Gills work in a way similar to lungs and extract oxygen from water to allow the fishes to breath. The fish pulls in water through the mouth and pushes it out through the gills, which absorb oxygen from the water and exchange it for carbon dioxide, which is expelled from the body. Gills allow the fishes to breath underwater.
Fishes are known for their ability to breath under water and this is only possible because of an organ known as gills. What are gills actually? Well, gills are tissues that are made of filaments. The filaments have small rows and columns of specialized cells that are grouped together, known as the epithelium. The filaments are responsible for the exchange of oxygen, carbon dioxide, acids and ammonia.
Gills work in a similar manner to lungs. Lungs have small sacs (known as alveoli) that are approximately 70% made of capillaries. These capillaries are responsible for carry deoxygenated blood from the body and taking oxygenated blood back to the body. Gills perform the same function. The fish intakes water through the mouth and pushes it out of the gills, the capillaries take the oxygen from the water and replaces it with carbon dioxide that is then pushed out of the body.
Most fishes have four gills on each side. They have a tree like structure, where there is a main bark, which has smaller branches attached to it and that branch as even smaller branches. This arrangement of cells allows for a very large surface area when the gills are immersed in water. The large surface area is important as it allows the fishes to absorb more oxygen from the water. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
In air, 21% of the air is oxygen, which roughly translates to about 210,000 parts per million. However, water only has about 4-8 parts per million of dissolved oxygen, which reduces the amount the oxygen that the fishes can absorb. Since fishes are cold blooded creatures, they tend to have a lower metabolism, which helps them survive on lower available oxygen. On the other hand, warm blooded creatures require more oxygen to survive and if fishes were warm blooded, they would have drowned.
Gills are only functional in water and not above it because of the density of water that prevents the lungs from collapsing and lying on top of each other. When the fish are taken out of water, the air pushes the gills close together, resulting in the fish to suffocate. The gills also have chloride cells that limits the intake of sodium from the water and removes excess sodium from the fishes’ body.
Higher vertebrates do not develop gills, but they do have four slits (known as gill arches), these form during fetal development, and later form into essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus.