McKenna's Pharmacology, 2e

2

Drugs and the body

Learning objectives Upon completion of this chapter, you should be able to:

1. Describe how body cells respond to the presence of drugs that are capable of altering their function. 2. Outline the process of dynamic equilibrium that determines the actual concentration of a drug in the body. 3. Explain the meaning of half-life of a drug and calculate the half-life of given drugs. 4. List at least six factors that can influence the actual effectiveness of drugs in the body. 5. Define drug–drug, drug–alternative therapy, drug–food and drug–laboratory test interactions.

Test your current knowledge of drugs and the body with a PrepU Practice Quiz!

Glossary of key terms absorption: what happens to a drug from the time it enters the body until it enters the circulating fluid; intravenous administration causes the drug to directly enter the circulating blood, bypassing the many complications of absorption from other routes active transport: the movement of substances across a cell membrane against the concentration gradient; this process requires the use of energy biotransformation agonist: drug that interacts directly with a receptor site to cause an effect chemotherapeutic agents: synthetic chemicals used to interfere with the functioning of foreign cell populations; this term is frequently used to refer to the drug therapy of neoplasms, but it also refers to drug therapy affecting any foreign cell competitive antagonist: drug that interacts with a receptor, blocking the ability of another drug to combine with it distribution: movement of a drug to body tissues; the places where a drug may be distributed depend on the drug’s solubility, perfusion of the area, cardiac output and binding of the drug to plasma proteins effective concentration: the concentration a drug must reach in the tissues that respond to the particular drug to cause the desired effect endocytosis: the process through which cells absorb molecules by engulfing them endogenous substances: naturally occurring chemicals that are vital to the normal functioning of the body, e.g. neurotransmitters, hormones and paracrines enzyme induction: process by which the presence of a chemical that is biotransformed by a particular enzyme system in the liver causes increased activity of that enzyme system excretion: removal of a drug from the body; primarily occurs in the kidneys, but can also occur through the skin, lungs, bile or faeces first-pass effect: a phenomenon in which drugs given orally are carried directly to the liver after absorption, where they may be largely inactivated by liver enzymes before they can enter the general circulation; oral drugs are frequently given in higher doses than drugs given by other routes because of this early breakdown glomerular filtration: the passage of water and water-soluble components from the plasma into the renal tubule half-life: the time it takes for the amount of drug in the body to decrease to half of the peak level it previously achieved hepatic microsomal system: liver enzymes tightly packed together in the hepatic intracellular structure, responsible for the biotransformation of chemicals, including drugs loading dose: use of a higher dose than that which is usually used for treatment to allow the drug to reach the effective concentration sooner antagonist: drug that interacts with a receptor but does not cause an effect, instead blocks it biotransformation: the process by which drugs are changed into new, less active chemicals