Figure 17-27 Effect of a decrease in cardiac output or an increase in oxygen consumption on mixed venous and arterial oxygen content. Mixed venous blood () either perfuses ventilated alveolar (ALV O₂ ) capillaries and becomes oxygenated end-pulmonary capillary blood (c') or perfuses whatever true shunt pathways exist and remains the same in composition (desaturated). These two pathways must ultimately join together to form mixed arterial (a) blood. If cardiac output (T) decreases or oxygen consumption (V̇O₂ ) increases, or both the tissues must extract more oxygen per unit volume of blood than under normal conditions. Thus, the primary effect of a decrease in T or an increase in V̇O₂ is a decrease in mixed venous oxygen content. The mixed venous blood with a decreased oxygen content must flow through the shunt pathway as before (which may remain constant in size) and lower the arterial content of oxygen. Thus, the secondary effect of a decrease in T or an increase in V̇O₂ is a decrease in arterial oxygen content.