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 most other vertebrates, two lungs are located near the backbone on either side of the heart. Their function is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, a process of gas exchange in the respiratory system.
The air that enters, or ventilates, the lungs enters the body through the mouth or nose, and travels through the pharynx, larynx, and trachea (windpipe). The trachea divides into two bronchi one for the right and one for the left lung, which then progressively subdivide into a system of smaller secondary and tertiary bronchi and smaller bronchioles. This division ends in alveoli, which are thin-walled sacs where gas exchange of carbon dioxide and oxygen, takes place.
Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal systems 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.
In humans, the lungs are located on either side of the heart in 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.
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.
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. On inspiration, air travels through the trachea of a bird into the 8-9 air sacs. Air then travels continuously from the air sacs at the back of the bird, through the lungs of the bird, and to the air sacs at the front of the bird. From here, air is expelled. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals. 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.
The lungs of birds are honey-comb-like and contain millions of tiny passages called parabronchi. Small sacs called called atria radiate from the walls of the tiny passages, and are the site of gas exchage. Gas exchange occurs by diffusion in these walls, as gas travels between the lumen of each parabronchus and blood vessels.
The air sacs, which hold air, do not contribute much to gas exchange, despitei being thin-walled, as they are poorly vascularized. The air sacs expand and contract due to changes in the volume in a bird's thorax and abdomen. 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.
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.
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 alveoli in mammals, but much larger and fewer in number. These 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.
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.
Both crocodilians and monitor lizards have developed lungs similar to those of birds, providing an unidirectional airflow and even possessing air sacs. 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.
Reptilian 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), 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.
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 restrict their size: all are small and rather thread-like in appearance, maximizing skin surface relative to body volume. 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 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 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.
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".
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.
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. 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. 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.
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