The primary task of the lungs is respiration. Respiration is the exchange of gases between an organism and its environment with the utilization of O2 and production of CO2. The most important physiological measure is the compliance of the intact respiratory system. The movement of O2 and CO2 in and out of the capillaries both in the lungs and in the peripheral tissues depends on gas diffusion. Positive pressure ventilation results in most of the ventilation being directed into the upper rather than the lower lung. The blood flow gradient due to gravity favors the dependent lung during one-lung ventilation (OLV). If the non-dependent lung is not ventilated any blood flow to it becomes shunt flow. This results in a larger alveolar-to-arterial oxygen tension difference with a lower PaO2 for a given oxygen concentration under identical circumstances, when compared to two-lung ventilation in the same position.