Chapter 2: Drugs and the Body

Drugs and the Body establishes the foundational principles of how medications interact with the living system, divided into two essential areas: pharmacodynamics, which studies the mechanism of how a drug affects the body, and pharmacokinetics, which describes how the body processes the drug, including absorption, distribution, biotransformation (metabolism), and excretion. Pharmacodynamically, drugs function primarily in one of four ways: replacing missing chemicals, stimulating cellular activity, slowing cellular activity, or interfering with foreign cells (known as chemotherapeutic agents). Many drugs act by binding to specific receptor sites on cell membranes, operating like a key in a lock, which then activates internal enzyme systems to produce an effect. Drugs that activate these sites are called agonists, while those that block normal stimulation are antagonists. Ideally, chemotherapeutic agents possess selective toxicity, targeting only systems unique to foreign or abnormal cells, thereby minimizing harm to healthy human tissue. The goal of pharmacokinetics is achieving a critical concentration—the specific drug level needed at reactive tissues to produce a therapeutic effect; this may necessitate a loading dose if quick results are required. Drug concentration is maintained through a dynamic equilibrium involving the rates of absorption, distribution, metabolism, and excretion. Absorption is heavily influenced by the administration route; for instance, intravenous (IV) drugs bypass the absorption phase entirely, achieving immediate effect. Conversely, oral drugs must contend with the first-pass effect, where they are carried directly to the liver after absorption and may be largely inactivated before entering general circulation, often requiring higher dosing. Distribution is the drug’s movement to tissues, affected by perfusion and the extent of protein binding; drugs that bind tightly to plasma proteins are released slowly, resulting in a longer duration of action. Biotransformation, primarily occurring in the liver via the hepatic microsomal system (cytochrome P450 enzymes), converts active drugs into less toxic, more water-soluble metabolites for removal. Excretion, mainly via the kidneys through processes like glomerular filtration, removes the drug from the body. The drug's half-life—the time required for the drug amount in the body to decrease by one half—helps determine appropriate dosing schedules. Numerous factors influence drug effects, including patient weight (dosing standard being 150 pounds), age (immature systems in children, less efficient processes in older adults), gender, genetic makeup (pharmacogenomics), disease state (pathological factors), psychological factors (e.g., the placebo effect), and acquired tolerance or accumulation. Finally, the nurse must consider various interactions—including drug–drug, drug–alternative therapy (e.g., herbal products), drug–food (e.g., grapefruit juice interfering with liver enzymes for up to 48 hours), and drug–laboratory test interactions—which can dangerously alter drug effectiveness or increase toxicity.