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The countercurrent system maximizes the extraction of oxygen from the water and secretion of wastes. Organisms use countercurrent systems in different ways. [An Example] Later we will see how fishes use a countercurrent system to inflate their swim bladders, but in the gill, it is used to increase blood/water exchange.
Oxygen in water flowing over the gill will diffuse from an area of high pressure into the blood, an area of low pressure, until the oxygen content of both water and blood is equal. A countercurrent system places the water with the highest oxygen content in contact with the blood with the highest oxygen content and as blood and water flow through lamellae exchange occurs over the entire respiratory surface. This maintains the greatest pressure gradient possible over the entire blood/water interface maximizing the flow of oxygen into the blood.
A parallel (non-countercurrent system) puts the water with the highest oxygen pressure immediately into contact with the blood with the lowest oxygen pressure and does not maintain a continual gradient. A parallel system has a maximum transfer efficiency of 50%. If contact time is long enough, half the pressure difference between the incoming water and blood can be transferred, that is, the oxygen diffuses until the pressures meet in the middle. On the other hand, a countercurrent system can have a transfer efficiency of 80% or more, depending on contact time.
Skates and rays have gill openings on the underside of the body and have spiracles (2) on top of the head that bring water into the buccal cavity when the mouth is buried in the sand.
In halibut and flounder, water is taken in on the side of the mouth and channeled through the dorsal operculum.
Agnatha have gill tissue arranged in a series of radiating ridges inside expanded pouches inside the buccal cavity. This branchial basket is elastic and contractile. Contraction and expansion of the basket brings water in and out of either the mouth or the gill slits (lamprey) or pharyngeal cutaneous duct (hagfish) bidirectionally. They do not have the more efficient unidirectional flow of other fishes. [ha!]
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