Drug pharmacology is classically divided into two disciplines, pharmacodynamics and pharmacokinetics.
Pharmacodynamics can be defined as what drugs do to the body. It describes the desired and undesired effects of drugs, as well as the cellular and molecular changes leading to these effects.
Pharmacokinetics can be defined as what the body does to drugs. It describes where drugs go, how they are transformed, and the cellular and molecular mechanisms underlying these processes.
Systemic drug pharmacokinetics has four phases: absorption, distribution, metabolism, and excretion.
Absorption is the phase in which drug is transferred from the administration site (e.g., digestive tract, lung, muscle) into the bloodstream. Intravenous drugs have no absorption phase because they are delivered directly into the bloodstream.
Distribution is the phase in which drug is transferred to tissue sites throughout the body.
Metabolism refers to the physiochemical processes by which substances in a living organism are synthesized (anabolism) or altered (catabolism); but in the context of anesthetic drugs only drug alteration is pertinent. Finally, excretion is the phase in which changed or unchanged drug is transferred from tissues or blood into some vehicle (e.g., bile, exhaled air, urine) for removal from the body.
Redistribution is lowering of drug concentration in one compartment (highly perfused organs e.g., brain) by delivery into another compartment. Highly lipid soluble drugs given by intravenous or inhalation routes are initially distributed to organs with high blood flow. Later, less vascular but more bulky tissues (such as muscle and fat) take up the drug plasma concentration falls and the drug is withdrawn from these sites. If the site of action of the drug was in one of the highly perfused organs, redistribution results in termination of the drug action. The greater the lipid solubility of the drug, the faster its redistribution is. For example, the anesthetic action of thiopentone is terminated in a few minutes due to redistribution. However, when the same drug (thiopentone) is given repeatedly or continuously over long periods, the low perfusion and high capacity sites are progressively filled up and the drug becomes longer acting due to cumulative effect.