* It may initially seem illogical for the heart to need special arteries feeding it. When the walls of the heart—the heart muscle—require the energy and oxygen stores in the blood, you might imagine that these could simply be absorbed from the vast amounts of blood passing through the chambers of the heart. But instead it has evolved that heart muscle is fed by arteries coursing from the main aorta. As an analogy, consider people working at a city’s water reservoir. Every time they get thirsty, they might go over to the edge of the reservoir with a bucket and pull up some water to drink. (Instead, the usual solution is to have a water fountain in the office, fed indirectly by that reservoir just outside.)