lung
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(lŭng) 
n.
n.
- Either of two spongy, saclike respiratory organs in most vertebrates, occupying the chest cavity together with the heart and functioning to remove carbon dioxide from the blood and provide it with oxygen.
- A similar organ in some invertebrates, including spiders and terrestrial snails.
at the top of (one's) lungs
- As loudly as one's voice will allow.
[Middle English lunge, from Old English lungen, lungs.]
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Either of two light, spongy, elastic organs in the chest, used for breathing. Each is enclosed in a membrane (pleura). Contraction of the diaphragm and the muscles between the ribs draw air into the lungs through the trachea, which splits into two primary bronchi, one per lung. Each bronchus branches into secondary bronchi (one per lobe of lung), tertiary bronchi (one per segment of lung), and many bronchioles leading to the pulmonary alveoli. There oxygen in the inspired gas is exchanged for carbon dioxide from the blood in the surrounding capillaries. Adequate tissue oxygen supply depends on sufficient distribution of air (ventilation) and blood (perfusion) in the lungs. Lung injuries or diseases (e.g., emphysema, embolism, pneumonia) can affect either or both.
For more information on lung, visit Britannica.com.
Paired, air-filled respiratory sacs, usually in the anterior or anteroventral part of the trunk of most tetrapods. They lie within the coelom and are covered by peritoneum. In mammals they are within special chambers of the coelom known as pleural cavities and the peritoneumis termed pleura.
Amphibian lungs are often simple sacs, with only small ridges on the internal walls. In higher forms the lungs become more and more subdivided internally, thus increasing greatly the surface areas across which the respiratory exchange takes place. However, even in many reptiles the lungs may be quite simple. Birds have especially complex lungs with a highly differentiated system of tubes leading into and through them to the air sacs which are contained in many parts of the bird's body. Mammalian lungs are simpler, but in them the internal subdivision into tiny sacs or alveoli is extreme; there may be over 350,000,000 of them in one human lung.
In humans the two lungs lie within the chest, separated by the heart and mediastinum. The right lung has three lobes and the left lung two. A bronchus, an artery, and a vein enter each lung medially at the hilum; each branches again and again as it enters the lobules and smaller divisions of the lungs (see illustration). The terminal airways or bronchioles expand into small clusters of grapelike air cells, the alveoli. The alveolar walls consist of a single layer of epithelium and collectively present a huge surface. A small network of blood capillaries in the walls of the alveoli affords surfaces for the actual exchange of gases. See also Respiration; Respiratory system.
The human lung. (After T. I. Storer and R. L. Usinger, General Zoology, 4th ed., McGraw-Hill, 1965)
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The name ‘lungs’ is derived from their lightness in weight, since they contain air, and the butcher refers to them as ‘lights’ for this reason. Adult lungs will float in water, but lungs from a fetus who has not breathed will sink. ‘Pulmonary’, from the Latin, refers to the lungs and is used in medical terminology, as in pulmonary function tests or pulmonary disease.
When the lungs are taken out of the chest they partially collapse. This is because they contain elastic fibres in the walls of the airways and alveoli (air sacs) ; open a festive balloon and it will empty itself. In addition, a thin layer of liquid lining the alveoli exerts surface tension, tending to collapse the lungs, although this surface tension is greatly decreased by the presence of surfactant. A soap bubble will collapse when pricked, although it has no elastic material; the surface tension of the film provides force enough.
The function of the lungs is to provide an enormous surface for gas exchange, with oxygen entering the body and carbon dioxide leaving it. The surface has to be protected against physical and environmental assault, in this respect resembling the internal gills of fishes and differing from the external gills of, for example, tadpoles. Thus the lungs are encased in structures — the chest wall and diaphragm — which will also provide the means of its ventilation. Amphibia inflate their lungs by pumping air in from the mouth; mammals, far more active and usually much larger, suck air into the lungs by muscular effort which creates a negative pressure around them.
Lungs: woodcut from Mundinus Anatomica, Marburg, 1541. Wellcome Institute Library, London
Our knowledge of the structure and func-tion of the lungs has depended on two major technological advances over the past three centuries: the light microscope and, later, the electron microscope. For almost two thousand years it was thought that the lungs were generally similar in structure to the liver, spleen, and pancreas, the only important difference being that air could enter the lungs through the trachea to mix physically with blood, cooling it by passing into the blood vessels. The superb anatomical dissections of Leonardo da Vinci (1452-539) and Versalius (1514-64) were regarded as consistent with this view. It was the use of the microscope by Malpighi (1628-94) that demonstrated for the first time the air-filled alveoli, the blind ends of the air passages into the lungs. He described them as ‘an almost infinite number of orbicular bladders’. Malpighi also showed that the blood capillaries in the lungs were vessels with walls that separated blood from gas, and allowed the passage of blood through the lungs, as deduced but not demonstrated by William Harvey. However, Malpighi knew nothing of gas exchange and thought the function of alveolar ventilation (the flow of air in and out of the alveoli) was to stir and mix the blood in the capillaries. From the 1950s onwards, the electron microscope displayed in detail the structures of the walls of the alveoli and of the tracheobronchial tree, extending to ‘ultrastructure’ within cells, knowledge of which had advanced meanwhile as light microscopy improved.
