Humans have two lungs, a right lung, and a left lung. They are situated within the thoracic cavity
of the chest
. The right lung is bigger than the left, which 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 the lower respiratory tract
that begins at the trachea
and branches into the bronchi
, and which receive airbreathed in
via the conducting zone
. The conducting zone ends at the terminal bronchioles
. These divide into the respiratory bronchioles
of the respiratory zone
which divide into alveolar ducts
that give rise to the alveolar sacs
that contain the alveoli
, where gas exchange takes place. Alveoli are also sparsely present on the walls of the respiratory bronchioles and alveolar ducts. Together, the lungs contain approximately 2,400 kilometres (1,500 mi) of airways and 300 to 500 million alveoli. 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. This sac also divides each lung into sections called lobes
. The right lung has three lobes and the left has two. The lobes are further divided into bronchopulmonary segments
and pulmonary lobules
. The lungs have a unique blood supply, receiving deoxygenated blood from the heart in the pulmonary circulation
for the purposes of receiving oxygen and releasing carbon dioxide, and a separate supply of oxygenated blood to the tissue of the lungs, in the bronchial circulation
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
. When the lungs are formed the fetus
is held in the fluid-filled amniotic sac
and so they do not function to breathe. Blood is also diverted from the lungs through the ductus arteriosus
. At birth
however, air begins to pass through the lungs, and the diversionary duct closes, so that the lungs can begin to respire. The lungs only fully develop in early childhood.
The lungs are located in the chest
on either side of the heart
in the rib cage
. They are conical in shape with a narrow rounded apex at the top, and a broad concave base that rests on the convex surface of the diaphragm
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
. The lungs stretch from close to the backbone
in the rib cage to the front of the chest
and downwards from the lower part of the trachea to the diaphragm.
The left lung shares space with the heart, and has an indentation in its border called the cardiac notch of the left lung
to accommodate this.
The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces. The medial surfaces of the lungs face towards the centre of the chest, and lie against the heart, great vessels
, and the carina
where the trachea divides into the two main bronchi.
The cardiac impression
is an indentation formed on the surfaces of the lungs where they rest against the heart.
Lobes and segments
Lobes and bronchopulmonary segments 
Each lung is divided into lobes by the infoldings of the pleura as fissures. The fissures are double folds of pleura that section the lungs and help in their expansion.
The main or primary bronchi enter the lungs at the hilum and initially branch into secondary bronchi
also known as lobar bronchi that supply air to each lobe of the lung. The lobar bronchi branch into tertiary bronchi
also known as segmental bronchi and these supply air to the further divisions of the lobes known as bronchopulmonary segments
. Each bronchopulmonary segment has its own (segmental) bronchus and arterial supply
Segments for the left and right lung are shown in the table.
The segmental anatomy is useful clinically for localising disease processes in the lungs.
A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue.
3D anatomy of the lung lobes and fissures.
The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower lobe by two fissures
, one oblique and one horizontal.
The upper, horizontal fissure
, separates the upper from the middle lobe. It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the sternal
end of the fourth costal cartilage
; on the mediastinal
surface it may be traced back to the hilum
The lower, oblique fissure
, separates the lower from the middle and upper lobes and is closely aligned with the oblique fissure in the left lung.
The mediastinal surface of the right lung is indented by a number of nearby structures. The heart sits in an impression called the cardiac impression. Above the hilum of the lung is an arched groove for the azygos vein
, and above this is a wide groove for the superior vena cava
and right brachiocephalic vein
; behind this, and close to the top of the lung is a groove for the brachiocephalic artery
. There is a groove for the esophagus behind the hilum and the pulmonary ligament
, and near the lower part of the esophageal groove is a deeper groove for the inferior vena cava
before it enters the heart.
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).
