托福口语 Task 4

Today we're going to talk about a fascinating adaptation in the animal kingdom called 'countercurrent exchange.' This is a mechanism that allows animals to conserve heat or other substances by transferring them between fluids flowing in opposite directions. One classic example is seen in the legs of certain birds, like ducks, that stand on ice. Warm blood flows from the bird's body down to its feet through arteries, while cold blood returns from the feet to the body through veins. As these vessels run close together, heat from the warm arterial blood is transferred to the cold venous blood, warming it before it returns to the body. This means the bird's feet can be very cold, but its core stays warm, minimizing heat loss. Another example is in fish gills. As water flows over the gills, it moves in the opposite direction to the blood flow inside the gill capillaries. Oxygen from the water diffuses into the blood because the blood has a lower oxygen concentration. Since the water and blood flow in opposite directions, the oxygen gradient is maintained along the entire gill, allowing the fish to extract a much higher percentage of oxygen than if they flowed in the same direction. So countercurrent exchange is a highly efficient system for conserving vital resources in different environments.