The airways below the larynx consist of (i) the trachea, a tube that extends almost to the middle of the chest; (ii) the bronchi (bronchial tree), formed by the trachea splitting into two and then each branch dividing again; (iii) the bronchioles — thin and short distensible airways that again divide many times to form (iv) the alveolar ducts from which (v) the alveoli arise. This multiple division results in about 23-5 generations of airway, with geometrically increasing numbers and total cross-sectional areas, and decreasing diameters. For example, from one trachea with a diameter of about 180 mm in an adult, by the tenth generation we have 1000 bronchi, each with a diameter of about 1.3 mm; by the twentieth generation we have 1 million bronchioles, each with a diameter of about 0.5 mm; and right at the end there are about 300 million alveoli. The diameter of the alveoli, like that of the bronchi and bronchioles, varies with the degree of lung inflation, in the range 0.1-0.3 mm. The alveolar surface area may be 30-100 m2 — often described as the size of a tennis court.
In the human embryo the lungs first begin to develop at about 3 weeks after fertilization. The alimentary tract develops earlier, and a ‘bud’ from its ventral surface progressively extends into the chest to form the airways and lungs. This common embryological source of the lungs and gut is reflected in adult structure and function; thus there are similarities in that both have smooth muscle in their walls, glandular secretion of mucus from their linings, and a nerve supply from the autonomic nervous system, although of course other differences in structure and function are considerable. At about 16 weeks alveoli begin to appear, looking rather like glands with thick walls. Only at about 20-25 weeks does the lung begin to resemble the adult tissue. Even at full-term birth the lungs, although functionally adequate, are not fully developed anatomically, having fewer bronchial branches and a far smaller alveolar surface area than they eventually acquire.
The airways: trachea, bronchi, and bronchioles
These are the conducting airways, and do not take part in gas exchange. Their function is to condition the air we breathe in and to conduct it to the alveoli. If inspired air reached the alveoli directly, if it were cold it would cool the tissue, if hot it would heat it, and if dry it would parch and destroy the alveolar walls. Only if we breathed air at 37°C and 100% humidity — a rare occurrence — would we avoid tissue damage. When we breathe through the mouth, as in exercise or with nasal blockage, we have eliminated the air conditioning role of the nose, and the mouth is much less efficient for this purpose. If the inspired air is cold and dry it will be raised to body temperature and full humidity by the first few generations of bronchi. This makes the airway lining itself (the mucosa) cold and dry, but protects the alveoli. On breathing out the mucosa will take up heat and water vapour from the expired air, restoring it to normal. Not only are the alveoli protected, but loss of heat and water from the body as a whole is minimized.
The walls of the trachea and bronchi have several layers. On the inner lining surface, the ‘luminal’ side, there is a layer of epithelium as a kind of skin. Most of the epithelial cells are ciliated, with microscopic ‘hairs’ (cilia) that continuously sweep any surface material towards the larynx, where it is coughed up or swallowed. This is the ‘ciliary escalator’. Other cells secrete mucus, the slimy liquid that constitutes phlegm and lies on the cilia. Just under the epithelium is a dense blood capillary network that provides nutrition for the epithelium and glands, and may be the site of uptake of inhaled pollutants and drugs. Deeper in the wall are the submucosal glands, the main source of the mucus that lines the airways. The glands are stimulated to secrete by many factors, the most important being pollutants, including cigarette smoke, and viral or bacterial infections of the airways. Smoker's cough brings up the mucus thus secreted, and in chronic bronchitis there is the overproduction of mucus that characterizes the disease and is due to local pathological changes. The secreted mucus normally has several important defensive effects. It will create a barrier and take up soluble pollutants and smoke particles, slowing down their entry into the body and protecting the epithelium from their harmful effects, and eliminating them via the ciliary escalator. It will stimulate cough as an even more rapid means of their removal. In health the mucus sheet is very thin and difficult to measure; it is probably about 0.02-0.05 mm thick. Even in disease when the output of mucus is greatly increased, it remains too thin to block the airways, unless there is associated inflammation.
