OBJECTIVE To describe system requirements for determination of the oxygen status of the blood using the oxygen status algorithm, a computer program. To define the oxygen extractivity, a term we propose, of the arterial blood and the oxygen extraction tension. To describe the different causes of tissue hypoxia, and the clinical interpretation of mixed venous oxygen tension and oxygen consumption rate. METHODS Previous physiological and clinical studies related to oxygen status of the blood. RESULTS The oxygen status algorithm calculates the oxygen extraction tension and generates the oxygen graph as an aid in interpreting oxygen status of the patient. A cybernetic scheme explains the causes of tissue hypoxia and forms the basis for the interpretation of changes in the mixed venous oxygen tension. A diagram with the mixed venous oxygen tension on the abscissa and the oxygen consumption rate on the ordinate illustrates the oxygen flux dependent oxygen consumption rate. A graph shows the relationship between mixed venous oxygen tension and oxygen delivery. CONCLUSIONS The oxygen status of arterial blood comprises three groups of quantities related to arterial oxygen tension, hemoglobin oxygen capacity, and hemoglobin oxygen affinity. Disturbances in one of these groups may be compensated by opposite changes in one or both of the other. The oxygen extraction tension indicates the degree of compensation, and mixed venous oxygen tension is the key parameter in evaluating the presence of a state of oxygen flux-dependent oxidative metabolism.