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