Deeper in the airway wall there is cartilage and smooth muscle. The cartilage stabilizes the airways and prevents their collapse during vigorous acts of breathing, such as coughing. The smooth muscle has not been shown to have a physiological role, unlike that in the intestines, which is responsible for the squeezing movement of peristalsis, but possibly it adjusts the diameter of the airways to make them optimally efficient for conducting gas to the alveoli.
The trachea and bronchi contain many sensory nerves, in general of two types. In the smooth muscle are receptors that signal the degree of stretch and therefore of inflation of the airways and lungs, and control the pattern of breathing — its rate and depth, probably to make it as efficient as possible. If the vagus nerves that carry sensory information from the bronchi are cut, in most animals the breathing becomes slow, deep, and mechanically inefficient. Secondly, in the epithelium there is a network of fine nerve fibres, with finger-like projections reaching almost to the airway lumen, that respond to inhaled pollutants and inflammatory mediators and set up a range of reflex responses. The most striking is the cough, but there is also reflex mucus secretion and smooth muscle contraction. The nerves look like, and act as, tripwires and sensing rods just under the surface, ready to respond to any adverse intruder.
The smallest air-conducting vessels, the bronchioles, are distinguished by having no cartilage in their walls, no mucus cells in their epithelium, and few or no submucosal glands. When the lungs inflate they probably distend equally with the alveoli, but there is little gas exchange in them. If they are inflamed, as in bronchiolitis, the alveoli they supply collapse, with a stiffening of the lungs and a failure of gas exchange.
Because the airways take no part in gas exchange, they are sometimes referred to as the ‘anatomical deadspace’. At rest their volume is about 150 ml in a healthy adult. If an average tidal volume of 500 ml is inhaled, at the end of inspiration only 350 ml will have entered the alveoli, and 150 ml will remain in the airways. The ventilation used for gas exchange will be only 350/500ths — or 70% — of the total ventilation. The rest could be called wasted ventilation but, as described earlier, it has an essential function in conditioning the inspired air. When we breathe out, the first 150 ml is unchanged ‘fresh’ air, followed by 350 ml of air from the alveoli, rich in carbon dioxide and partly depleted of oxygen.
Asthma
In asthma the contraction of bronchial smooth muscle can have a profound effect by narrowing the airways; a greater muscular effort is then required to inflate the lungs. The smooth muscle contracts in response to two main stimuli, chemical and nervous, and either can cause the wheezing associated with asthma. Most types of asthma involve inflammation, with release of chemical mediators like histamine, bradykinin, and substance P, which diffuse to the smooth muscle and make it contract. In addition, nervous signals can come down from the brain via the vagus nerves, when asthma and wheezing are induced by emotional factors in susceptible subjects.
It used to be thought that asthma was solely due to smooth muscle contraction narrowing the airways. This view was supported by the effectiveness of treatment by smooth muscle relaxants such as salbutamol. But asthma is now considered to be an inflammation of the airways, with multiple effects that all narrow the airways: smooth muscle contraction, thickening of the mucosa by oedema because of leaking blood vessels, and secretion of mucus into the airway lumen. The use of anti-inflammatory drugs has become general.
The alveolar ducts and alveoli
Here gas exchange takes place. There are about 15 million alveolar ducts, and each gives rise to about 20 alveolar air sacs. Each of these alveoli is surrounded by a network of blood capillaries — a bit like a balloon in a close-fitting string bag except that the alveoli are not spherical, and they share the ‘string’ with the adjacent alveoli all around (see figure for shape in cross-section). The entire output of the right heart goes through the alveolar blood vessels and then into the left heart. The enormous alveolar surface, up to 300 m2, promotes gas exchange between blood and air, since the rate of diffusion of a gas depends on the surface area, the thinness of the diffusion barrier, and the solubility of the gas (Fick's Law). The alveolar wall is extremely thin, from 0.2-0.5 μm, depending on the degree of inflation of the lungs. The barrier to diffusion has three components. On the surface of the alveoli is a thin layer of secretion, containing surfactant, the detergent phospholipid that lowers the surface tension of the lungs and allows them to be stretched by relatively low pressures. The surfactant layer is about 0.15 μm thick. Then there is the epithelial cell layer of the alveoli. This consists of two types of cells, those which mainly provide a mechanical sheet (type I) and those that secrete surfactant (type II). Together they constitute the lining ‘skin’ of the alveoli. The capillary endothelium is the third component of the barrier. Cells of a different type, the alveolar macrophages, are found within the cavities of the alveoli; their function is to ingest and remove solid particles, such as those of smoke.
