Your brain is an energy hog. It weighs less than 2 percent of your total weight yet consumes one fifth of your body's energy. The brain draws its fuel—oxygen and glucose—from blood delivered by a whopping 400 miles of blood vessels. Lined up end to end, all that vasculature would extend from New York City to Montreal.
These blood vessels are astonishingly dynamic. They tune the flow of blood to respond to the brain's needs from moment to moment. When certain brain areas work hard at something, more blood flows to those regions to help them refuel. Vessels do this by dilating near the spots that need a supply boost. This widening coaxes blood to reroute, much as customers in a busy store redistribute themselves whenever a new checkout line opens.