Describe countercurrent exchange in the gills of fish. How does it increase the effectiveness of gills?
Correct Answer:
Countercurrent exchange in the gills of fish is a system where water flows over the gills in the opposite direction to the flow of blood within the gill capillaries. This arrangement maximizes the diffusion of oxygen from water into the blood because it maintains a concentration gradient along the entire length of the gill.
Explanation:
Countercurrent exchange is a vital adaptation in fish that enhances the efficiency of gas exchange in their gills. In this system, water containing dissolved oxygen flows over the gill filaments in one direction, while blood flows through the capillaries in the opposite direction. This setup ensures that the oxygen concentration in the water is always slightly higher than that in the blood across the entire surface of the gill.
The key to understanding how countercurrent exchange improves efficiency lies in the continuous concentration gradient between water and blood. As water enters the gills, it has a high oxygen concentration. At the same point, the blood flowing in the opposite direction has a lower oxygen concentration. As both fluids move in opposite directions, oxygen continuously diffuses from the water (higher concentration) into the blood (lower concentration). Because this gradient is maintained along the entire length of the gill capillaries, fish can extract up to 80 to 90 percent of the oxygen from water.
If the blood and water flowed in the same direction (a concurrent system), the gradient would rapidly disappear, and oxygen transfer would stop once equilibrium is reached. In such a case, only about 50 percent of the oxygen could be absorbed.
The countercurrent exchange system is therefore crucial for fish, especially because water holds much less oxygen than air. By optimizing the amount of oxygen that enters the bloodstream, fish can meet their metabolic needs efficiently even in low-oxygen environments. This system exemplifies how structural adaptations support physiological functions in aquatic organisms.
