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.