|MadSci Network: General Biology|
How do fish process oxygen from the water?
Most fish obtain oxygen from the water using a very efficient system: gills. Compared with air, water contains relatively little oxygen. A given volume of water contains only few percents of the oxygen in the same volume of air! Moreover in the gills, oxygen crosses cell surface from the water to the blood only by a passive phenomenon called diffusion. As a consequence, the gills must have a very large area. In some active fish species, the gill area is over 18 times that of the body surface are of the fish! Also because water is dense and viscous, a lot of energy is needed to force water to flow around gills. Thus, unlike most terrestrial animals which breathe air in a tidal fashion (in and out), water flow over the fish gill is almost always unidirectional. The advantage is that in the gill, blood flow is arranged in the direction opposite to water flow. This way, blood leaving the gill has the same relative quantity of oxygen as that in the incoming water.
Schematically, this gas exchanger consists of branchial arches each bearing a series of regular comb-like gill filaments. Each of these filaments has many smaller structures called lamellae that themselves give rise to smaller lamellae running parallel to the flow of water. In very active fish like tuna there may be more than 5 million of these very small lamellae! These lamellae have thin walls through which oxygen passes from the water to the blood. But fish can also obtain oxygen from the water with other systems. For example, most fish hatch at a very small size. These very small individuals, called larvae, do not have functional gills and depend on oxygen diffusing across the body surface.
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