Volume of Distribution
A pharmacokinetic parameter that describes the theoretical volume into which a drug distributes in the body relative to its plasma concentration. Peptides typically have a small volume of distribution because their size and hydrophilicity limit tissue penetration beyond the extracellular fluid.
Technical Context
Vd is a theoretical volume — it equals the amount of drug in the body divided by the plasma concentration. A Vd close to plasma volume (~3L) indicates the drug remains mainly in blood. A Vd close to extracellular fluid volume (~14L) indicates distribution into interstitial fluid. Very large Vd (>total body water ~42L) indicates extensive tissue binding. Most peptides have small Vd (0.05-0.3 L/kg) because their size and hydrophilicity restrict distribution. Albumin-bound peptides have Vd approximately equal to albumin's Vd (~0.1 L/kg). The combination of small Vd and moderate-to-low clearance contributes to the long half-lives of albumin-bound peptide drugs.