Cold feet Brrrrr. It can make you shiver to watch ducks or other waterfowl in the winter as they swim in icy water or waddle across a frozen pond. If we humans tried this with bare feet, it wouldn’t be long before we were chilled through and through, and our body temperature would drop dangerously low. The fact that the same thing doesn’t happen to ducks, which usually have an even higher body temperature than we do, is not due to their warming plumage alone. To curb temperature loss through their feet, nature has furnished these birds with a very special mechanism. The blood vessels that transport warm blood from the body to the extremities nestle very closely against those that lead back into the body. So, the cold blood from the feet flows right past the warm blood. It’s like having a heater for the cold blood. That’s why, in winter, ducks don’t have to expend much energy warming up the cold blood from their feet inside the body. The principle they are unconsciously using is called countercurrent exchange.
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