The Countercurrent Exchange System

In the gill lamellae, blood flows in the opposite direction to the water flowing over them. This arrangement is called a countercurrent exchange system.As water passes along a lamella it loses oxygen. At the same time, blood entering the lamella from the other end starts with very little oxygen and picks it up as it travels. The result is that, at every point along the lamella, the blood meets water that has a higher oxygen concentration than the blood itself:• blood that has just entered (lowest oxygen) meets water that is about to leave, which has lost some oxygen but still has more than the blood
• blood that is about to leave (highest oxygen) meets water that has just entered, which is richest in oxygenA concentration gradient is maintained along the whole length of the lamella, so oxygen diffuses into the blood all the way along. The blood can leave with an oxygen concentration close to that of the incoming water, and about 80% of the oxygen in the water can be absorbed.Compare a parallel flow (concurrent) system, in which blood and water travel in the same direction. The gradient is steep at the start but the two quickly reach equilibrium, after which no net diffusion occurs. The blood can never contain more than about half of the oxygen, so only about 50% is extracted.Worked example: incoming water is 100% saturated and blood enters at 0%. With parallel flow they level off near 50%. With countercurrent flow the blood can leave at about 80%, because it always meets water that is richer in oxygen.