Countercurrent Heat Exchange

In mammals and birds, warm blood flows toward an animal’s feet through arteries, and colder blood returns to the body through veins. The veins and arteries in a bird’s featherless legs are close to each other. Before the warm arterial blood gets to the birds’ feet, the veins absorb much of their heat and return it to the body. This exchange of heat keeps a bird’s feet colder, often just above freezing. Birds have little muscle or nerve tissue in their exposed legs and feet, so there is no need to waste energy keeping them warm.