If you carefully observe the goldfish in the fish tank, you will find that the fish are always drinking water, sometimes even gulping. Maybe you'll worry: won't fish break their belly after drinking so much water? Of course, there is no need to worry. The fish are safe and still swimming freely. So why do fish keep drinking water?
In fact, what we see is not that the fish is "drinking water", but that it is breathing. Fish can't live without water, just as we can't live without air. But why don't we "drink water" like fish? That's because our living environment is full of air. We can easily breathe the oxygen in the air to meet the needs of our body. The fish is different. There is not so much oxygen in the water. The oxygen that can supply the fish to breathe is dissolved in the water. Therefore, if the fish wants to breathe, it can only rely on constantly "drinking water". Of course, this is also due to the special organ of fish - gills.
The gills of fish are like human lungs and respiratory organs. The gills of most fish are located on both sides of the body and behind the eyes. The gills of fish are mainly composed of gill filaments, gill rakes and gill arches. Gill filaments are like the teeth of a comb. There are many gaps in the middle, and each gill filament is covered with gill slices, which contain many capillaries. When water flows through the gill filaments, oxygen is absorbed through the capillaries on the gill slices, and then supplied to the needs of the fish body through blood circulation. The water is filtered by the gill filaments and returned to the water body where the fish lives.
Gills are like a sieve, leaving useful things and filtering out unused things. This is the whole breathing process of fish: first "drink" a lot of water from the mouth, and then let the water flow out of the gills. When the water flows through the gill filaments, the oxygen dissolved in the water is absorbed by the capillaries, and the water is discharged. It can be seen that although the surface that fish "drink" into is water, it is actually oxygen.
In addition, there is a great feature of fish gills, that is, when in the water, each gill filament and gill piece are fully opened to expand the contact area with the water body as much as possible, which increases the opportunity for fish to absorb oxygen from the water. Therefore, although the dissolved oxygen content in water is relatively low, fish can still obtain enough oxygen for themselves.
However, the gills of fish can only absorb oxygen in the water, so fish can't live without water. This is why fish should "drink water" and must keep "drinking". Of course, in this process, it is not difficult for us to find that from the beginning to the end, the water is not drunk into the stomach by the fish. Therefore, although the fish drinks a lot of water, its stomach is not broken.