The left lung is divided into two lobes, an upper and a lower lobe, by the oblique fissure, which extends from the costal
to the mediastinal surface of the lung both above and below the hilum
The left lung, unlike the right, does not have a middle lobe, though it does have a homologous
feature, a projection of the upper lobe termed the lingula
. Its name means "little tongue". The lingula on the left lung serves as an anatomic parallel to the middle lobe on the right lung, with both areas being predisposed to similar infections and anatomic complications.
There are two bronchopulmonary segments
of the lingula: superior and inferior.
The mediastinal surface of the left lung has a large cardiac impression
where the heart sits. This is deeper and larger than that on the right lung, at which level the heart projects to the left.
On the same surface, immediately above the hilum, is a well-marked curved groove for the aortic arch
, and a groove below it for the descending aorta
. The left subclavian artery
, a branch off the aortic arch, sits in a groove from the arch to near the apex of the lung. A shallower groove in front of the artery and near the edge of the lung, lodges the left brachiocephalic vein
. The esophagus
may sit in a wider shallow impression at the base of the lung.
The weight of the left lung, by standard reference range
, in men is 110–675 g (0.243–1.488 lb)
in women 105–515 g (0.231–1.135 lb).
The left lung (left) and right lung (right). The lobes of the lungs can be seen, and the central root of the lung is also present.
Cross-sectional detail of the lung
image of collagen fibres in a cross sectional slice of mammalian lung tissue.
3D Medical Illustration Showing Different Terminating Ends of Bronchial Airways
The connective tissue of the lungs is made up of elastic and collagen fibres that are interspersed between the capillaries and the alveolar walls. Elastin
is the key protein
of the extracellular matrix
and is the main component of the elastic fibres.
Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known as lung compliance
. It is also responsible for the elastic recoil
needed. Elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions.
The connective tissue links all the alveoli to form the lung parenchyma which has a sponge-like appearance. The alveoli have interconnecting air passages in their walls known as the pores of Kohn
Pulmonary neuroendocrine cells
are found throughout the respiratory epithelium including the alveolar epithelium,
though they only account for around 0.5 per cent of the total epithelial population.
PNECs are innervated airway epithelial cells that are particularly focused at airway junction points.
These cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. Cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas.
In the bronchi there are incomplete tracheal rings
and smaller plates of cartilage that keep them open.:472
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.:472
The absence of cartilage in the terminal bronchioles gives them an alternative name of membranous bronchioles
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 an acinus
which includes the respiratory bronchioles, the alveolar ducts, alveolar sacs
, and alveoli.
This is also called the terminal respiratory unit.
An acinus measures up to 10 mm in diameter.
A primary pulmonary lobule
is that part of the acinus that includes the alveolar ducts, sacs, and alveoli but does not include the respiratory bronchioles.
The unit described as the secondary pulmonary lobule
is the lobule most referred to as the pulmonary lobule
or respiratory lobule
This lobule is a discrete unit that is the smallest component of the lung that can be seen without aid. The secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules.
The lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. The respiratory bronchioles supply the alveoli in each acinus and is accompanied by a pulmonary artery branch. Each lobule is enclosed by an interlobular septa. Each acinus is incompletely separated by an interlobular septa.
The respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli.
The walls of the alveoli are extremely thin allowing a fast rate of diffusion. The alveoli have interconnecting small air passages in their walls known as the pores of Kohn
Alveoli and their capillary networks.
Alveoli consist of two types of alveolar cell
and an alveolar macrophage
. The two types of cell are known as type I
and type II
(also known as pneumocytes).
Types I and II make up the walls and alveolar septa
. Type I cells provide 95% of the surface area of each alveoli and are flat ("squamous
"), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape.
Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.
Type I are squamous epithelial cells
that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange.
These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated basement membranes
Type I cells are not able to divide, and consequently rely on differentiation
from Type II cells.
Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and lung surfactant
Type II cells are able to divide and differentiate to Type I cells.
The alveolar macrophages
have an important immunological
role. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.
