The mixed venous PO2(PvO2) as a parameter of peripheral O2 supply correlates at rest and during exercise with the coefficient of oxygen delivery (COD = CaO2 x CO/VO2). PvO2 is determined by the O2 capacity, pulmonary and cardiac function. Values under 34 mm Hg at rest and a pronounced drop of PvO2 during exercise indicate a restriction of either pulmonary or cardiac function or both. The effect of respiratory disorder on PvO2 is striking only when the PaO2 is below 65 mm Hg. Besides PvO2, the meaning of the mean peripheral O2 capillary pressure (PacO2) for the peripheral O2 supply should also be considered.