The countercurrent principle.

Countercurrent flow is when two things are flowing in the opposite direction to increase the exchange of products between them.

This in the gas exchange system of a fish- blood is flowing the opposite way to water so that more CO2 will leave the blood and enter the water, and more O will leave the water and enter the blood.

If blood and water were flowing in the same direction then at the beginning (nearest the mouth) there would be a lot more O in the water than in the blood so O would move into the blood. By the end of the gill, the O level in the blood would have risen, but that in the water has been lowered so there is a small, or non existent, concentration gradient. In this case O would only be exchanged at the start of the gills and along the rest of the length, diffusion would not take place.

If the blood is running in the opposite direction to the water, then at the beginning of the gill the water would have a high O concentration and the blood would also have a high O concentration (because it has already been past water on its way along the gill) but it would still have a lower concentration than the water because the waters concentration is so high when it has just entered so diffusion would occur. At the end of the gills when the O in the water had depleted, the blood has a very low oxygen content (as it has just been round the body) so there would still be diffusion. In this way the gills can extract O from water along the full length of the gills.

lillianwaller

The diagram shows how even though the O concentration in the water goes down, it is still higher than the blood at all times so diffusion occurs.


This video is helpful:
https://www.youtube.com/watch?v=cVFqME-NW9s