Drug distribution can be defined as the movement of drug between blood and extravascular tissues. As drug absorption occurs, drug transfers to the blood, resulting in a concentration gradient across the capillaries, allowing filtration of drug into the interstitial fluid. The accumulated drug in the interstitial fluid drives its passive diffusion into tissues and organs. Passive diffusion is normally the driving force in drug distribution, although transporters can be involved in certain tissues, such as the brain. With time, the process of elimination removes drug from the blood, reversing the concentration gradients between blood and tissues, resulting in the net movement of drug from tissue back into the bloodstream.