There is a large presence of microorganisms in the lungs known as the lung microbiome or microbiota. The lung microbiome interacts with the airway epithelial cells. The microbiome is complex in healthy people, and altered in diseases such as asthma
. The lung microbiome is dynamic and significant changes can take place in COPD following infection with rhinovirus
. The interaction between the microbiome and the epithelial cells is of probable importance in the maintenance of stable homeostasis. Fungal genera
that are commonly found in the lung microbiota, known as the lung mycobiome
, and Aspergillus
The lungs as main part of respiratory tract
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
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.
The lobes of the lung are subject to anatomical variations
A horizontal interlobar fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. An accessory fissure was also found in 14% and 22% of left and right lungs, respectively.
An oblique fissure was found to be incomplete in 21% to 47% of left lungs.
In some cases a fissure is absent, or extra, resulting in a right lung with only two lobes, or a left lung with three lobes.
A variation in the airway branching structure has been found specifically in the central airway branching. This variation is associated with the development of COPD
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.
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.
In the embryo this structure is developed in the process of branching morphogenesis
, and is generated by the repeated splitting of the tip of the branch. In the development of the lungs (as in some other organs) the epithelium forms branching tubes.The lung has a left-right symmetry and each bud known as a bronchial bud
grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles.
The branching is a result of the tip of each tube bifurcating.
The branching process forms the bronchi, bronchioles, and ultimately the alveoli.
The four genes mostly associated with branching morphogenesis in the lung are the intercellular signalling protein
– sonic hedgehog
(SHH), fibroblast growth factorsFGF10
and FGFR2b, and bone morphogenetic protein BMP4
. FGF10 is seen to have the most prominent role. FGF10 is a paracrine signalling
molecule needed for epithelial branching, and SHH inhibits FGF10.
The development of the alveoli is influenced by a different mechanism whereby continued bifurcation is stopped and the distal tips become dilated to form the alveoli.
At the end of the fourth week the lung bud divides into two, the right and left primary bronchial buds
on each side of the trachea.
During the fifth week the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. These give rise to the lobes of the lungs, three on the right and two on the left. Over the following week, the secondary buds branch into tertiary buds, about ten on each side.
From the sixth week to the sixteenth week, the major elements of the lungs appear except the alveoli
From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. By week 26 the terminal bronchioles have formed which branch into two respiratory bronchioles.
During the period covering the 26th week until birth the important blood–air barrier
is established. Specialised type I alveolar cells
where gas exchange
will take place, together with the type II alveolar cells
that secrete pulmonary surfactant
, appear. The surfactant reduces the surface tension
at the air-alveolar surface which allows expansion of the alveolar sacs. The alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts,
and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.
Vitamin A deficiency
The developing lung is particularly vulnerable to changes in the levels of vitamin A
. Vitamin A deficiency
has been linked to changes in the epithelial lining of the lung and in the lung parenchyma. This can disrupt the normal physiology of the lung and predispose to respiratory diseases. Severe nutritional deficiency in vitamin A results in a reduction in the formation of the alveolar walls (septa) and to notable changes in the respiratory epithelium; alterations are noted in the extracellular matrix and in the protein content of the basement membrane. The extracellular matrix maintains lung elasticity; the basement membrane is associated with alveolar epithelium and is important in the blood-air barrier. The deficiency is associated with functional defects and disease states. Vitamin A is crucial in the development of the alveoli which continues for several years after birth.
, the baby's lungs are filled with fluid secreted by the lungs and are not inflated. After birth
the infant's central nervous system
reacts to the sudden change in temperature and environment. This triggers the first breath, within about 10 seconds after delivery.
Before birth, the lungs are filled with fetal lung fluid.
After the first breath, the fluid is quickly absorbed into the body or exhaled. The resistance
in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies other changes
which result in an increased amount of blood entering the lung tissues.
At birth the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present.
The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung.
Alveolar septa have a double capillary network
instead of the single network of the developed lung. Only after the maturation of the capillary network can the lung enter a normal phase of growth. Following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged.
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.