Scanning electron microscope view of lung alveoli, magnified × 750, showing the way in which their shape is retained as their size alters with changes in lung volume. (a) At full capacity (b) At the volume representing the end of a normal expiration. Note that the alveoli are not circular in section but have flat walls, common to adjacent alveoli, with curvatures at the junctions. Compare the capillaries in the stretched alveolar walls in (a) with the rounded capillaries in (b). Adapted from Albertine, K. H., Williams, M. C., and Hyde, D. M. (2000). Fig. 1.22, p. 17. In Textbook of respiratory medicine, 3rd edn. (ed. J. F. Murray and J. A. Nadel). W. B. Saunders, Philadelphia
Carbon dioxide is over twenty times more soluble than oxygen in body liquids, and diffuses twenty times more quickly out of the body than oxygen enters. In any diseases where alveolar gas exchange is decreased, for example when the alveolar wall is thickened by alveolitis, the first changes in blood gas transfer will be with oxygen, and the patient may develop quite severe hypoxia before the blood carbon dioxide begins to increase.
For many years at the beginning of the twentieth century there was intense scientific dispute as to whether the alveoli of the human lungs could secrete oxygen into the bloodstream. The argument was that at high altitudes the oxygen pressure was so low that it could not maintain blood oxygen pressure without active transport through the epithelium. Perhaps the indirect methods to test the problem were not sensitive enough for a clear solution — and it was known that some fishes could actively secrete oxygen into their swim bladders, taking advantage of the properties of their haemoglobin that did not seem to apply to human haemoglobin. The problem was finally solved when more sensitive analysis showed that human lungs, and presumably those of other mammals, could not secrete oxygen and that all gas exchanges could be explained by passive diffusion. Even in the absence of oxygen secretion, some climbers can just get to the top of Mount Everest without added oxygen.
— John Widdicombe
See respiratory system. See also breathing; carbon dioxide; development and growth: birth and infancy; oxygen.
One of a pair of respiratory organs in the thorax. The lungs consist of air tubes terminating in alveoli where gaseous exchange takes place. The tubes are connected to the air by way of the bronchi and trachea. The lungs are fibrous elastic sacs which can be expanded and compressed by movements of the diaphragm and ribcage during ventilation. The lungs and its airways are a site of water evaporation, an important factor in water balance and thermoregulation.
Lungs
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lungs, elastic organs used for breathing in vertebrate animals, excluding most fish, which use gills, 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 trachea, 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 pleura. 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 respiration.
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A pair of organs, the principal parts of the respiratory system, at the front of the cavity of the chest, or thorax. In the lungs, oxygen from the air that is inhaled is transferred into the blood, while carbon dioxide is removed from the blood and exhaled.
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Either of the two main organs of respiration, lying on either side of the heart, within the chest cavity. The lungs supply the blood with oxygen inhaled from the outside air, and they dispose of waste carbon dioxide in the exhaled air, as a part of the process of respiration. They are usually divided into lobes, the left lung has up to three (cranial, middle and caudal), while the right lung has up to four (cranial, middle, caudal and accessory). Horse lungs are least subdivided; cat and dog lungs are deeply fissured into lobes.
The lungs are made of elastic tissue filled with interlacing networks of tubes and sacs carrying air, and with blood vessels carrying blood. The bronchi, which bring air to the lungs, branch out within the lungs into many smaller tubes, the bronchioles, which culminate in clusters of tiny air sacs called alveoli, whose total runs into millions. The alveoli are surrounded by a network of capillaries. Through the thin membranes of the capillaries, the air and blood make their exchange of oxygen and carbon dioxide. See also pulmonary, respiratory.
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- accessory l. — develop from an embryonic lung bud in an abnormal site, e.g. neck, abdomen.
- l. birth changes — include dilation of the alveoli and the bronchial tree, marked pulmonary vasodilation, decreased resistance to blood flow through the lungs, constriction of the ductus arteriosus, removal of fluid from the fetal bronchial tree.
- l. breath sounds — see breath sounds.
- l. bud — blunt end of the respiratory diverticulum which grows ventrally out of the proximal end of the foregut, then extends caudally and divides into two, forming the origins of the bronchial tree.
- l. consolidation — see consolidation, hepatization.
- l.-digit syndrome — an uncommon condition in cats in which a primary lung tumor metastasizes to, usually multiple, digits as well as other sites.
- ectopic l. — edematous, lobulated masses of lung tissue in the abdominal or thoracic cavities or in subcutaneous sites.
- l. factor — closely related ipomeanols produced in rotting sweet potatoes by the catabolic activities of the fungus Perilla frutescens and other fungi of phytoalexins in the tubers. The factor is not toxic until it is activated by pulmonary microsomal enzymes.
