lungs


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lungs,

elastic organs used for breathing in vertebrate animals, excluding most fish, which use gillsgills,
external respiratory organs of most aquatic animals. In fishes the gills are located in gill chambers at the rear of the mouth (pharynx). Water is taken in through the mouth, is forced through openings called gill slits, and then passes through the gill clefts, spaces
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, and a few amphibian species that respire through the skin. The word is sometimes applied to the respiratory apparatus of lower animals.

The human lungs are paired organs, located on either side of the heart and occupying a large portion of the chest cavity from the collarbone to the diaphragm. Air enters the body through a series of passages, beginning with the nose or mouth. It travels to the chest cavity through the tracheatrachea
or windpipe,
principal tube that carries air to and from the lungs. It is about 4 1-2 in. (11.4 cm) long and about 3-4 in. (1.9 cm) in diameter in the adult.
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, which divides into two bronchi, each of which enters a lung. The bronchi divide and subdivide into a network of countless tubules. The smallest tubules, or bronchioles, enter cup-shaped air sacs known as alveoli, which number about 700 million in both lungs. Each alveolus is surrounded by a net of capillaries. As blood flows through these vessels, carbon dioxide passes into the alveoli, and oxygen diffuses into the bloodstream. The capillaries are part of a vast network of pulmonary blood vessels that connect the lungs directly to the heart via the large pulmonary arteries and veins. The alveoli are clustered in groups, or lobules, and the lobules are clustered into lobes.

In humans, the left lung has two lobes; the right lung three. The lungs are covered by a thin membrane called the pleurapleura
, membranous lining of the upper body cavity and covering for the lungs. The pleura is a two-layered structure: the parietal pleura lines the walls of the chest cage and covers the upper surface of the diaphragm, and the pulmonary pleura, or visceral layer, tightly covers
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. They are expanded and contracted (thereby inhaling and exhaling air) by the combined movement of the diaphragm and the rib cage, which is alternately raised (expansion) and lowered (contraction) by the chest muscles. In recent years, smoking has been found to cause severe and sometimes fatal diseases of the lung, such as cancer and emphysema. Pneumonia is an inflammation of the lung tissue caused by various agents or organisms such as viruses. Asthma, a hypersensitivity or allergic response to some stimuli, covers a range of severity and is characterized by bronchial spasms and difficult breathing. See respirationrespiration,
process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO2
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.

Lungs

 

the organs used for air breathing in some fishes (Dipnoi, Crossopterygii, and Cladistia), terrestrial vertebrates, and man; gas is exchanged between the air in the lungs and the blood in the pulmonary capillaries.

In Dipnoi, the lungs are paired pouches with alveolate walls that perform the respiratory function (the Australian lungfish has a secondarily unpaired lung). The respiratory surface of the lung is increased by trabeculae that protrude into the lung cavity and give its walls alveolarity. In Cladistia, the lungs are paired and smooth-walled.

The lungs of terrestrial vertebrates have their origin in the lungs of ancient crossopterygians; as in lunged fishes, they take the form of paired outpouchings of the ventral wall of the pharynx and are supplied with blood from the pulmonary artery, which is homologous with the sixth branchial artery of fishes. The lungs are located in the anterior part of the body cavity; in mammals and man, they are located in the thorax, which is separated from the abdominal cavity by the diaphragm. Externally, the lungs are covered with a serous membrane called the pleura. The lungs are usually paired, although in apodal amphibians, lizards, and snakes, one of the lungs (most often the left) is reduced. In Plethodontidae, lungs are absent.

Air enters the lungs through respiratory passages lined with ciliated epithelium. In amphibians, the air is forced into the lungs from the oral cavity by the rhythmic contractions of the floor of the cavity, while the rima glottidis opens and the nares close. In Amniota, with the development of a movable rib cage, the air is sucked into the lungs when the thorax is actively expanded. In mammals, the volume of the thoracic cavity is also increased during inhalation by means of the contraction of the diaphragm.

In amphibians, the lungs, smooth-walled (in gilled animals) or alveolate pouches, begin immediately at the larynx or trachea. In lower reptiles (Gatteria), the walls of the lungs are alveolate. In some lizards (monitors), turtles, and crocodiles, the lungs have a spongy structure; in proliferating, the trabeculae divide the lung cavity into chambers and cells, within which there develop secondary trabeculae. These define a system of smaller cells, significantly increasing the lung’s respiratory surface. The central passage, or intrapulmonary bronchus, remains clear. In a number of reptiles, the lungs have outpouchings called lung books.

