The endothelium acts as a barrier between blood and rest of the body tissue. It is selectively permeable for certain chemicals and white blood cells to move across from blood to tissue or for waste and carbon-dioxide to move from tissue to blood.
The blood-brain barrier (BBB) is formed by microvascular endothelial cells, pericytes and astrocytes. It prevents the entry of most large hydrophilic molecules and many potentially harmful toxins from the blood into the brain. On the other hand, it also prevents the entry of many therapeutic agents into the brain making it difficult to develop drugs that can efficiently cross the blood-brain-barrier. Considerable efforts are made to better mimic and understand the functions of blood brain barrier systems to increase the efficacy of drug development.