- l. fluke — see paragonimus.
- l. hilus — that part of the lung that is not covered by pleura and through which blood vessels, bronchi, nerves and lymphatics enter and leave the lung.
- l. lobe torsion — occurs uncommonly in dogs and cats, most often of the right middle lobe. May occur spontaneously, following trauma, or in association with pleural effusion. Impaired venous return causes engorgement and rapid necrosis. Clinical signs include coughing and hemoptysis.
- l. meridian points — acupuncture points on the lung meridian.
- l. mites — see pneumonyssus, halarachne, orthohalarachne.
- l. perforation — may cause lung hemorrhage, emphysema, hemothorax or pneumothorax, or any combination of these conditions.
- l. puller — appliance for pulling the pluck, the heart and lungs on the trachea, out of the thorax at the abattoir.
- l. puncture — see lung perforation (above).
- l. Qi deficiency — in acupuncture terminology is a deficiency of Qi or energy in the lungs manifested by recurrent illness, weak cough, rapid shallow respiration, dry cracked muzzle.
- l. reflexes — hering–breuer reflexes.
- l. resonance — resonant sound achieved on percussion of the chest wall over normal lung.
- shock l. — see shock lung.
- l. sounds absent — breath sounds audible on auscultation over normal lung are absent over consolidated, neoplastic and collapsed lung.
- stiff l. — one with decreased compliance.
- total l. capacity — the sum of the potential air spaces in the bronchioles and the alveoli.
- l. volume — see total lung capacity (above), volume.
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n
One of a pair of light, spongy organs in the thorax, constituting the main component of the respiratory system. The lungs provide the tissue surface necessary for the exchange of gases between the environment and the blood. Oxygen is extracted from inspired air, and carbon dioxide is dispersed from the venous system back into the environment.
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categories related to 'lung'
- Respiratory System - lung: either of two organs for breathing that takes carbon dioxide from blood and returns oxygen to it
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For other uses, see Lung (disambiguation).
"Lunged" redirects here. For other uses, see Lunge.
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart. Their principal function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. This exchange of gases is accomplished in the mosaic of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called alveoli.
To completely explain the anatomy of the lungs, it is necessary to discuss the passage of air through the mouth to the alveoli. Once air progresses through the mouth or nose, it travels through the oropharynx, nasopharynx, the larynx, the trachea, and a progressively subdividing system of bronchi and bronchioles until it finally reaches the alveoli where the gas exchange of carbon dioxide and oxygen takes place.[2]
The drawing and expulsion of air (ventilation) is driven by muscular action; in early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, whereas in reptiles, birds and mammals a more complicated musculoskeletal system is used.
Medical terms related to the lung often begin with pulmo-, such as in the (adjectival form: pulmonary) or from the Latin pulmonarius ("of the lungs"), or with pneumo- (from Greek πνεύμων "lung").
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Mammalian lungs
Further information: Human lung
The lungs of mammals have a spongelike and soft texture and are honeycombed with epithelium, having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are a typical example of this type of lung.
Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward, increasing volume and thus decreasing pressure, causing air to flow into the airways. Air enters through the oral and nasal cavities; it flows through the pharynx, then the larynx and into the trachea, which branches out into the main bronchi and then subsequent divisions. During normal breathing, expiration is passive and no muscles are contracted (the diaphragm relaxes). The rib cage itself is also able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is transported into or expelled out of the lungs. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.[3]
Anatomy
In humans, the trachea divides into the two main bronchi that enter the roots of the lungs. The bronchi continue to divide within the lung, and after multiple divisions, give rise to bronchioles. The bronchial tree continues branching until it reaches the level of terminal bronchioles, which lead to alveolar sacs. Alveolar sacs are made up of clusters of alveoli, like individual grapes within a bunch. The individual alveoli are tightly wrapped in blood vessels and it is here that gas exchange actually occurs. Deoxygenated blood from the heart is pumped through the pulmonary artery to the lungs, where oxygen diffuses into blood and is exchanged for carbon dioxide in the hemoglobin of the erythrocytes. The oxygen-rich blood returns to the heart via the pulmonary veins to be pumped back into systemic circulation.
Human lungs are located in two cavities on either side of the heart. Though similar in appearance, the two are not identical. Both are separated into lobes by fissures, with three lobes on the right and two on the left. The lobes are further divided into segments and then into lobules, hexagonal divisions of the lungs that are the smallest subdivision visible to the naked eye. The connective tissue that divides lobules is often blackened in smokers. The medial border of the right lung is nearly vertical, while the left lung contains a cardiac notch. The cardiac notch is a concave impression molded to accommodate the shape of the heart.
