Various distribution models exist to describe how a substance moves through the body with time. For example, if it distributes to two compartments (e.g., plasma and bone), a two-compartment model can describe it. The body has structural barriers to the distribution of substances into certain areas. The two most extensive are the blood-brain barrier and placental barrier. These barriers selectively inhibit the passage of certain toxicants into the central nervous system or to the developing fetus.