One or more of the several components of the oxygen transport system may function abnormally in critical illness. Arterial hypoxemia is an important feature of acute respiratory failure. Its prominence may obscure other limitations in oxygen availability such as low cardiac output, anemia, or an increased red cell affinity state. These several components of the oxygen transport system can be influenced by therapeutic maneuvers, but the result may not necessarily be a net benefit. For example, red blood cell transfusion therapy may correct anemia, but increase the red blood cell affinity state so as to adversely affect cardiac function. Treatment programs require consideration of the interaction of these several variables affecting oxygen transport.