Each lobe is surrounded by a pleural cavity, which consists of two pleurae. The parietal pleura lies against the rib cage, and the visceral pleura lies on the surface of the lungs. In between the pleura is pleural fluid. The pleural cavity helps to lubricate the lungs, as well as providing surface tension to keep the lung surface in contact with the rib cage.
Lungs are to a certain extent 'overbuilt' and have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. Such excess capacity is one of the reasons that individuals can smoke for years without having a noticeable decrease in lung function while still or moving slowly; in situations like these only a small portion of the lungs are actually perfused with blood for gas exchange. Destruction of too many alveoli over time leads to the condition emphysema, which is associated with extreme shortness of breath. As oxygen requirements increase due to exercise, a greater volume of the lungs is perfused, allowing the body to match its CO2/O2 exchange requirements. Additionally, due to the excess capacity, it is possible for humans to live with only one lung, with the one compensating for the other's loss.
The environment of the lung is very moist, which makes it hospitable for bacteria. Many respiratory illnesses are the result of bacterial or viral infection of the lungs. Inflammation of the lungs is known as pneumonia; inflammation of the pleura surrounding the lungs is known as pleurisy.
Vital capacity is the maximum volume of air that a person can exhale after maximum inhalation; it can be measured with a spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease.
The lung parenchyma is strictly used to refer solely to alveolar tissue with respiratory bronchioles, alveolar ducts and terminal bronchioles.[4] However, it often includes any form of lung tissue, also including bronchioles, bronchi, blood vessels and lung interstitium.[4]
Non respiratory functions
In addition to their function in respiration, the lungs also:
- Alter the pH of blood by facilitating alterations in the partial pressure of carbon dioxide
- Filter out small blood clots formed in veins
- Filter out gas micro-bubbles occurring in the venous blood stream such as those created during decompression after underwater diving .[5]
- Influence the concentration of some biologic substances and drugs used in medicine in blood
- Convert angiotensin I to angiotensin II by the action of angiotensin-converting enzyme
- May serve as a layer of soft, shock-absorbent protection for the heart, which the lungs flank and nearly enclose.
- Immunoglobulin-A is secreted in the bronchial secretion and protects against respiratory infections.
- Maintain sterility by producing mucus containing antimicrobial compounds.[6] Mucus contains glycoproteins, e.g. mucins, lactoferrin,[7] lysozyme, lactoperoxidase.[8][9] We find also on the epithelium Dual oxidase 2[10][11][12] proteins generating hydrogen peroxide, useful for hypothiocyanite endogenous antimicrobial synthesis. Function not in place in cystic fibrosis patient lungs.[13][14]
- Ciliary escalator action is an important defence system against air-borne infection.The dust particles and bacteria in the inhaled air are caught in the mucous layer present at the mucosal surface of respiratory passages and are moved up towards pharynx by the rhythmic upward beating action of the cilia.
- Provide airflow for the creation of vocal sounds.
- Thermoregulation (panting)
- The blood volume of the lungs is about 450 milliliters, about 9 percent of the total blood volume of the entire circulatory system,Under various physiological and pathological conditions, the quantity of blood in the lungs can vary from as little as one-half normal up to twice normal.loss of blood from the systemic circulation by hemorrhage can be partly compensated for by the automatic shift of blood from the lungs into the systemic vessels[15]
Avian lungs
Avian lungs do not have alveoli as mammalian lungs do, they have Faveolar lungs. They contain millions of tiny passages known as parabronchi. There are air vesicles, called atria, which project radially from the walls of the parabronchi. The gas exchange tissues are set into the walls of the atria and gases travel via diffusion between the gas exchange tissues and the lumen of each parabronchus. There are two categories of parabronchi. The paleopulmonic parabronchi are found in all birds and air flows through them in the same direction – posterior to anterior during inhalation and exhalation. Some bird species also have neopulmonic parabronch where the air flow is bidirectional. The paleopulmonic unidirectional airflow is in contrast to the mammalian system, in which the direction of airflow in the lung is tidal, reversing between inhalation and exhalation.
