Scott McCandless, age ll, of Gladstone, mo., for is question:
How do the gills of a fish work?
The breathing process is basically the same in both fish and land dwellers. But one all important factor makes the difference between life and death. We can take our vital oxygen from the air and the fish cannot. We cannot take our oxygen from the water and the fish can.
A true fish has a pair of gills, one on each side of his head. Each is covered by a saucer shaped gill case where you would expect to find an ear. The front rim of the gill case is fixed to the head, and the opposite side of the rim opens and closes like a flap. The stiff gill case shelters and protects the true gill which is a mass of fringed tissue.
As we breathe, we take in oxygen from the air and return waste carbon dioxide. As a fish breathes, he takes in oxygen from the water and returns waste carbon dioxide. Our air fills countless small pockets in our spongy lungs, and these air pores are lined with the thinnest of thin walls. On one side of these walls is air, on the other side are tiny blood vessels. The busy red blood cells grab the oxygen they need and return their waste carbon dioxide right. Through the thin walls of the air pockets.
The fringes of a fish's gills also are lined with thinner than thin walls. Gills also have networks of fine blood vessels. And the busy red blood cells of the fish grab oxygen from the water and return waste carbon dioxide through the thin walls of the gills. The basic process is the same for lung breathers and gill breathers.
However, lung breathers can use only gaseous oxygen from the air, and the gill breathing fish must have his oxygen dissolved in water. If you watch, he seems to be swallowing gulps of water. This is the first step in his breathing process. The water is sent through two openings into the fringed gills, and the dissolved oxygen is absorbed through their thin walls. Waste carbon dioxide is returned and flows out with the waste water through the gill flaps.
Water is made from hydrogen and oxygen, but fish cannot breathe the oxygen in water molecules. They must have a supply of free oxygen dissolved in water, and the world's water contains enough free oxygen to supply an abundant fish population.
Seaweeds add oxygen to the water, just as land plants add oxygen to the air. W8ves and tossing breakers also gather oxygen when they splash up into the air. Rapids and waterfalls mingle and dissolve oxygen from the air. Stagnant waters, as a rule, have less free oxygen. This is why fish tend to be more plentiful in turbulent than in stagnant water.