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 system where two fluids flowing in opposite directions pass close to each other, allowing for efficient transfer of heat or substances. Let me give you two examples. First, consider the Arctic wolf. In freezing temperatures, its legs are often exposed to the cold, but the blood flowing down to the paws and the blood returning to the heart run side by side in tiny vessels. As warm blood moves down, it passes heat to the colder blood coming up, so the paws stay just warm enough to function, while the heart remains warm. Second, think of a tuna fish. To swim fast, its muscles need to be warm. The tuna's muscles produce heat, and this heat is conserved through countercurrent exchange in its blood vessels. Warm blood from the muscles passes its heat to the cooler blood coming from the gills, raising the muscle temperature above that of the surrounding water. This allows the tuna to be a powerful predator even in cold ocean waters.