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	:''for the village in Tibet, China see ]''

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	]'', 20th ed. 1918.</ref>]]
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	The '''lung''' is the essential ] in air-breathing vertebrates, the most primitive being the ]. Its principal function is to transport ] from the ] into the ], and to excrete ] from the bloodstream into the atmosphere. This exchange of gases is accomplished in the mosaic of specialized ] that form millions of tiny, exceptionally thin-walled air sacs called ]. The lungs also have non respiratory functions.

	Medical terms related to the lung often begin with '''''pulmo-''''', from the ] ''pulmonarius'' ("of the lungs"), or with '''''pneumo-''''' (from ] πνεύμω "lung")<ref>{{cite web \| url = http://www.kmle.com/search.php?Search=pneumo-\| title = ''KMLE Medical Dictionary Definition of pneumo-'' \| author = }}</ref><ref>{{cite web \| url = http://www.kmle.com/search.php?Search=pulmo\| title = ''KMLE Medical Dictionary Definition of pulmo-'' \| author = }}</ref>

	==Respiratory function==
	] production from ] requires oxygen and produces carbon dioxide as a by-product, creating a need for an efficient means of oxygen delivery ''to'' cells and excretion of carbon dioxide ''from'' cells. In small organisms, such as single-celled bacteria, this process of gas exchange can take place entirely by ]. In larger organisms, this is not possible; only a small proportion of cells are close enough to the surface for oxygen from the atmosphere to enter them through diffusion. Two major ]s made it possible for organisms to attain great ]: an efficient ] that conveyed ]es to and from the deepest tissues in the body, and a large, internalized ] that centralized the task of obtaining oxygen from the atmosphere and bringing it into the body, whence it could rapidly be distributed to all the circulatory system.

	In air-breathing vertebrates, respiration occurs in a series of steps. Air is brought into the animal via the airways — in reptiles, birds and mammals this often consists of the ]; the ]; the ]; the ] (also called the windpipe); the ] and ]s; and the terminal branches of the ]. The lungs of mammals are a rich lattice of alveoli, which provide an enormous surface area for gas exchange. A network of fine ] allows transport of ] over the surface of alveoli. Oxygen from the air inside the alveoli diffuses into the bloodstream, and carbon dioxide diffuses from the blood to the alveoli, both across thin alveolar ].

	The drawing and expulsion of air is driven by ] action; in early ]s, air was driven into the lungs by the ] muscles, whereas in ]s, ]s and ]s a more complicated ] is used. In the mammal, a large muscle, the ] (in addition to the internal intercostal muscles), drive ventilation by periodically altering the intra-thoracic ] and ]; by increasing volume and thus decreasing pressure, air flows into the airways down a pressure gradient, and by reducing volume and increasing pressure, the reverse occurs. During normal ]ing, expiration is passive and no muscles are contracted (the diaphragm relaxes).

	Another name for this inspiration and expulsion of air is ]. Vital capacity is the maximum volume of air that a person can exhale after maximum inhalation. A person's vital capacity can be measured by a spirometer (spirometry). In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease.

	==Non respiratory functions==
	In addition to respiratory functions such as ] and regulation of ] ], the lungs also:
	* influence the concentration of biologically active substances and drugs used in medicine in arterial blood
	* filter out small ]s formed in ]s
	* serve as a physical layer of soft, ]-absorbent protection for the ], which the lungs flank and nearly enclose.
	* filter out gas micro-bubbles occurring in the ] blood stream during ] ].<ref>Wienke B.R. : "Decompression theory"</ref>

	==Mammalian lungs==
	{{further\|]}}
	The lungs of mammals have a spongy texture and are honeycombed with ] having a much larger surface area in total than the outer surface area of the lung itself. The ] are typical of this type of lung.

	Breathing is largely driven by the muscular ] at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the diaphragm ], forcing the air out more quickly and forcefully. The ] 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 sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a '''bellows lung''' as it resembles a blacksmith's ].

