ABSTRACT
The biological membranes in some organs have special characteristics that make the membrane act as a barrier to protect the organ from the distribution of many substances in the blood. An example of these barriers is the blood brain barrier (BBB) that is characterized by the presence of tight junctions between the endothelial cells of the blood capillaries supplying the blood to the brain. The BBB prevents the distribution of many polar compounds in the blood to the brain tissues. Only lipophilic compounds can distribute across the BBB by passive diffusion. In this case, the distribution of polar compounds across these biological barriers is slow and the rate determining step for the distribution process is the rate of drug diffusion across the barrier. This is called permeability-limited distribution. Consequently, nonpolar drugs usually distribute to the tissues faster than polar drugs. Hence, the equilibrium between the drug in the blood and the drug in tissues is achieved faster for nonpolar drugs compared to the more polar drugs.
