As we said before, fish force water through their gills in order to extract as must oxygen from the water as possible.

The reason they don’t inhale and exhale water; and instead allow a unidirectional flow of water over the gills is due to energy.

It costs more energy to inhale and exhale water than air, as water is more dense and viscous than air.

The animation to the left shows the flow of water into the mouth and out the gills.

The image to the right are the gills of a fish.

Notice how they are not simply a solid structure but divided into fine filaments. This increases the surface area to volume ratio. One forms a gill plate.

There are multiple gill plates stacked on top of each other.

Counter current gas exchange is the bases of how gills work.

Similar to the alveoli in humans, capillaries carry red blood cells close to the surface, releasing carbon dioxide out into the water and absorbing oxygen.

This is because there high concentration of oxygen and low concentration of  carbon dioxide in the water.

(Observed in the animation bellow)

Counter current gas exchange

Counter current gas exchange is where the water flows in the opposite direction to where the blood flows. This is seen in the animation above.

The reason for this, is so the maximum amount of diffusion can occur.

Click here for questions on the fish respiratory system