Title: Pharmacokinetics Content: Pharmacokinetics is a branch of pharmacology which studies what the body does to a drug. Pharmacokinetics looks at how a substance enters, moves through and exits the body. It relates how the dose delivered affects the concentration within the body. It is closely related to another branch of pharmacology, pharmacodynamics, which describes how a drugs affects the body. == ADME == When developing new drugs, it is important to know exactly what happens to the chemical once administered. There are four important parameters of pharmacokinetics, usually known by the acronym ADME: Absorption, Distribution, Metabolism and Excretion. This model was developed in order to simplify perception of the many processes that take place once a drug enters an organism. * Absorption - How is the medication absorbed (through the skin, the intestine, the mouth)? * Distribution - How does it spread through the organism? * Metabolism - Is the medication converted chemically inside the body, and into what. Are these new substances active? Could they be toxic? * Excretion - How does the organism get rid of the chemical (through the bile, urine, breath, skin)? === Absorption === Absorption describes the movement of a drug from the dosage form to the site of action, usually the blood. The bioavailability (F) of the drug is the fraction of the drug being administered that reaches the blood circulation. ==== Factors affecting the absorption ==== * Route of administration * Physicochemical properties of the drug, for example, how soluble or acidic it is. * Patient related factors: sex, age, disease state, blood flow and supply to the site of adsorption ==== Route of administration ====  === Excretion === This is how a drug or its metabolite is removed from the body. Usually this occurs in the kidneys (urine), but can also take place in the biliary system (faeces), gastrointestinal tract (GIT), skin (sweat) and lungs (expired air). ==== Factos effecting drug excretion ==== Many of the products of hepatic metabolism are water soluble metabolites of lipid soluble drugs, these enter the systemic circulation and are transported to the kidney where they are eventually excreted in the urine. Drugs and metabolites that are in free aqueous solution may also be excreted in sweat or expired air. Where the drug or metabolite is not freely water soluble, it is normally excreted into the gut via the bile duct from when it is excreted in the faeces. One complication of this route of excretion is the enterohepatic shunt. In this process the drug is excreted into the guy from the bile duct, but is then reabsorbed and transported back to the liver. The consequence of this is that the presence of the drug or metabolite within the body is extended, which, if the metabolite is pharmacologically active, may extend the duration of action of the drug. Some chemicals, for example lead, accumulate within the tissues. == Factors effecting plasma drug concentration == The actual concentration of drug present within the plasma (bloodstream) following administration is dependent on absorption, distribution, metabolism and excretion. Plasma concentrations of a drug after administration of identical doses either by intravenous injection or orally. Only about half of the dose administered orally actually enters circulation, however the plasma half-life of the drug is approximately 3 hours both orally and via IV. In most therapeutic circumstances, drugs are not given as a single dose but as multiple doses. This has profound effects on the plasma concentrations achieved. The state of constant maxima and minima is called "steady state". This is the point where rate of drug availability equals rate of elimination, and the optimum therapeutic effect is reached. Category:Pharmacology Category:Drugs