Pharmacokinetics

Pharmacokinetics is the branch of pharmacology concerned with the time course of drug movement through the body: absorption from the site of administration, distribution to tissues and organs, metabolism by enzymatic transformation (chiefly in the liver), and excretion via renal, biliary, or pulmonary routes. The field treats the organism as a compartmental system — blood as a central compartment, tissues as peripheral compartments — and models drug concentration over time using systems of differential equations that are formally identical to models in dynamical systems theory and control theory.

The practical objective is to predict the concentration-time profile of a drug, which determines whether therapeutic thresholds are reached and whether toxic thresholds are exceeded. But the compartmental model is an idealization: real organisms exhibit nonlinear metabolism (enzyme saturation), time-dependent changes (induction or inhibition of metabolic enzymes by the drug itself), and enormous inter-individual variability driven by genetics, age, disease state, and concurrent medications. Pharmacokinetics is the science of average behavior and the art of managing deviation from that average.