The journey doesn't end in the blood
When you study pharmacokinetics, you meet four steps spelled out by ADME: Absorption, Distribution, Metabolism, Excretion. Absorption gets the drug into the blood. Distribution is what happens next — the drug leaves the bloodstream and spreads into the tissues, organs, and fluids of the body. This is drug distribution, and it is far more than a detail. It decides how much of the drug actually reaches the site where it needs to act, how long it lingers in the body, and whether a blood sample even reflects what's happening inside the heart, brain, or fat.
Picture the body as a set of connected pools — plasma, then the watery space between cells, then the inside of cells, then fat. The drug enters the plasma pool first, then trickles outward until the pools reach a kind of balance. The blood you can sample is only the first, smallest pool. Most of the drug may be sitting somewhere you can't easily measure.
Two forces that move a drug around
What carries a drug from blood into tissue? Two things matter most. First, blood flow — tissue perfusion. Organs that receive a lot of blood (brain, heart, liver, kidney) see the drug almost immediately. Poorly perfused tissues (fat, resting muscle) get their share slowly, sometimes over hours. Second, the drug's ability to cross membranes, mostly by passive diffusion across the fatty cell walls. A drug that dissolves easily in fat — high lipophilicity — slips through membranes and reaches deep into tissues. A water-loving drug tends to stay in the watery compartments.
- The drug is absorbed and enters the plasma — the central pool.
- Highly perfused organs (brain, heart, kidney, liver) equilibrate within minutes.
- Muscle, skin, and other moderately perfused tissues fill more slowly.
- Fat and poorly perfused tissues take up lipophilic drugs last — and release them last.