Wading birds in northern climates are able to walk in pools of water on ice without dying of hypothermia because they have countercurrent heat exchangers in their legs. These exchangers allow heat in warm blood going to the feet to be transferred to cool blood returning from the feet. Why are such countercurrent heat exchangers so efficient at conserving heat?
A) The gradient between the two currents is maintained even as one current cools and the other current heats.
B) There is a large temperature differential from one end of each current to the other.
C) Blood in the arteries and veins flow in the same direction, so there is no loss to friction.
D) All of the above apply.
E) A and B only apply.

Question

Quizplus · Accepted Answer

Countercurrent heat exchange is efficient at conserving heat because it maintains a constant gradient between the two currents, even as one current cools and the other heats, and there is a large temperature differential from one end of each current to the other. However, blood in the arteries and veins do flow in opposite directions, which increases the efficiency of heat transfer but also creates frictional losses, so choice C is not correct.

Wading Birds in Northern Climates Are Able to Walk in Pools