How the Brain is Protected

The brain is protected by a series of bone, membrane, and fluid. The outermost protection is the cranium, or skull, which protects the brain from blows that could easily damage it. Then there are three membranes called meninges. The first of the three, dura mater, lines the inside of the cranium. The thinner arachnoid membrane lies just beneath this. Finally, there is the pia mater that covers the brain directly. It follows the folds of the brain and contains blood vessels to carry blood to and from the cerebral cortex. Separating the arachnoid and pia mater is a clear liquid called cerebrospinal fluid. This forms a cushion between the soft brain tissue and the hard cranial bones. This same fluid also fills four ventricles (spaces) inside the brain. Tissues inside the ventricles produce cerebrospinal fluid, which flow through the spaces between the pia mater and arachnoid, and is absorbed into the blood by small vessels in the arachnoid.

The blood-brain barrier protects brain tissue from contact with large molecules in the bloodstream, since this could cause damage. Subtances in blood reach the body tissues by passing through the thin walls of tiny blood vessels called capillaries, mostly passing through the spaces between the cells making up the wall. However, in capillaries located in the brain, the cells are packed together more closely and have enzymes that allow only certain molecules to pass through. These capillaries form the blood-brain barrier.