In the countercurrent exchange system found in fish, how does the blood flow?

In a countercurrent exchange system, the blood flows in opposite directions to the oxygen source, which in the case of fish, is the water passing over their gills. This arrangement enhances the efficiency of oxygen uptake. As oxygen-rich water flows over the gill membranes, the blood in the gill capillaries is moving in the opposite direction. This creates a concentration gradient that allows oxygen to continuously diffuse from the water into the blood, even as the blood becomes increasingly saturated with oxygen.

This counterflow mechanism ensures that there is always a higher concentration of oxygen in the water relative to the blood, maximizing the amount of oxygen that can be absorbed as water passes through the gills. It is an effective adaptation for aquatic respiration, particularly in environments where dissolved oxygen levels may fluctuate. The other flow directions proposed, such as the same direction or circular patterns, would not facilitate this efficient transfer of gases.