venous blood gas [L, venosus, full of veins; AS, blod + Gk, chaos, gas] , the oxygen and carbon dioxide levels in venous blood. Venous blood gas is measured by various methods to assess the adequacy of oxygenation and ventilation and to determine the acid-base status. The oxygen tension of venous blood normally averages 40 mm Hg; the dissolved oxygen content, 0.1% by volume; the total oxygen content, 15.2%; and the oxygen saturation of venous hemoglobin, 75%. The carbon dioxide tension normally averages 46 mm Hg; the dissolved carbon dioxide content, 2.9% by volume; and the total carbon dioxide content, 50%. The normal average pH of venous plasma is 7.37. Venous blood gas in an extremity pertains chiefly to that limb. A sample from a central venous catheter is usually an incomplete mix of venous blood from various parts of the body; a sample of completely mixed venous blood may be obtained from the pulmonary artery for an accurate determination of venous blood gas.
