Fish extract dissolved oxygen molecules from the surrounding water. The oxygen content of water is much lower compared to air, so fish have special adaptations which enable them to make the most of the available oxygen. These adaptations are gills.
1. Gill filaments have lamellae which increase the surface area available for diffusion, while keeping the diffusion pathway short.
2. The water flow through the fish’s mouth as well as the blood in gill capillaries follow the countercurrent principle. As seen in the above diagram, water and blood flow against each other, rather than along each other. This ensures that oxygen diffusion can take place along the whole length of the flow, not just for half of it – before the concentration of oxygen in the blood and in the water becomes equal.
This is easily exemplified (and an acceptable form of explanation in an exam) by a number table. The upper row is the oxygen concentration in the blood, while the lower is the one in the water. Along the flow, oxygen enters the bloodstream from the water, so that the concentration in blood increases, while the concentration in water decreases.
Here’s a video which explains nicely how fish carry…