Fish breathe underwater through their gills because they allow them to extract oxygen from the water and release the carbon dioxide produced by their respiration.
Fish breathe underwater thanks to their gills, small organs typically located behind the head. The gills function like a kind of oxygen filter: they capture the water, which contains dissolved oxygen, and circulate it over a very thin surface, richly supplied with many tiny blood vessels. It is here that oxygen easily passes from the water into the fish's blood. In exchange, the fish directly releases carbon dioxide into the water. This system allows fish to breathe peacefully underwater without ever needing to surface for a breath of air.
The gills of fish work somewhat like our lungs, but underwater. When water flows over the gill filaments, filled with tiny blood vessels, the dissolved oxygen diffuses directly into the fish's blood. At the same time, the carbon dioxide present in the fish's blood travels in the opposite direction to join the water and be expelled. All of this occurs thanks to a natural phenomenon called diffusion, where gases simply move from an area of high concentration to an area of lower concentration. Fish optimize this gas transfer through the counter-current mechanism: blood flows in one direction while water flows in the opposite direction, thereby maximizing oxygen absorption. Simple and effective!
Fish use their gills to extract dissolved oxygen from the water, but in the open air, the gills stick together and lose all their effectiveness. In fact, wet gills provide a large surface area for gas exchange in water, a kind of high-performance sponge. But out of the water, this thin wet layer dries quickly, and the gills collapse, becoming unable to absorb oxygen from the air effectively. As a result, they suffocate very quickly. This can be compared to a dry sponge that can no longer clean effectively: without water, the gills simply no longer work.
Water is essential for fish as it keeps their gills moist and open. In the open air, the gill filaments stick together and lose their exchange surface, preventing oxygen from being properly captured. Furthermore, it is thanks to the continuous flow of water that dissolved oxygen easily crosses the very thin gill membranes, ensuring effective respiration. Without water, these delicate membranes dry out quickly and become unable to ensure the gas exchange necessary for the fish's survival.
Cold water contains more dissolved oxygen than warm water, which is why some species of fish are more active and abundant in cool or cold waters.
Sharks are required to swim constantly to ensure a continuous flow of water through their gills, as many of them do not have a mechanism that allows them to perform this function while at rest.
Goldfish can survive brief periods without oxygen thanks to a special mechanism that allows them to temporarily produce energy under anaerobic conditions (absence of oxygen).
Some species of fish, such as catfish or eels, have complementary respiratory organs that allow them to breathe temporarily out of water, especially in challenging conditions or temporary ponds.
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