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)