Drug distribution within the body is a dynamic process involving the movement of a drug in two directions across various compartments: from the bloodstream into tissues (tissue uptake) and from tissues back into the bloodstream (tissue release or redistribution). This process is passive and primarily driven by two variables: the concentration gradient between the bloodstream and the extravascular tissues and the drug's ability to cross the cell membrane.

Initially, the free drug in the bloodstream efficiently permeates the walls of capillaries, allowing it to enter the extravascular space known as the extracellular fluid. From there, the drug must cross the cell membrane to reach the intracellular fluid inside the cells. This step, crucial for the drug to exert its effects, is the rate-limiting process in drug distribution.

Two factors significantly influence this rate-limiting step. The first factor is the perfusion rate, which determines how quickly the drug is delivered to the extracellular tissue. Perfusion rate relates to the blood flow to the specific tissue, affecting the speed at which the drug reaches its target. The second factor is the drug's membrane permeability, which indicates how easily the drug can traverse the cell membrane. Membrane permeability is vital because it governs the drug's ability to enter cells and interact with cellular components.

Several other factors come into play when considering drug distribution. Drug-plasma protein binding impacts distribution significantly, as drugs bound to plasma proteins, such as albumin, are generally inactive and unable to cross cell membranes, limiting their availability for tissue uptake. Organ or tissue size impacts how much drug can be accommodated, and physiological barriers, such as the blood-brain barrier, restrict the passage of certain substances, ensuring the protection of sensitive organs like the brain. Individual differences, such as genetic variations, metabolism rates, and overall health, can influence drug distribution patterns.

In the second direction, drugs return from the tissues to the bloodstream. This movement occurs when drug concentrations in the bloodstream decrease due to metabolism, elimination, or distribution to other tissues. Tissue release is driven by the concentration gradient in the reverse direction. It depends on factors such as the tissue’s affinity for the drug, the presence of binding proteins, and the drug’s ability to diffuse back through cellular and capillary membranes.

Researchers use the concept of the volume of distribution to quantify the extent of drug distribution within the body. This hypothetical volume represents the body fluid space where a drug is dispersed to achieve the observed blood concentration.