Arterial blood-gas composition is determined by ventilation, pulmonary blood flow, and by how ventilation is matched to blood flow in the lungs. In healthy adults there are regional differences in both ventilation and blood flow in the lungs and the distribution of blood flow tends to parallel that of ventilation. Ventilation and blood flow can become mismatched in a variety of disease processes that affect the lungs. Mismatching of ventilation and perfusion causes decreased PaO2, may change PaCO2, and increases AaDO2 difference. Many different types of interventions are frequently necessary to treat mismatching of ventilation and perfusion.