How the countercurrent flow mechanism works in fish

How the countercurrent flow mechanism works in fish respiration

Gill filaments are supported by gill arches. Each of them is covered with folds of tissue called lamella. The lamellas form a curtain through which water passes as it moves from the buccal cavity to the opercular cavity. Gills will function efficiently only if water is kept moving across them in the same direction, from anterior to posterior. So the lamellas are occasions for gas exchange.

Blood flows across each lamella within a dense network of capillaries. In both bony fish and cartilaginous fish, the branchial artery flows from the inside of filaments to the outside. Water flows from the outside of lamella to the inside.

The direction of water flows across the lamella is opposite to the blood, which is called countercurrent flow. The blood can absorb dissolved oxygen as much as 85 percent. But if they are the same direction, the dissolved oxygen is only a fifth of the above.

The same direction In the beginning, there was a big difference between a high concentration of oxygen in water and a low concentration in blood. Oxygen is exchanged in lamella. The concentration of oxygen in water decreases gradually, and the concentration of oxygen in blood rises gradually, until the concentration of oxygen in water and blood is equal. At this point, the blood no longer gets oxygen from the water.

The opposite directions In this case, the blood flows in the opposite direction to the water. Although the blood has a high concentration of oxygen, it can still absorb oxygen from the water.

The countercurrent flow of water and blood at the exchange surface of the secondary lamellae ensures that the partial pressure of oxygen in the water always exceeds that of the blood, thereby maximizing the efficiency of oxygen diffusion into the blood.

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