Summary
Highlights
Pharmacokinetics (PK) describes what the body does to a drug, from administration to excretion. It is divided into four main parameters abbreviated as ADME: absorption, distribution, metabolism, and elimination.
Absorption is the movement of a drug into the bloodstream. Methods include passive diffusion, facilitated diffusion, active transport, and endocytosis. Bioavailability refers to the fraction of the drug that reaches the systematic circulation, with IV administration providing 100% bioavailability as compared to oral or other routes.
Distribution involves the passage of drugs from the blood into bodily tissues. Factors like blood flow, polarity, molecular size, and protein binding affect this process. The volume of distribution (Vd) determines the drug's propensity to remain in the plasma versus entering extravascular tissues.
Elimination removes drugs from the body, primarily via kidneys and the biliary system. First-order kinetics involve a constant percentage of drug removal over time, while zero-order kinetics involve a constant amount (milligrams) of drug removal regardless of concentration.