When a drug is absorbed and enters the systemic circulation, it is naturally distributed throughout the fluid and tissues in the body. Drug distribution is a subject that is covered in a branch of pharmacology called pharmacokinetics. The drug distribution is usually varied, and depends on several factors such as: Blood perfusion, Tissue binding (since drug binding is linked to the lipid content), Regional Ph., Cell membrane permeability, Additionally, the rate at which a drug enters into a tissue depends on: the flow of blood to the tissue, the mass of the tissue, the barriers existing between the blood and the tissue. The drug will eventually reach a distribution equilibrium, when the rate of drug entry and exit between the blood and the tissue is equal. At this point, when equilibrium has been reached, the concentration of the drug in the tissues and extracellular fluids is reflected by the concentration of the drug in the blood plasma. However, drug distribution is a dynamic process, because it occurs simultaneously with other pharmacokinetic processes such as drug metabolism and excretion. Water-soluble drugs remain in the blood, but fat-soluble drugs are concentrated in the fatty tissues.
