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WHY DOES FISH BREATH IN WATER? WHY CAN'T THEY BREATH IN LAND?
There are many species of fish and other aquatic animals that obtain oxygen from water by use of a system of external gills. Gills are highly-branched networks of capillaries that bring the blood close to the oxygenated water. The capillaries have an enormous surface area, but at the same time a very thin wall. This is to minimize the distance between the red blood cells inside the capillaries and water itself. This design is adapted to increasing the rate of diffusion of the gas. Diffusion rate is proportional to the surface area of the capillary bed and inversely proportional to the distance it has to diffuse. Large, thin walled gills are ideal for this, but they are also fragile, and will collapse when placed in air. In water, the fragile gills are buoyant and while the capillaries are filled with blood, they are supported. In air, the gills collapse under their own mass, and the capillaries shut down, so the fish starves for oxygen even though it is immersed in oxygen-rich air. A fish in air will survive for a long time if a tube with running oxygenated water is placed into the mouth and water flows out through the gills, at least as long as the skin remains moist.

Because external gills are fragile, most fish protect them with a bony covering called the operculum. Water enters the mouth, passes over the gills, and leaves through the gill opening. Muscles open and close the operculum and by timing the opening of the mouth in alternation with the opening of the gills, the fish pumps the water past the gills. When water flows in the mouth and out the gill openings, it runs counter to the flow of blood. Countercurrent flow tends to maximize the exchange of gasses by diffusion. If the direction of flow of water were the same as the direction of the flow of blood, there would be less efficient exchange

External gills without covers do occur in some fish and some amphibians, especially in the larval stages. The lack of gill coverings may be needed when oxygen demands are extremely high..


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