Blood Brain Barrier
The Blood Brain Barrier separates blood that circulates throughout the body from brain fluid in the central nervous system. Its most important function is to prevent such things as infections and toxins from entering the brain via the bloodstream. It is formed by blood capillaries (the smallest blood vessels) in the brain that very tightly packed together (and thus, have tighter junctions) than other capillaries in the body which strictly control what can pass through into the brain fluid.
Like the capillaries throughout your body the brain capillaries branch out like a tree delivering nutrients and taking waste away from the cells in your brain. The capillaries are surrounded by tight junctions that close off the vessels from the brain fluid. The junctions are very picky when it comes to what may pass through the barrier into the brain. Water, various gases, and some lipid soluble molecules move through the barrier naturally by a process known as diffusion. Diffusion occurs when something moves from an area of high concentration to low concentration. The barrier also actively moves amino acids and glucose that the brain cells need for nutrition. The blood brain barrier also helps maintain a constant environment for the brain so that it can properly function at all times.