	==Anatomy==
	In humans, it is the two main bronchi (produced by the bifurcation of the trachea) 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 sacks. Alveolar sacs are made up of clusters of ], 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 ] is pumped through the ] to the lungs, where oxygen ] into blood and is exchanged for carbon dioxide in the ] of the ]. The oxygen-rich blood returns to the heart via the pulmonary veins to be pumped back into systemic circulation.

	], bronchial tree, and ] (Cardiac notch labeled at bottom left).\|350px]]
	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 ], with three lobes on the right and two on the left. The lobes are further divided 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 and city dwellers. The medial border of the right lung is nearly vertical, while the left lung contains a ]. The cardiac notch is a concave impression molded to accommodate the shape of the heart.
	Lungs are to a certain extent 'overbuilt' and have a tremendous reserve volume as compared to the oxygen exchange requirements when at rest. This is the reason 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. As oxygen requirements increase due to ], a greater volume of the lungs is perfused, allowing the body to match its ]/] exchange requirements.

	The environment of the lung is very moist, which makes it hospitable for ]. Many respiratory illnesses are the result of bacterial or ] ] of the lungs.

	==Avian lungs==
	] lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as para-bronchi, connected at both ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the para-bronchi and into air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion, a process of crosscurrent exchange.

	Avian lungs contain two sets of air sacs, one towards the front, and a second towards the back. Upon inspiration, air travels backwards into the rear (caudal) sac, and a small portion travels forward past the para-bronchi and oxygenating the blood into the cranial air sac. On expiration, deoxygenated air held in the cranial air sack is exhaled, and the still-oxygenated air stored in the caudal sack moves over the parabronchi and is exhaled, with some remaining in the cranial sac. The complex system of air sacs ensures that the airflow through the avian lung always travels in the same direction - posterior to anterior. This 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 ], and this also allows them to sustain a higher ] than an equivalent weight mammal. Because of the complexity of the system, misunderstanding is common and it is incorrectly believed that that it takes two breathing cycles for air to pass entirely through a bird's respiratory system. A bird's lungs do not store air in either of the sacs between respiration cycles, air moves continuously from the posterior to anterior air sacs throughout respiration. This type of lung construction is called ''']s''' as distinct from the bellows lung possessed by most other animals.

	==Reptilian lungs==
	] lungs are typically ventilated by a combination of expansion and contraction of the ribs via axial muscles and buccal pumping. ]s also rely on the ] 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.

	==Amphibian lungs==
	The lungs of most ]s and other ]s 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 ], amphibians employ ]. Note that the majority of salamander species are ]s and conduct respiration through their skin and the tissues lining their mouth.

	==Invertebrate lungs==
	Some ]s have "lungs" that serve a similar respiratory purpose, but are not evolutionarily related to, vertebrate lungs. Some ]s have structures called "]s" used for atmospheric gas exchange. The ] uses structures called ] lungs to breathe air and indeed will drown in water, hence it breathes on land and holds its breath underwater. The ] are an order of snails and slugs that have developed "lungs".

	==Origins==

	The lungs of today's terrestrial ]s and the ]s of today's ] have evolved from simple sacs (outpocketings) of the esophagus that allowed the organism to gulp air under oxygen-poor conditions. Thus the lungs of vertebrates are ] to the gas bladders of fish (but not to their ]s). This is reflected by the fact that the lungs of a ] 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 ] in more "primitive" ]s, and is lost in the higher orders. (This is an instance of correlation between ].) There are currently no known animals which have both lungs and a gas bladder.

	==See also==
	{{commonscat\|lungs}}
	{{wiktionary}}
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	* ]
	* ]
	* ]
	* ]
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	* ]
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	==Further reading==
	* {{McGrawHillAnimation\|biochemistry\|Oxygen%20Carbon%20Dioxide}}
	* Lung Function Fundamentals. http://www.anaesthetist.com/icu/organs/lung/lungfx.htm
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	==References==
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	==Footnotes==
	<references/>
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Revision as of 21:04, 24 September 2007