lung

Your lungs are not just empty bags, but filled full with the tiny convolutions of the tubules bronchioles and alveoli. In fact, the lungs resemble thick sponges more than anything else. Why?

One of the big principles in the structure and function of biological organisms is the surface area to volume ratio. Any organism has chemical needs (food, oxygen, water, etc.) as well as a means of interfacing with the outside world to get at those needs. An organism is limited in size by the rate at which it can conduct this interface. In the case bacteria, it is the plasma membrane, which is permeable to water, certain gasses and certain ions and has the ability to take up nutrients and expel waste. Thus, a bacterium needs a certain amount of plasma membrane to "support" the needs of the volume it contains.

The problem with this setup is that surface area doesn't grow in proportion to volume. If you took a happy spherical bacterium and doubled its diameter, you would find that while the surface area increases by a factor of 4, the volume increases by a factor of 8. This is why there aren't 80kg bacteria: the surface area would never be able to conduct the chemical business required by the volume it contains.

But how is it that I (80kg) can respirate, exchange ions and gather nutrients if my ostensible surface area is so small?

My system of interface is the lungs, which because of their structure have a surface area far larger than might be expected. In fact, the average set of lungs has the surface area of a tennis court. The intestines, too, are highly convoluted at the microscopic level to allow nutrients to absorb into the blood at maximum rate. These adaptations are what allow large organisms to survive, and, in the case of we mammals, are what allow us to achieve a small enough surface are to volume ratio to be successfully endothermic.

An organ found in air-breathing vertebrate animals which functions in air respiration. It transfers atmospheric oxygen from the air to the blood, and carbon dioxide and other waste gases from the blood to the air.

Most animals have a pair; one species of lungfish has just one, and other primitive fish may have lunglike air bladders (the swim bladder, found in many bony fishes, evolved from the lung).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

Lung (?), n. [OE. lunge, AS. lunge, pl. lungen; akin to D. long, G. lunge, Icel. & Sw. lunga, Dan. lunge, all prob. from the root of E. light. See Light not heavy.] Anat.

An organ for aerial respiration; -- commonly in the plural.

My lungs began to crow like chanticleer. Shak.

⇒ In all air-breathing vertebrates the lungs are developed from the ventral wall of the esophagus as a pouch which divides into two sacs. In amphibians and many reptiles the lungs retain very nearly this primitive saclike character, but in the higher forms the connection with the esophagus becomes elongated into the windpipe and the inner walls of the sacs become more and more divided, until, in the mammals, the air spaces become minutely divided into tubes ending in small air cells, in the walls of which the blood circulates in a fine network of capillaries. In mammals the lungs are more or less divided into lobes, and each lung occupies a separate cavity in the thorax. See Respiration.

Lung fever Med., pneumonia. -- Lung flower Bot., a species of gentian (G. Pneumonanthe). -- Lung lichen Bot., tree lungwort. See under Lungwort. Lung sac Zool., one of the breathing organs of spiders and snails.

© Webster 1913.