Once a drug has been absorbed into the blood, it is distributed around the body. This initially involves a journey round the blood supply, with subsequent distribution to the various tissues and organs fed by that blood supply. Although the drug is evenly distributed throughout the blood supply, this does not mean that the drug is evenly distributed around the body, since the blood supply is richer to some areas of the body than to others. Drugs that are excessively hydrophobic are often absorbed into fatty tissues and removed from the blood supply. The existence of the blood-brain barrier makes it possible to design drugs that will act at various parts of the body, but have no activity in the brain, thus reducing any central nervous system side effects. The mother’s blood provides the fetus with essential nutrients and carries away waste products, but the chemicals must pass through the placental barrier.