Abstract
Gas exchange in the lung depends on tidal breathing, which brings new oxygen to and removes carbon dioxide from alveolar gas. This maintains alveolar partial pressures that promote passive diffusion to add oxygen and remove carbon dioxide from blood in alveolar capillaries. In a lung model without ventilation and perfusion (V̇AQ̇) mismatch, alveolar partial pressures of oxygen and carbon dioxide are primarily determined by inspiratory pressures and alveolar ventilation. Regions with shunt or low 
ratios worsen arterial oxygenation while alveolar dead space and high lung units lessen CO2 elimination efficiency. Although less common, diffusion limitation might cause hypoxemia in some situations. This review covers the principles of lung gas exchange and therefore mechanisms of hypoxemia or hypercapnia. In addition, we discuss different metrics that quantify the deviation from ideal gas exchange.
Keywords
ventilation-perfusion ratio - respiratory dead space - respiratory physiology - pulmonary ventilation - hypoxia - hypercapnia
