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Lung

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For other uses, see Lung (disambiguation).
This article is about lungs in general. For human lungs, see Human lung.
"Lunged" redirects here. For other uses, see Lunge.
Sketch of the human lungs.
The human lungs flank the heart and great vessels in the chest cavity[1]
Air enters and leaves the lungs via a conduit of cartilaginous passageways—the bronchi and bronchioles. In this image, lung tissue has been dissected away to reveal the bronchioles[1]

The lung is the essential respiratory organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, two lungs are located near the backbone on either side of the heart. Their function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, a process called respiration.

The air that enters, or ventilates, the lungs enters the body through the mouth or nose, and sequentially progresses through the pharynx, larynx, and trachea (windpipe). The trachea divides into two bronchi for the right and left lung, which then progressively subdivide into a system of bronchi and bronchioles. This division ends in alveoli, which are thin-walled sacs where gas exchange of carbon dioxide and oxygen, or perfusion, takes place.[2]

The negative inspiratory force of breathing is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal system to support and foster breathing. In humans, the primary muscle that drives breathing is the diaphragm. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians.

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").

Anatomy

Humans

Main article: Human lung
Illu bronchi lungs.jpg

In humans, the lungs are located on either side of the chest, with the left lung sharing the left side of the space with the heart, and sitting in an impression called the cardiac notch. The lungs are surrounded by the pleural cavity, a lining of two lubricated layers that allows the negative pressure of breathing to be maintained without friction. The negative inspiratory force in the chest is due to the action of the diaphragm, a muscle below the lungs which separates the chest from the abdomen.

Human lungs can be affected by a variety of diseases. Many respiratory illnesses are because 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. Lung diseases can arise suddenly, such as a pneumothorax or hemothorax, in which fluid or air is trapped in the pleural cavity and compresses the lung. Diseases can also be chronic, such as emphysema, a common complication of smoking caused by inflammation and the progressive inability of alveoli to expand and contract with respiration. Fibrotic diseases of the lung occur when the lung is inflamed for a long period of time, whether because of a person's occupation (such as Coalworker's pneumoconiosis) or rarer causes, such as a person's medication. Smoking and occupational exposure to harmful substances are also key risk factors for some forms of lung cancer.[3]

The function of human lungs is often measured by lung function tests such as spirometry. These measure how much a person is able to inhale (total lung capacity) or exhale (vital capacity). How well and how quickly a person's lungs expel air helps indicate the health of their lungs and whether, if sick, the disease is obstructive (caused by a difficulty getting air to the alveoli, such as in asthma or choking) or restrictive.

Birds

Cranial sinus and postcranial air sac systems in birds. All pneumatic spaces are paired except the clavicular air sac, and the lungs are shaded. Abbreviations: aas, abdominal air sac; atas, anterior thoracic air sac; cas, cervical air sac; clas, clavicular air sac; hd, humeral diverticulum of the clavicular air sac; lu, lung; pns, paranasal sinus; ptas, posterior thoracic air sac; pts, paratympanic sinus; t, trachea.
Bird respiration air flow schematic

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 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.[4]

Unlike mammals, the lungs of birds do not have alveoli like those of mammals: birds have honey-comb-like, faveolar lungs[citation needed], which contain millions of tiny passages called parabronchi. Small sacs called called atria radiate from the walls of the tiny passages. Gas exchange occurs by diffusion in these walls, as gas travels between the lumen of each parabronchus and blood vessels.

Unlike mammals, in which air enters and leaves the lungs like a bellows, the flow of air in bird lungs is in one direction only. Air moves continuously from the posterior to the anterior of the lungs throughout respiration, and is not stored in the lung. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals.[5] This means that they 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.[5]

This typical system is one of two types of parabronchi found in birds, called paleopulmonic parabronchi and is found in all birds. Some bird species also have a lung structure where the air flow is bidirectional, called neopulmonic parabronchi.

Reptiles

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.[4]

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.[4]

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.[4]

Both crocodilians and monitor lizards have developed lungs similar to those of birds, providing an unidirectional airflow and even possessing air sacs.[6] The now extinct pterosaurs have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of Pteranodontia, the hindlimbs.

Amphibians

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 can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Unlike higher vertebrates, who use a breathing system driven by negative pressure where the lungs are inflated by expanding the rib cage, amphibians employ positive pressure system, forcing air down into the lungs by buccal pumping.[7] 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.[8]

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 restrict their size, all are small and rather thread-like in appearance, maximizing skin surface relative to body volume.[9] The only other known lungless tetrapods are 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.[4]

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.[4]

Invertebrates

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".


Function

Lungs of a raccoon being manually inflated during a dissection

The lungs of mammals including those of humans, have a soft, spongelike texture and are honeycombed with epithelium, having a much larger surface area in total than the outer surface area of the lung itself.

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 use of the intercostal muscles, together with 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.[10]

Evolutionary origins

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.[11] 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 carps, trouts, herrings, catfish, eels have retained the physostome condition with the sack being open to the esophagus. In more basal bony fish, such as the gar, bichir, bowfin and the lobe-finned fish, the bladders have evolved to primarily function as lungs.[11] 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 the physostome gas bladders, which can serve as both buoyancy organ and with the pneumatic duct to the gut also serve as lungs. This condition is found 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

Further reading

Footnotes

  1. ^ a b Gray's Anatomy of the Human Body, 20th ed. 1918.
  2. ^ Wienberger, Cockrill, Mandel. Principles of Pulmonary Medicine. Elsevier Science.[verification needed]
  3. ^ PMID 26289596 (PubMed)
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  4. ^ a b c d e f Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 330–334. ISBN 0-03-910284-X. 
  5. ^ a b Ritchson, G. "BIO 554/754 - Ornithology: Avian respiration". Department of Biological Sciences, Eastern Kentucky University. Retrieved 2009-04-23. 
  6. ^ Unidirectional Airflow In The Lungs Of Birds, Crocs And Now Monitor Lizards
  7. ^ Janis, C.M.; Keller, J.C. (2001). "Modes of ventilation in early tetrapods: Costal aspiration as a key feature of amniotes" (PDF). Acta Palaeontologica Polonica 46 (2): 137–170. Retrieved 11 May 2012. 
  8. ^ Brainerd, E. L. (1999). New perspectives on the evolution of lung ventilation mechanisms in vertebrates. Experimental Biology Online 4, 11-28. http://www.brown.edu/Departments/EEB/brainerd_lab/pdf/Brainerd-1999-EBO.pdf
  9. ^ Duellman, W. E.; Trueb, L. (1994). Biology of amphibians. illustrated by L. Trueb. Johns Hopkins University Press. ISBN 0-8018-4780-X. 
  10. ^ 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]
  11. ^ a b Colleen Farmer (1997). "Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates" (PDF). Paleobiology.