence_358

In some salamanders even these are lacking, and the lung has a smooth wall. Lung_sentence_359

In caecilians, as in snakes, only the right lung attains any size or development. Lung_sentence_360

Lungfish Lung_section_38

The lungs of lungfish are similar to those of amphibians, with few, if any, internal septa. Lung_sentence_361

In the Australian lungfish, there is only a single lung, albeit divided into two lobes. Lung_sentence_362

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. Lung_sentence_363

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. Lung_sentence_364

Invertebrates Lung_section_39

Further information: Respiratory system of gastropods Lung_sentence_365

Some invertebrates have lung-like structures that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Lung_sentence_366

Some arachnids, such as spiders and scorpions, have structures called book lungs used for atmospheric gas exchange. Lung_sentence_367

Some species of spider have four pairs of book lungs but most have two pairs. Lung_sentence_368

Scorpions have spiracles on their body for the entrance of air to the book lungs. Lung_sentence_369

The coconut crab is terrestrial and uses structures called branchiostegal lungs to breathe air. Lung_sentence_370

They cannot swim and would drown in water, yet they possess a rudimentary set of gills. Lung_sentence_371

They can breathe on land and hold their breath underwater. Lung_sentence_372

The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian. Lung_sentence_373

Pulmonates are mostly land snails and slugs that have developed a simple lung from the mantle cavity. Lung_sentence_374

An externally located opening called the pneumostome allows air to be taken into the mantle cavity lung. Lung_sentence_375

Evolutionary origins Lung_section_40

The lungs of today's terrestrial vertebrates and the gas bladders 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. Lung_sentence_376

These outpocketings first arose in the bony fish. Lung_sentence_377

In most of the ray-finned fish the sacs evolved into closed off gas bladders, while a number of carp, trout, herring, catfish, and eels have retained the physostome condition with the sack being open to the esophagus. Lung_sentence_378

In more basal bony fish, such as the gar, bichir, bowfin and the lobe-finned fish, the bladders have evolved to primarily function as lungs. Lung_sentence_379

The lobe-finned fish gave rise to the land-based tetrapods. Lung_sentence_380

Thus, the lungs of vertebrates are homologous to the gas bladders of fish (but not to their gills). Lung_sentence_381

See also Lung_section_41

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