Lung

The lung is an organ of the respiratory system of air-breathing vertebrates. Its function is to exchange carbon dioxide from blood with oxygen from air. The process in which this happens is called "external respiration" or breathing ("internal respiration" is the use of oxygen in the internal processes of cells).

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.

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, enclosed by a double-walled sac and protected by the bony structure of the rib cage. The inner layer of the sac adheres tightly to 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 filled with fluid called the pleural cavity; this allows the inner and outer layers to slide over each other, but prevents them from being separated easily. When one or both layers of this sac are penetrated by injury, a collapsed lung[?] can result.

Breathing is largely driven by the diaphragm below, that is a muscle that expands and contracts 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 bronchial tubes[?] that end in alveoli, which are tiny sacs surrounded by capillaries filled with blood. Oxygen is carried in blood by hemoglobin.

Human lungs

Humans have two lungs. 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 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 seriously affected by diseases such as asthma, emphysema, congestive heart failure, and respiratory infections[?] such as pneumonia and bronchitis[?].

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. This is because there is less oxygen in the air at altitude, so the lungs gradually expand to process more air. When someone from sea level travels up to the higher parts of the earth (eg. the Andes, Mexico City, Tibet and the Himalayas) they will often develop a condition called altitude sickness because their lungs cannot process enough oxygen for their bodies needs. Some famous athletes (examples anyone?) have come from regions of very high altitude, and their enlarged lungs have contributed to their success.

Tidal volume is the amount of air taken into the lungs in a single breath. In the average adult, tidal volume is about 500 cm³.

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

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.

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.

Arachnid lungs

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

Sources:

• Dr D.R. Johnson: Introductory anatomy, respiratory system (http://www.leeds.ac.uk/chb/lectures/anatomy7)
• Franlink Institute Online: The Respiratory System (http://sln.fi.edu/biosci/systems/respiration)

Also, Lung (龍) is the generic Chinese name for a dragon. In Chinese mythology there were four types: the celetial dragon Tien-Lung[?], the treasure dragon Futs-Lung, earth dragon Ti-Lung[?] and rain dragon Shen-Lung[?].