The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left. Lung function testing
is carried out to evaluate lung volumes
The lungs possess several characteristics which protect against infection. The respiratory tract is lined by respiratory epithelium
or respiratory mucosa, with hair-like projections called cilia
that beat rhythmically and carry mucus
. This mucociliary clearance
is an important defence system against air-borne infection.
The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia.:661–730
The lining of the lung also secretes immunoglobulin A
which protects against respiratory infections; goblet cells
which also contains several antimicrobial compounds such as defensins
, and antioxidants
A rare type of specialised cell called a pulmonary ionocyte
that is suggested may regulate mucus viscosity has been described.
In addition, the lining of the lung also contains macrophages
, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis
; and dendritic cells
which present antigens to activate components of the adaptive immune system
such as T-cells
The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the mouth
and behind the mouth in the oropharynx
, and larger particles are trapped in nasal hair
New research suggests a role of the lungs in the production of blood platelets.
Gene and protein expression
About 20,000 protein coding genes
are expressed in human cells and almost 75% of these genes are expressed in the normal lung.
A little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. The highest expression of lung specific proteins are different surfactant
such as SFTPA1
, and napsin
, expressed in type II pneumocytes. Other proteins with elevated expression in the lung are the dynein
in ciliated cells, and the secreted SCGB1A1
protein in mucus-secreting goblet cells
of the airway mucosa.
Inflammation and infection
of the lung due to a pulmonary embolism
A lung contusion
is a bruise caused by chest trauma. It results in hemorrhage of the alveoli causing a build-up of fluid which can impair breathing, and this can be either mild or severe. The function of the lungs can also be affected by compression from fluid in the pleural cavity pleural effusion
, or other substances such as air (pneumothorax
), blood (hemothorax
), or rarer causes. These may be investigated using a chest X-ray
or CT scan
, and may require the insertion of a surgical drain
until the underlying cause is identified and treated.
Obstructive lung diseases
3D still image of constricted airways as in bronchial asthma.
Lung tissue affected by emphysema using H&E stain
The breakdown of alveolar tissue, often as a result of tobacco-smoking leads to emphysema, which can become severe enough to develop into COPD. Elastase
breaks down the elastin
in the lung's connective tissue that can also result in emphysema. Elastase is inhibited by the acute-phase protein
, alpha-1 antitrypsin
, and when there is a deficiency
in this, emphysema can develop. With persistent stress from smoking, the airway basal cells
become disarranged and lose their regenerative ability needed to repair the epithelial barrier. The disorganised basal cells are seen to be responsible for the major airway changes that are characteristic of COPD
, and with continued stress can undergo a malignant transformation. Studies have shown that the initial development of emphysema is centred on the early changes in the airway epithelium of the small airways.
Basal cells become further deranged in a smoker's transition to clinically defined COPD.
Restrictive lung diseases
(collapsed lung) is an abnormal collection of air in the pleural space
that causes an uncoupling of the lung from the chest wall
The lung cannot expand against the air pressure inside the pleural space. An easy to understand example is a traumatic pneumothorax, where air enters the pleural space from outside the body, as occurs with puncture to the chest wall. Similarly, scuba divers
ascending while holding their breath with their lungs fully inflated can cause air sacs (alveoli
) to burst and leak high pressure air into the pleural space.
As part of a physical examination
in response to respiratory symptoms of shortness of breath
, and cough
, a lung examination
may be carried out. This exam includes palpation
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. The posterior fields can be listened to from the back and include: the lower lobes (taking up three quarters of the posterior fields); the anterior fields taking up the other quarter; and the lateral fields under the axillae
, the left axilla for the lingual, the right axilla for the middle right lobe. The anterior fields can also be auscultated from the front.
Abnormal breathing sounds
heard during a lung exam can indicate the presence of a lung condition; wheezing
for example is commonly associated with asthma
Lung function testing
are used to measure functional residual capacity
Functional residual capacity cannot be measured by tests that rely on breathing out, as a person is only able to breathe a maximum of 80% of their total functional capacity.
