The lungs serve as a major site of administration for a number of agents given for both local and systemic effects. Such drugs can be inhaled as gases (e.g., volatile anesthetics) or as aerosols (suspended liquid droplets or solid particles). Absorption of agents from the lung is facilitated by the large surface area of the pulmonary alveolar membranes (50-100 m2), the limited thickness of these membranes (approximately 0.2 and the high blood flow to the alveolar region.
Pulmonary absorption of volatile anesthetics across the alveolar-capillary barrier is very rapid because of the relatively high lipid-water partition coefficients and small molecular radii of such agents. The driving force for diffusion is a combination of the blood-air partition coefficient (which is a measure of the capacity of blood to dissolve drug) and the difference in partial pressure between the alveoli and the arterial and venous blood. Agents with high blood-air partition coefficients require more drug to be dissolved in the blood for equilibrium to be reached.
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