The lungs of birds, which are conjoined to the dorsal side of the thorax, are compact and only slightly mobile. The primary bronchus and several secondary bronchi leave the lungs to form air sacs, which furnish a secondary ventilation, along with gas exchange, during inspiration and expiration. In birds, the secondary bronchi are united by parabronchi. These branch radially into bronchioles, which break up into a network of what is known as air capillaries. The air capillaries are woven about with a network of blood-bearing capillaries.

In mammals, a numerously branching intrapulmonary bronchial tree with an ever-diminishing branchial diameter ends in thin-walled bronchioles, which enter alveolar passages with terminal enlargements (the alveolar sacs and alveoli). It is here that gas exchange occurs. The alveolar passages and sacs that emerge from a given terminal bronchiole form a pulmonary acinus. A group of acini, joined together by connective tissue, form pulmonary lobules. These, in turn, join pulmonary lobes, whose number in the right lung is always greater (to six) than in the left (to three).

The respiratory organs of some invertebrates are also lunglike. In Pulmonata, this refers to a part of the mantle cavity that is richly supplied with blood vessels and serves for air breathing; in Holothuria, to the branched processes of the cloaca that perform the respiratory function, called the respiratory tree; and in arachnids, to the pouchlike cavities taken up by sheetlike processes (derivatives of branchial pedicels).

REFERENCES

Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1974.
Masenov, T. M. Biodinamika legkikh u mlekopitaiushchikh. Alma-Ata, 1968.
T. M. MASENOV
In man, the lungs are paired organs of respiration in the thoracic cavity. They have the shape of a half cone with a rounded apex and a concave base (which is a function of the shape of the diaphragm). The inner, or medial, surface of the lung faces the heart and is the site of the pulmonary hilus, the point at which the primary bronchus and the pulmonary artery enter, at which the pulmonary veins exit, and at which the lymph nodes that collect lymph from the lung, the nerve plexus, and the bronchial arteries and veins are located.
Above and posterior to the hilus of the left lung is the aorta. The apex of the lung shows the least mobility during respiration. The lower edge of the lung drops and rises by 1–2 cm during normal respiration and by 6–10 cm when respiration is forced.
Lung tissue is soft and, at the same time, elastic and resilient. The right lung is shorter and wider than the left. The left lung consists of a superior and an inferior lobe; the right lung, of a superior, a medial, and an inferior lobe. There are more or less deep fissures between the lobes. There are ten bronchopulmonary segments in each lung, sections of the pulmonary parenchyma that are each supplied with an independent bronchus and a branch of the pulmonary artery. Segmental veins pass at the boundaries of the segments, where there are no large arteries or bronchi. The segments have the shape of irregular cones or pyramids, with the apexes directed toward the hilus. The segments are subdivided into lobules (about 1,000 per lung). The lobules are 9–27 mm high, depending on whether they are located on the surface of the organ or deeper within it. The intralobular bronchi divide into bronchioles. These bronchioles, in turn, continue branching to become respiratory bronchioles, which pass into the alveolar passages. The alveolar passages communicate with two, three, or four alveolar sacs.
Groups of connected alveolar passages, or acini, are considered the structural units of the lung. There are about 96 acini per lobule; in both lungs together, there is a total of about 800,000 acini and more than 700 million alveoli. The respiratory surface area varies from 30 sq m to 100 sq m (with a deep breath).
Each lung is covered with pleura, and the pleural cavity contains a small amount of fluid, which allows the lung to slide easily in the thoracic cavity during respiratory movement.
Venous blood enters the lung through the pulmonary artery, a branch of the pulmonary trunk that originates at the right ventricle of the heart. The pulmonary artery branches, as do the bronchi. The terminal arterioles break up into capillaries, which pass into the interalveolar septa and entwine the alveoli. The alveolar walls are formed of respiratory epithelium, the surface of which is covered with elastic fibers. Here the blood is enriched with oxygen and gives up its carbon dioxide.
Innervation of the lungs is the function of nerve fibers that are sympathetic (conducting the impulses that dilate the bronchi and constrict the blood vessels) and parasympathetic (constricting the bronchi, intensifying the secretory activity of the bronchial glands, and dilating the blood vessels). Fluid and certain of the products of gas exchange are discharged through the lungs.
The lungs participate in the maintenance of constant body temperature; the regulation of blood coagulation; the metabolism of proteins, fats, and carbohydrates; and the defense of the body against bacteria entering with the air (by elaborating immune antibodies). The most common diseases of the lungs are pneumonia, tumors, emphysema, and tuberculosis.
IA. O. OL’SHANSKII
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