TOPICS: Perfusion-limited gas exchange, diffusion-limited gas exchange, concentration gradient, alveolar-capillary gradient, oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), nitrous oxide (N2O), pulmonary embolism, circulation, diffusion barrier, emphysema, interstitial lung disease, diffusing capacity for carbon monoxide (DLCO), bicarbonate (HCO3-), carbonic anhydrase, partial pressure of carbon dioxide (PCO2), partial pressure of arterial oxygen (PaO2), mixed venous oxygen tension (PvO2), mixed venous carbon dioxide tension (PvCO2)