By utilizing a unidirectional flow of air, avian lungs are able to extract a greater concentration of oxygen from inhaled air. Birds are thus equipped to fly at altitudes at which mammals would succumb to hypoxia. This also allows them to sustain a higher metabolic rate than most equivalent weight mammals.[16] Note than some species of small bats have a higher mean total morphometnc pulmonary diffusing capacity for oxygen than equivalent weight birds but this is the exception and is not the rule.[17]
The lungs of birds are relatively small, but are connected to 8-9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. The air sacs, although thin walled, are poorly vascularized, and do not themselves contribute much to gas exchange, but they do act like bellows to ventilate the lungs. The air sacs expand and contract due to changes in the volume of the combined thorax and abdominal cavity. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronized with movement of the flight muscles.[18]
Because of the complexity of the system, misunderstanding is common and it is incorrectly believed that it takes two breathing cycles for air to pass entirely through a bird's respiratory system. Air is not stored in either the posterior or anterior sacs between respiration cycles, air moves continuously from the posterior to the anterior of the lungs throughout respiration. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals.[16]
Reptilian lungs
Reptilian lungs are typically ventilated by a combination of expansion and contraction of the ribs via 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), which in turn pulls the bottom of the lungs backward, expanding them. Turtles, which are unable to move their ribs, instead use their forelimbs and pectoral girdle to force air in and out of the lungs.[18]
The lung of most reptiles has a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to, but much larger and fewer in number than, mammalian alveoli, and give the lung a sponge-like texture. In tuataras, snakes, and some lizards, the lungs are simpler in structure, similar to that of typical amphibians.[18]
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.[18]
Amphibian lungs
The lungs of most frogs and other amphibians are simple balloon-like structures, with gas exchange limited to the outer surface area of the lung. This is not a very efficient arrangement, but amphibians have low metabolic demands and also frequently supplement their oxygen supply by diffusion across the moist outer skin of their bodies. Unlike mammals, which use a breathing system driven by negative pressure, amphibians employ positive pressure.[18] The majority of salamander species are lungless salamanders, which respirate through their skin and tissues lining their mouth. The only other known lungless tetrapods are also amphibians; the Bornean Flat-headed Frog (Barbourula kalimantanensis) and Atretochoana eiselti, a caecilian.
The lungs of amphibians typically have a few narrow 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.[18]
Lungfish
The lungs of lungfish are similar to those of amphibians, with few, if any, internal septa. In the Australian lungfish, there is only a single lung, albeit divided into two lobes. Other lungfish and Polypterus, however, have two lungs, which are located in the upper part of the body, with the connecting duct curving round 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.[18]
Invertebrate lungs
Some invertebrates have "lungs" that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids have structures called "book lungs" used for atmospheric gas exchange. The Coconut crab uses structures called Branchiostegal lungs to breathe air and indeed will drown in water, hence it breathes on land and holds its breath underwater. The Pulmonata are an order of snails and slugs that have developed "lungs".
Origins of the vertebrate lung
The lungs of today's terrestrial vertebrates and the gas bladders of today's fish are believed to have evolved from simple sacs (outpocketings) of the esophagus that allowed early fish to gulp air under oxygen-poor conditions.[19] These outpocketings first arose in the bony fish; in some of the ray-finned fish the sacs evolved into gas bladders, while in other ray-finned fish (such as the gar, bichir and amia) as well as the lobe-finned fish they evolved into lungs.[19] 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). This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the esophagus and in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the higher orders. (This is an instance of correlation between ontogeny and phylogeny.) No known animals have both a gas bladder and lungs.
See also
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Wikimedia Commons has media related to: lungs |
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Look up lung in Wiktionary, the free dictionary. |
- Alveolar-capillary barrier
- American Lung Association
- Asthma
- Borders of the lung
- Bronchitis
- Bronchus
- Cardiothoracic surgery
- Chronic obstructive pulmonary disease
- Drowning
- Dry drowning
- Emphysema
- Gills
- Left lung
- Liquid breathing
- Lung cancer
- Lung volumes
- Mechanical ventilation
- Pneumothorax
- Pneumonia
- Pulmonary contusion
- Pulmonology
- Right lung
- Tuberculosis
Further reading
- Lung Function Fundamentals. http://www.anaesthetist.com/icu/organs/lung/lungfx.htm
- Dr D.R. Johnson: Introductory anatomy, respiratory system
- Franlink Institute Online: The Respiratory System
- Lungs 'best in late afternoon'
- Chronic Respiratory Disease - leading research and articles on respiratory disease.
- Avian lungs and respiration
Footnotes
- ^ a b Gray's Anatomy of the Human Body, 20th ed. 1918.
- ^ Wienberger, Cockrill, Mandel. Principles of Pulmonary Medicine. Elsevier Science.[verification needed]
- ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright1 (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. OCLC 32308337.[page needed]
- ^ a b medilexicon.com > Medical Dictionary - 'Parenchyma Of Lung' In turn citing: Stedman's Medical Dictionary. 2006
- ^ Wienke B.R. : "Decompression theory"[verification needed]
- ^ Travis SM, Conway BA, Zabner J, et al. (May 1999). "Activity of abundant antimicrobials of the human airway". American Journal of Respiratory Cell and Molecular Biology 20 (5): 872–9. PMID 10226057. http://ajrcmb.atsjournals.org/cgi/pmidlookup?view=long&pmid=10226057.
