To carry blood throughout the brain, you will need pipes of all diameters and lengths. In this 1-millimeter-square view of the cerebral cortex of a living rat, large blood vessels along the surface lead to capillaries that extend deep into the brain.

To create this image, a fluorescent sugar molecule was injected into the rat's bloodstream; here the blood-filled vessels appear white. Neuroscientist Andy Shih of the University of California at San Diego uses this imaging technique to measure vessel diameters and track blood flow rates, which change constantly depending on the needs of the local neurons.

He has found that when blood flow is compromised--as by a stroke--nearby vessels pick up the slack, helping brain tissue recover.