การฟัง TOEFL

🎧 บทถอดเสียง

Narrator: Listen to part of a lecture in a biology class. Professor: Today we're going to talk about a fascinating adaptation in the animal kingdom—countercurrent exchange. This mechanism is crucial for animals living in extreme environments. For example, consider the arctic fish. In freezing waters, their blood would normally lose heat rapidly, but countercurrent exchange allows them to retain heat. Here's how it works: warm blood from the fish's core flows out to the gills through arteries, while cold blood from the gills returns through veins that run parallel but in opposite directions. As the warm arterial blood passes the cold venous blood, heat transfers to the venous blood, warming it before it reaches the core. Meanwhile, the arterial blood cools down, but it's already heading to the gills, where it will be re-warmed by the environment? Actually, no—the gills are the site of heat loss, but because the arterial blood is already cool, the temperature difference at the gills is minimized, reducing heat loss. This system is so efficient that it allows these fish to thrive in waters that would be lethal to most other species. Now, can anyone think of another example? Yes, in the legs of wading birds or even in the flippers of whales. The principle is the same: conserving body heat in cold conditions.