- ^ Rogan MP, Taggart CC, Greene CM, Murphy PG, O'Neill SJ, McElvaney NG (October 2004). "Loss of microbicidal activity and increased formation of biofilm due to decreased lactoferrin activity in patients with cystic fibrosis". The Journal of Infectious Diseases 190 (7): 1245–53. doi:10.1086/423821. PMID 15346334.
- ^ Wijkstrom-Frei C, El-Chemaly S, Ali-Rachedi R, et al. (August 2003). "Lactoperoxidase and human airway host defense". American Journal of Respiratory Cell and Molecular Biology 29 (2): 206–12. doi:10.1165/rcmb.2002-0152OC. PMID 12626341.
- ^ Conner GE, Salathe M, Forteza R (December 2002). "Lactoperoxidase and hydrogen peroxide metabolism in the airway". American Journal of Respiratory and Critical Care Medicine 166 (12 Pt 2): S57–61. doi:10.1164/rccm.2206018. PMID 12471090.
- ^ Fischer H (October 2009). "Mechanisms and Function of DUOX in Epithelia of the Lung". Antioxidants & Redox Signaling 11 (10): 2453–65. doi:10.1089/ARS.2009.2558. PMC 2823369. PMID 19358684. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2823369.
- ^ Rada B, Leto TL (2008). "Oxidative innate immune defenses by Nox/Duox family NADPH Oxidases". Contributions to Microbiology. Contributions to Microbiology 15: 164–87. doi:10.1159/000136357. ISBN 978-3-8055-8548-4. PMC 2776633. PMID 18511861. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2776633.
- ^ Rada B, Lekstrom K, Damian S, Dupuy C, Leto TL (October 2008). "The Pseudomonas toxin pyocyanin inhibits the Dual oxidase-based antimicrobial system as it imposes oxidative stress on airway epithelial cells". Journal of Immunology 181 (7): 4883–93. PMC 2776642. PMID 18802092. http://www.jimmunol.org/cgi/pmidlookup?view=long&pmid=18802092.
- ^ Moskwa P, Lorentzen D, Excoffon KJ, et al. (January 2007). "A Novel Host Defense System of Airways Is Defective in Cystic Fibrosis". American Journal of Respiratory and Critical Care Medicine 175 (2): 174–83. doi:10.1164/rccm.200607-1029OC. PMC 2720149. PMID 17082494. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2720149.
- ^ Conner GE, Wijkstrom-Frei C, Randell SH, Fernandez VE, Salathe M (January 2007). "The Lactoperoxidase System Links Anion Transport To Host Defense in Cystic Fibrosis". FEBS Letters 581 (2): 271–8. doi:10.1016/j.febslet.2006.12.025. PMC 1851694. PMID 17204267. //www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1851694.
- ^ GUYTON&HALL Medical physiology 12th edition
- ^ a b Ritchson, G. "BIO 554/754 - Ornithology: Avian respiration". Department of Biological Sciences, Eastern Kentucky University. http://www.people.eku.edu/ritchisong/birdrespiration.html. Retrieved 2009-04-23.
- ^ Maina, John N.; A. S. King (July 1, 1984). "Correlations between structure and function in the design of the bat lung: a morphometric study". Journal of Experimental Biology 111 (1): 43–61. http://jeb.biologists.org/content/111/1/43.full.pdf. Retrieved 14 January 2012.
- ^ a b c d e f g Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 330–334. ISBN 0-03-910284-X.
- ^ a b Colleen Farmer (1997). "Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates". Paleobiology. http://www.biology.utah.edu/farmer/publications%20pdf/1997%20Paleobiology23.pdf.
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This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Translations:
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Lung
Dansk (Danish)
n. - lunge, åndehul
Ελληνική (Greek)
n. - (ανατ.) πνεύμονας (κν. πλεμόνι)
Português (Portuguese)
n. - pulmão (m) (Anat.)
Русский (Russian)
легкое, прибор для искусственного дыхания
Svenska (Swedish)
n. - lunga (äv. bildl.), (attr.) lung-
中文（简体）(Chinese (Simplified))
肺, 空地, 肺脏
中文（繁體）(Chinese (Traditional))
n. - 肺, 空地, 肺臟
العربيه (Arabic)
(الاسم) رئه, رئه ميكانيكيه
עברית (Hebrew)
n. - אונת ריאה, ריאה
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