Day 78: Hypothermia and frostbite are subjects with which most winter outdoor enthusiasts will be familiar, so the question arises: How do those little bitty birds survive? Why don't their feet freeze? The first part of the answer is relatively simple. A bird's body temperature is significantly higher (106 degrees) than that of a human. The second and most notable factor is that they are possessed of a "countercurrent" circulatory system in which the veins carrying cooled blood are intertwined with the arteries transporting warm blood from the body's interior. As much as 85 percent of the warmth from the outgoing blood transfers to that returning before it re-enters the body. This system of intertwined veins and arteries exists in both the legs and in the wings. Humans don't have this evolutionary advantage, so we must wear appropriate clothing to protects our extremities.
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