Gills themselves have a car radiator-like appearance. Most fish have 4 gills on each side, consisting of a main bar-like structure that has numerous branches as that of a tree, and those branches consisting of even smaller branch-like structures.
This arrangement of cells allows for a very large surface area when the gills are immersed in water. Functionally, the mechanism for pumping water over the radiator-like gills seems to vary depending on the species of fish.
In general, this is achieved by the fish lowering the floor of the mouth and widening the outer skin flap that protects the gills, called the operculum. This increase in volume lowers the pressure within the mouth causing the water to rush in. The pressure is then increased compared to the outside of the mouth and the water is forced through the operculum opening and across the gills.
Gills themselves need a very large surface area to provide the fish with the necessary oxygen demands. Water, on the other hand, only has about parts per million of dissolved oxygen that the gills can extract. Cold blooded animals also tend to have a lower metabolism than their warm blooded counterparts. This aids them in their ability to handle environments of low available oxygen. Should the same size fish be warm blooded, the metabolism of the little swimmer would be increased to the point that the available oxygen would not be sufficient and little Nemo would perish.
While the large gill surface area allows for sufficient exchange of carbon dioxide and oxygen, it at the same time exposes the same large blood volume to the hypertonic that is, saltier than thou sea water, creating a situation in which fish must have a backup mechanism for expelling excess sodium that has been incidentally absorbed. Conversely, freshwater fish need to have an opposite mechanism allowing them to excrete excess water to keep their sodium levels appropriately high.
Never mind about those anadromous gypsies who trounce back and forth, able to thrive in both fresh and salt water environments. We will just call them show offs and leave it at that. To deal with this sodium problem, inside the gill resides nifty little cells called chloride cells. These cells allow for the extrusion of any unwanted sodium. Fishes also need oxygen, but rather than getting it from air, they have to get it from water.
But there is less oxygen available in water than air. And to make matters worse for the poor fishes, water is thicker than air, so it takes much more work to move it around. Rather than breathing in and out through the mouth, fish use a one-way system, passing water in one direction over their gills.
Water goes in the mouth, across the gills and out through the opercula the bony covering protecting their gills. But gills and lungs are more similar than you might think. Both have really big surface areas which increases the amount of water or air that touches the gill or lung tissue, and so increases the amount of oxygen available.
This means the capillaries come into close contact with the air or water outside, letting oxygen pass across the thin walls and into the blood. At the same time, carbon dioxide, which is a waste product from our bodies, passes out. Hello, curious kids!
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