The ultimate goal of the respiratory system is to exchange oxygen (O2) and carbon dioxide (CO2), to meet the metabolic needs of the body. In order to effectively transfer both gases, ventilation and blood flow must be adequately apportioned and matched within the lungs. Of the four classic mechanisms determining abnormal arterial blood respiratory gases – i.e. alveolar hypoventilation, impaired alveolar-end capillary diffusion to O2, increased intrapulmonary shunt and ventilation-perfusion (VA/Q ) mismatching – the last is by far the most common cause of impaired pulmonary gas exchange in respiratory disease. All the abnormalities alluded to above except alveolar hypoventilation may be viewed as intrapulmonary determinants of pulmonary gas exchange. Other key extrapulmonary factors of respiratory blood gases include the fractional concentration of O2 in the inspired gas, the hemodynamic status (cardiac output) and the metabolic demands (O2 consumption) of the body.