Albumin, the blood's parking lot
As a drug travels in the plasma, much of it sticks reversibly to large proteins floating there. The most important is albumin, the most abundant plasma protein, which binds acidic and neutral drugs; a separate protein binds many basic drugs. This is plasma protein binding. A drug molecule glued to albumin is like a car parked in a lot — it's still in the blood, but it can't go anywhere, can't reach its target, can't be filtered out by the kidney, and can't slip into tissue. It is held, dormant, in reserve.
The binding is reversible and constantly shifting. Bound and free molecules exist in equilibrium: as free drug leaves the blood, bound drug releases to replace it. So the proteins act as a buffer, slowly handing the drug back as the free portion is used up.
Only the free fraction works
The portion not stuck to protein is the free drug fraction. This is the only part that is pharmacologically alive. Only free drug can cross membranes into tissue, bind its receptor, be metabolized by the liver, and be cleared by the kidney. A drug that is 99% protein-bound has only 1% of its blood content doing anything at all. This is why total blood concentration can mislead: two patients with the same total level can have very different *free* levels — and very different effects.
When binding sites get crowded
Because albumin has a limited number of binding sites, drugs can compete. If drug A is heavily bound and you add drug B that grabs the same sites, B can knock A loose — drug displacement. Suddenly more of drug A is free, raising its active fraction. In theory this is a classic drug interaction. In practice, the body usually compensates: the freshly freed drug is also cleared faster, so the free level often settles back. The displacement matters most for drugs that are highly bound, have a narrow safety margin, and a small Vd — where a small shift in percent bound is a large shift in free drug.