The brain abides in a world of liquid that both cushions and
nourishes it. Nearly a fifth of the blood pumped by the heart surges through
the brain's intricate network of blood vessels to meet its unflagging demand for
oxygen and glucose.Specialized blood vessels within the brain called choroid plexuses produce protective cerebrospinal fluid. Each of the brain's four cavities, the ventricles, contains a choroid plexus. The cerebrospinal fluid continuously washes over the brain and spinal cord, suspending these organs in a liquid cushion that protects them from injury.
The blood-brain barrier, another protective feature, consists of a network of uniquely structured blood vessels. These capillaries are nearly impermeable to many harmful chemicals carried by the blood, but do allow oxygen, water, and glucose to enter the brain. The cells of these capillaries are more tightly joined than cells of other blood vessels. The vessels themselves are wrapped twice - first by a layer called a basement membrane, then by the fatty extensions, or end feet, of special glial cells. (National Geographic)