Pharmacokinetics is how a body processes a drug
Pharmacokinetics can be simply described as the study of 'what the body does to the drug' and includes:
• The rate and extent to which drugs are absorbed into the body and distributed to the body tissues
• The rate and pathways by which drugs are eliminated from the body by metabolism and excretion
• The relationship between time and plasma drug concentration.
The main processes involved in pharmacokinetics are absorption, distribution, and the two routes of drug elimination, metabolism and excretion. Together they are sometimes known by the acronym ‘ADME’. Distribution, metabolism and excretion are sometimes referred to collectively as drug disposition. Distribution is the process by which drugs move around the body. After entering the blood, drug molecules must cross capillary walls to enter the tissues, reach cell membranes and enter cells.
Excretion is the process by which drugs leave the body. Drugs that are sufficiently water-soluble will be excreted unchanged in the urine. Lipid-soluble drugs must be modified to water-soluble metabolites before excretion via the kidney or into the intestine via the bile.
Absorption is the process by which drug molecules gain access to the bloodstream from the site of drug administration. The speed of this process (the rate of drug absorption) and its completeness (the extent of drug absorption) depend on the route of administration. Enteral Drugs given by mouth are normally swallowed before being absorbed in the stomach or small bowel, after which they enter the portal venous system and pass through the liver before gaining access to the systemic circulation. Some drugs introduced into the alimentary tract are absorbed directly into the systemic circulation without passing through the liver (e.g. via the buccal, sublingual or rectal routes), thereby avoiding the potential hazards of gastric acid, binding to food, and metabolism by gut wall or liver enzymes (first-pass metabolism).
Absorption after an oral dose is a lengthy process, during which drug molecules may be damaged (e.g. denatured by gastric acid), sequestered (e.g. bound to food preventing absorption) or modified by first-pass metabolism. As a consequence of all these hazards, it is not surprising that absorption is frequently incomplete following oral administration. The proportion of a dose that reaches the systemic circulation unscathed is known as the bioavailability of the drug.
The rate of drug metabolism varies widely between individuals, influenced by genetic and environmental factors. This is the major reason for inter-individual differences in the plasma concentration of some drugs after a standard dose, which leads to wide variation in drug response. Faecal excretion is the preferred route of elimination for larger molecular-weight drugs, including those that are conjugated with glucuronide in the liver, and any drugs that are not absorbed. Molecules of drug or metabolite that enter the bile after liver metabolism are carried into the intestinal lumen, pass down the gut and are eliminated in the faeces. If they are still sufficiently lipid-soluble, some molecules of unchanged drug or metabolite may be reabsorbed and re-enter the portal vein
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