The barrier is not found in every part of the brain. The pituitary and hypothalamus, which must regulate circulating levels of hormones, are in contact with the contents of the blood. Metabolism describes the complex chemical reactions that change drugs to substances that can be eliminated from the body. Typically, liver enzymes metabolize lipid soluble drugs into water soluble waste products to be filtered out by the kidneys. These are usually ionized which makes re-absorption difficult.
Some drugs remain unchanged and are slowly filtered by the kidneys.
The plasma half-life of a drug is the time necessary for the blood levels of a drug to drop to half of the equilibrium level. The drop in drug level might result from re-distribution and storage of the drug. The drop might result from metabolism and elimination of the drug. Drug metabolism occurs primarily in the liver, but also in blood plasma, kidneys, lungs, intestines, skin, and cell tissues at the site of the receptors. C. Drug actions 1.