Fish gills use a design called 'countercurrent oxygen exchange' to maximize the amount of oxygen that their blood can pick up. They achieve this by maximizing the amount of time their blood is exposed to water that has a higher oxygen level, even as the blood takes on more oxygen.

What is the countercurrent exchange system and how does it work?

In countercurrent flow, the two flows move in opposite directions. Two tubes have a liquid flowing in opposite directions, transferring a property from one tube to the other. ... The counter-current exchange system can maintain a nearly constant gradient between the two flows over their entire length of contact.

What is countercurrent exchange in biology?

A biological mechanism in which there is an exchange of a substance between fluids flowing in opposite directions. Engineers have known that efficient and almost complete heat or other exchange could be achieved between two fluids flowing in opposite directions in separate tubes.