02-22-2009, 05:52 PM
They are different in structure and also in the way that gas exchange occurs.In gills, the water flows in a single direction, and the blood flow in the blood vessels occurs in the opposite direction. Thus optimal gas exchange is achieved (there are good illustrations in textbooks to understand this better). However, to take up oxygen effectively, gills need an enormous surface area, because oxygen is relatively scarce in water. Thus, the gill lamellae are subdivided into smaller lamellae, and these in turn into even smaller projections; these extremely thin laminas are supported in water, but collapse onto each other in air, because of surface tension and gravity.Lungs can be of 2 types: the basic model comprises paired sacs, with more or less elaborate inner walls for greater surface area, but also with a surfactant substance that decreases surface tension so that these sacs do not collapse. The air goes in and out of the lungs by way of a single duct, thus there isn't a continuous flow of oxygenated air. Less efficient, but air has so much oxygen that it works anyway. The exception are the lungs of birds, in which the blood vessels are at right angles with respect to air flow, and the latter is continuous and unidirectional.All respiratory surfaces need to be moist, and lungs are no exception. However, in this case the moisture is _inside_ the body, and the body surface is still mostly impermeable.Third choice... that would be perhaps the tracheae of insects. Aquatic arthropods have lungs of different types, and terrestrial chelicerates do have special gills too. But insects have a different system: ducts that pierce the body wall and project inwards, taking oxygenated air directly to the tissues (no participation of the circulatory system).
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