Drug Excretion: Pulmonary and Glandular Routes

Volatile drugs diffuse into exhaled air through simple diffusion in the lungs. The rate of pulmonary excretion depends on drug solubility, pulmonary blood flow, and respiration rate. Less blood-soluble gaseous anesthetics like nitrous oxide are rapidly excreted, while highly soluble compounds like alcohol are expelled more slowly.

Breast milk is more acidic than plasma, which facilitates the excretion of weakly basic drugs through passive diffusion. Many nonelectrolytes like ethanol enter breast milk quickly, reaching concentrations similar to plasma levels. This concentration difference means overconsumption of such agents can negatively impact the suckling infant.

Pulmonary drug excretion depends on pulmonary blood flow, respiration rate, and the substance's solubility in blood. Drugs with low blood solubility are excreted rapidly, while those with high blood and tissue solubility are excreted slowly. These factors determine how quickly volatile compounds like general anesthetics leave the body.

Yes, saliva is a useful alternative for measuring drug concentrations because salivary drug concentration closely mirrors the unbound drug concentration in plasma. Nonionized lipid-soluble drugs diffuse through the epithelial lining of salivary glands into drug-containing saliva. This method is especially valuable when obtaining blood samples is challenging.

Potent drugs like barbiturates, morphine, and ergotamine may induce toxicity in nursing infants because they concentrate in breast milk. These medications can reach significant levels in milk due to passive diffusion influenced by pH, molecular weight, lipid solubility, and ionization. Nursing mothers should avoid such medications when possible to protect infant safety.

Drug solubility in blood is the primary determinant of pulmonary excretion speed. Gaseous anesthetics with low blood solubility, such as nitrous oxide, are rapidly excreted through exhaled air. Conversely, highly soluble compounds like alcohol remain in the bloodstream longer and are expelled more slowly through respiration.

Nonionized lipid-soluble drugs undergo passive diffusion through the epithelial lining of salivary glands, resulting in drug-containing saliva secretion. The concentration of drugs in saliva reflects the unbound drug concentration in plasma. This passive process allows certain drug classes, such as cephalosporins, to achieve significantly higher concentrations in saliva.