Tema 5. FARMACOCINÉTICA; Excreción.

Pharmacokinetics and Excretion of Drugs

Overview of Drug Excretion

• The excretion of drugs or their metabolites from the body primarily occurs through the kidneys, which are responsible for renal excretion. This process involves three main mechanisms: glomerular filtration, tubular secretion, and tubular reabsorption.

Mechanisms of Renal Excretion

• Glomerular filtration allows all drugs dissolved in plasma with a molecular weight under 70,000 to pass into urine. Factors such as age, pregnancy, certain substances, and health conditions can influence this process.

• Tubular secretion can occur passively or via active transport. Generally, drugs utilize the same tubular secretion systems as endogenous substances; thus, it may lack selectivity regarding free or protein-bound drug fractions.

• Tubular reabsorption enables previously filtered or secreted drugs to return to systemic circulation. This typically happens through passive diffusion but can also involve active transport depending on the substance's nature.

Impact of pH on Reabsorption

• The reabsorption process is influenced by the pH of the medium; alkalinizing or acidifying urine can expedite the elimination of weak acids or bases during intoxication scenarios.

Alternative Routes of Drug Excretion

• Besides renal pathways, hepatobiliary excretion via bile is significant for high molecular weight compounds and may be beneficial in treating biliary infections. Other routes include feces and pulmonary excretion for inhaled anesthetics.

• Drug transfer through breast milk is generally minimal but poses risks if toxic medications are consumed in large quantities by nursing mothers.

Key Pharmacokinetic Concepts

• The plasma half-life indicates how long it takes for drug concentration to reduce by half; after five half-lives, a drug is considered eliminated from the body.