Pharmacokinetics: Absorption, Distribution, Metabolism, and Excretion of Drugs

Mechanisms of Drug Absorption and Factors Affecting Bioavailability

Pharmacokinetics is the branch of pharmacology concerned with the movement of drugs within the body. It is divided into four essential processes: absorption, distribution, metabolism, and excretion.

Absorption

Absorption refers to the movement of a drug from its site of administration into the bloodstream. Various mechanisms govern this process, including passive diffusion, facilitated diffusion, active transport, and endocytosis.

Factors Affecting Bioavailability

• Drug solubility and formulation
• Route of administration
• Gastrointestinal pH and enzymatic activity
• First-pass metabolism
• Presence of food and drug interactions

Distribution

Once absorbed, drugs distribute throughout the body, influenced by factors such as blood flow, protein binding, and lipid solubility.

Metabolism

Metabolism occurs mainly in the liver, where enzymes modify the drug for easier elimination. Phase I reactions involve oxidation, reduction, and hydrolysis, while Phase II reactions include conjugation.

Excretion

Drugs are eliminated via renal (urine), biliary (feces), or pulmonary (exhalation) pathways. Kidney function plays a crucial role in drug clearance.

Understanding pharmacokinetics helps clinicians optimize drug therapy and minimize adverse effects.