The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between 4 and 6 litres.
Females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Smokers
have a lower capacity than nonsmokers. Thinner persons tend to have a larger capacity. Lung capacity can be increased by physical training as much as 40% but the effect may be modified by exposure to air pollution.
Other lung function tests include spirometry
, measuring the amount (volume) and flow of air that can be inhaled and exhaled. The maximum volume of breath that can be exhaled is called the vital capacity
. In particular, how much a person is able to exhale in one second (called forced expiratory volume
(FEV1)) as a proportion of how much they are able to exhale in total (FEV). This ratio, the FEV1/FEV ratio, is important to distinguish whether a lung disease is restrictive
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.
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).
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 8 or 9 air sacs
that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.
The lungs of birds contain millions of tiny parallel passages called parabronchi
. Small sacs called atria
radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site of gas exchange
by simple diffusion.
The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).
The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. The air sacs expand and contract due to changes in the volume in the thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronised with movement of the flight muscles.
Parabronchi in which the air flow is unidirectional are called paleopulmonic parabronchi
and are found in all birds. Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. These are termed neopulmonic parabronchi
The lungs of most reptiles have a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to alveoli in mammals, but much larger and fewer in number. These give the lung a sponge-like texture. In tuataras
, and some lizards
, the lungs are simpler in structure, similar to that of typical amphibians.
Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. Amphisbaenians
, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung.
lungs typically receive air via expansion and contraction of the ribs driven by axial muscles
and buccal pumping. Crocodilians
also rely on the hepatic
piston method, in which the liver is pulled back by a muscle anchored to the pubic bone
(part of the pelvis) called the diaphragmaticus,
which in turn creates negative pressure in the crocodile's thoracic cavity, allowing air to be moved into the lungs by Boyle's law. Turtles
, which are unable to move their ribs, instead use their forelimbs and pectoral girdle
to force air in and out of the lungs.
) retains its larval form with gills into adulthood
The lungs of most frogs
and other amphibians
are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a positive pressure
system to get air to their lungs, forcing air down into the lungs by buccal pumping
. This is distinct from most higher vertebrates, who use a breathing system driven by negative pressure
where the lungs are inflated by expanding the rib cage.
In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the skull
, forcing the air into the lungs.
Due to the possibility of respiration across the skin combined with small size, all known lungless tetrapods
are amphibians. The majority of salamander species are lungless salamanders
, which respirate through their skin and tissues lining their mouth. This necessarily restricts their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume.
Other known lungless tetrapods are the Bornean flat-headed frog
and Atretochoana eiselti
, a caecilian
The lungs of amphibians typically have a few narrow internal walls (septa
) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honey-comb 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.
The lungs of lungfish
are similar to those of amphibians, with few, if any, internal septa. In the Australian lungfish
, there is only a single lung, albeit divided into two lobes. Other lungfish and Polypterus
, however, have two lungs, which are located in the upper part of the body, with the connecting duct curving around and above the esophagus
. The blood supply also twists around the esophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.
Book lungs of spider (shown in pink)
have lung-like structures that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids
, such as spiders
, have structures called book lungs
used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs.
Scorpions have spiracles
on their body for the entrance of air to the book lungs.
The coconut crab
is terrestrial and uses structures called branchiostegal lungs
to breathe air.
They cannot swim and would drown in water, yet they possess a rudimentary set of gills. They can breathe on land and hold their breath underwater.
The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.
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.
These outpocketings first arose in the bony fish
. In most of the ray-finned fish
the sacs evolved into closed off gas bladders, while a number of carp
, and eels
have retained the physostome
condition with the sac being open to the esophagus. In more basal bony fish, such as the gar
and the lobe-finned fish
, the bladders have evolved to primarily function as lungs.
The lobe-finned fish gave rise to the land-based tetrapods
. Thus, the lungs of vertebrates are homologous
to the gas bladders of fish (but not to their gills
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