The blood-brain barrier is a set of cells lining the blood vessels of the brain and nerves that prevent certain molecules that would otherwise be able to penetrate cells in the body from passing from the blood to the cerebro-spinal fluid. These cells prevent the passage of microscopic objects like bacteria, as well as large molecules and molecules that attach easily to water. However, it freely allows the passage of molecules that do not attach easily to water (such as oxygen, carbon dioxide and hormones) and also actively allows the passage of glucose and some essential proteins.