chapter two

Chapter 2: Drugs and the Body

Pharmacodynamics

  • Definition:

    • Pharmacodynamics is the study of interactions between chemical components of living systems and foreign chemicals (including drugs) that enter these systems.

    • When a new chemical enters a biological system, it can alter the function of cells, leading to multiple physiological changes.

Drug Actions

  • Possible Actions of Drugs:

    • Replacing or acting as a substitute for missing chemicals in the body.

    • Increasing or stimulating certain cellular activities to enhance physiological functions.

    • Depressing or slowing cellular activities, thereby reducing various physiological responses.

    • Interfering with the functioning of foreign cells, which may help combat infections or other pathological conditions.

Receptor Sites

  • Definition and Function:

    • Receptor sites are specific areas on cell membranes that react with certain chemicals and produce effects within the cell.

  • Types of Drugs Interacting with Receptor Sites:

    • Agonists:

    • Drugs that bind to receptor sites and induce a response in the cell.

    • Competitive Antagonists:

    • Drugs that bind to the same receptor sites as agonists but do not activate them, thereby blocking the action of the agonists.

    • Noncompetitive Antagonists:

    • Drugs that bind to different receptor sites and inhibit the action of agonists without competing for the same binding site.

    • Partial Agonists:

    • Drugs that bind to receptor sites but produce a weaker response compared to full agonists.

  • Drug-Enzyme Interactions:

    • Drugs may interact with enzymes in ways that affect metabolic processes.

  • Selective Toxicity:

    • The ability of a drug to selectively target pathological cells while sparing healthy cells.

Key Interaction Examples

  • Lock and Key Metaphor:

    • Drug A (Agonist): Binds to receptor sites, producing effects.

    • Drug B: Cannot bind to receptor sites, thus has no effect.

    • Drug C (Antagonist): Binds to the same receptor site as Drug A, preventing it from binding.

    • Drug D (Competitive Antagonist): Binds to a different receptor site but inhibits Drug A's actions.

Pharmacokinetics

  • Definition:

    • Pharmacokinetics is the study of the absorption, distribution, metabolism (biotransformation), and excretion of drugs.

Key Processes in Pharmacokinetics

  1. Onset of Drug Action:

    • Refers to the time it takes for a drug to begin its therapeutic effect.

  2. Drug Half-Life:

    • The time required for the concentration of a drug in the body to decrease to half its initial amount.

  3. Timing of Peak Effect:

    • Refers to the time when the drug reaches its maximum therapeutic effect.

  4. Duration of Drug Effects:

    • The length of time a drug maintains its therapeutic effects before additional doses are needed.

  5. Metabolism/Biotransformation of the Drug:

    • The chemical modification made by an organism on a chemical compound, primarily occurring in the liver.

  6. Site of Excretion:

    • Refers to organs responsible for removing drugs from the body, primarily the kidneys.

Pharmacokinetics Flow

  • Processes Involved:

    • Drug Dose Administration Method:

    • Drug can be administered through various routes (e.g., GI tract, muscle, subcutaneous tissue).

    • Drug Distribution:

    • Drugs travel through blood plasma to target tissues; distribution influenced by properties like lipid solubility and blood flow.

    • Tissue Storage Sites:

    • Drugs can accumulate in fat or protein tissues.

    • Excretion Mechanism:

    • Drugs are excreted primarily via kidneys, bile, lungs, and other pathways.

Important Pharmacokinetic Concepts

  • Critical Concentration:

    • The minimum amount of drug required to achieve a desired therapeutic effect.

  • Loading Dose:

    • Typically a higher initial dose administered to rapidly reach critical concentration.

  • Dynamic Equilibrium:

    • The state at which the amounts of drug absorbed, distributed, metabolized, and excreted balance out.

Dynamic Equilibrium

  • Components that Contribute to Dynamic Equilibrium:

    1. Absorption

    2. Distribution

    3. Biotransformation

    4. Excretion

Absorption

  1. Definition:

    • The process by which a drug moves