Oxygen is the essential final electron acceptor of the energy metabolism of higher animals. Under physiological conditions the mean oxygen tension (dissolved free oxygen concentration) is 74 to 104 mm Hg (104-146 µmol/L) in arterial blood and 34 to 46 mm Hg (48-64 µmol/L) in venous blood. 1-3 This can be altered in disease. For example, when the hematocrit is lowered but heart and lung function are normal as in anemia, arterial blood can be normoxic but hypoxemic because of normal oxygen tension but reduced hemoglobin-bound oxygen content. When the hematocrit and heart function are normal but lung function is impaired, arterial blood may be both hypoxic and hypoxemic. Venous blood, which normally is normoxic and normoxemic, may be hypoxic and hypoxemic when blood delivery is reduced by a local vascular stenosis or because of reduced cardiac output or when oxygen consumption is increased by disease. 2 For the liver, oxygen is essential as an electron acceptor in energy metabolism. It also regulates metabolic zonation in the normal liver and, under pathological conditions, is a modulator of liver disease.