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The lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. Its function is to exchange oxygen from air with carbon dioxide from blood. The process in which this happens is called "external respiration" or breathing. Medical terms related to the lung often start in pulmo- from the Latin word pulmones for lungs.

Table of contents
1 Mammalian lungs
2 Avian lungs
3 Amphibian lungs
4 Evolutionary origins
5 Arachnid lungs
6 See also
7 External links

Mammalian lungs

The lungs of mammals have a spongy 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 typical of this type of lung.

The lungs are located inside the thoracic cavity, protected by the bony structure of the rib cage and enclosed by a double-walled sac called pleura. The inner layer of the sac adheres tightly to the outside of the lungs and the outer layer is attached to the wall of the chest cavity. The two layers are separated by a thin space called the pleural cavity that is filled with pleural fluid; this allows the inner and outer layers to slide over each other, and prevents them from being separated easily.

Breathing is largely driven by the diaphragm below, a muscle that by contracting expands the cavity in which the lung is enclosed. The rib cage itself is also able to expand and contract to some degree.

As a result, air is sucked into and pushed out of the lungs through the trachea and the bronchial tubes or bronchi; these branch out and end in alveoli which are tiny sacs surrounded by capillaries filled with blood. Here oxygen from the air diffuses into the blood, where it is carried by hemoglobin.

The deoxygenated blood from the heart reaches the lungs via the pulmonary artery and, after having been oxygenated, returns via the pulmonary veins.

Human lungs

Frontal view of lungs cut open
Trachea branches into bronchi
Humans have two lungs, with the left being divided into 2 lobes and the right into 3 lobes. These contain approximately 1500 miles of airways and 300 million alveoli, having a total surface area of about 140m2 in adults (roughly the same area as a
tennis court).

The total lung capacity depends on the person's age, weight, sex and the degree of physical activity - it ranges between ca. 4,000-6,000 cm3. For example, females tend to have a 20-25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. Heavy smokers have a drastically lower capacity than nonsmokers. Lung capacity is also affected by altitude. A person who is born and lives at sea level will have a smaller lung capacity than a person who spends their life at a high altitude. In addition to the total lung capacity, one also measures the tidal volume, the volume breathed in with an average breath, about 500 cm3. For a detailed discussion of the various lung volumes, see the article on lung volumes.

Typical adult resting breathing pattern has a breath rate of 10-20 breaths per minute with 1/3 of the breath time in inspiration.

The following is a list of important medical conditions involving the lung. Many of these are caused or worsened by smoking.

It is now also possible to transplant lungss, even together with the heart.

Avian lungs

Birds have a significantly different structure to their lungs than mammals do. In addition to the lungs themselves, birds have posterior and anterior air sacs which are involved in controlling the air flow through the lungs. They have a flow through respiration system.

When a bird inhales air flows in through the trachea to the posterior air sac, and the air currently contained within the lungs flows into the anterior air sac. When the bird exhales, the fresh air now contained within the posterior air sac is driven into the lungs and the stale air now contained within the anterior air sacs is expelled through the trachea into the atmosphere. Two complete cycles of inhalation and exhalation are required for one "breath" of air to make its trip through the avian respiratory system. Avian lungs do not have alveoli like mammalian lungs do, instead consisting of millions of tiny tubes running the length of the lung.

The purpose of this complex system of air sacs is to ensure that the airflow through the avian lung is always travelling in the same direction - posterior to anterior. This is unlike the mammalian system, in which the direction of airflow in the lung reverses between inhalation and exhalation. By having the airflow moving in a single direction, avian lungs are able to employ a system of countercurrent exchange to extract oxygen much more efficiently from the air.

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.

Evolutionary origins

The lungs of vertebrates are closely related (i.e. homologous) to the swim bladders of fish (but not to their gills). The evolutionary origin of both are thought to be outpocketings of the upper intestines. This is reflected by the fact that the lungs of a fetus also develop from an outpocketing of the upper intestines (see ontogeny and phylogeny). The article on swim bladders contains further details about the evolutionary origin of these two organs.

Arachnid lungs

Spiders have structures called "book lungs", which are not evolutionarily related to vertebrate lungs but serve a similar respiratory purpose.

See also

External links

Also, lung (龍) is the generic Chinese name for a dragon.