Final answer:
Countercurrent exchange in fish gills is effective due to the continuous maintenance of a concentration gradient of gases across the entire gill surface, which is achieved by the countercurrent flow of water and blood. The large surface area provided by the folded gill surfaces facilitates maximum diffusion of oxygen into the blood and carbon dioxide out of it.
Step-by-step explanation:
Countercurrent exchange is a highly efficient respiratory mechanism in fish that maximizes the diffusion of gases across the gill membranes. As water flows over the gills, it moves in the opposite direction to blood flow within the gill vessels. This countercurrent flow ensures that the difference in oxygen concentration between the water and blood is maintained across the entire length of the gill, allowing for gas exchange at every point. This system is effective because oxygen-rich water is always flowing past oxygen-depleted blood, maximizing oxygen uptake and carbon dioxide release.
The folded surfaces of the gills provide a large surface area, enhancing the process. While in the water, oxygen dissolves at a much lower concentration compared to the atmosphere. Despite this, the efficient design of fish gills allows them to extract enough oxygen to survive. This process does not require energetic movement by the fish, as the gills are capable of extracting oxygen from the water even if the fish remains relatively still. Ultimately, this contributes to the overall energetic efficiency of aquatic organisms.