Overview
Clinical Meaning
Gas exchange occurs at the alveolar capillary membrane through passive diffusion driven by partial pressure gradients (Fick's law of diffusion).
FNP
FNP
Gas exchange occurs at the alveolar-capillary membrane through passive diffusion driven by partial pressure gradients (Fick's law of diffusion). Oxygen diffuses from alveoli (PAO2 ~100 mmHg) into pulmonary capillary blood (PvO2 ~40 mmHg), while CO2 diffuses from capillary blood (PvCO2 ~46 mmHg) into alveoli (PACO2 ~40 mmHg). CO2 diffuses 20 times more readily than O2 (higher solubility coefficient), so CO2 retention indicates severe hypoventilation or V/Q mismatch. The alveolar gas equation calculates expected PAO2: PAO2 = FiO2 × (Patm - PH2O) - PaCO2/R, where R = respiratory quotient (~0.8). The A-a gradient (PAO2 - PaO2, normally < 15 mmHg in young adults, increases with age) differentiates causes of hypoxemia: normal A-a gradient = hypoventilation (CNS depression, neuromuscular disease) or low FiO2 (altitude); elevated A-a gradient = V/Q mismatch (COPD, asthma, PE), shunt (ARDS, pneumonia, atelectasis), or diffusion impairment (pulmonary fibrosis). Ventilation-perfusion (V/Q) matching is critical: areas of high V/Q (dead space — ventilated but not perfused, as in PE) waste ventilation; areas of low V/Q (shunt effect — perfused but not ventilated, as in pneumonia) cause hypoxemia. True shunt (V/Q...
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