The walls of capillaries throughout the body and within the CNS are formed by endothelial cells, however, within the CNS the capillary endothelial cells are very tightly packed. This density of cells restricts the passage of substances out of the bloodstream much more than endothelial cells in capillaries elsewhere in the body. This is the basis of the [[Blood Brain Barrier - Anatomy & Physiology|blood brain barrier]]. | The walls of capillaries throughout the body and within the CNS are formed by endothelial cells, however, within the CNS the capillary endothelial cells are very tightly packed. This density of cells restricts the passage of substances out of the bloodstream much more than endothelial cells in capillaries elsewhere in the body. This is the basis of the [[Blood Brain Barrier - Anatomy & Physiology|blood brain barrier]]. |