This figure show one approach to a general classification of the components of oxygen supply and utilization. More specifically, it may be used to better understand the types of failure that can occur, leading to cellular hypoxia. Oxygen supply, represented by the macrocirculation, is the oxygen contained in the blood leaving the heart. It is calculated as cardiac output (CO) × oxygen carrying capacity of the hemoglobin (Hb) × arterial oxygen saturation (SaO2) × 13.4 (a constant). The microcirculation consists of the small arterioles and capillaries where oxygen exchange generally occurs, while the mitochondria are where the oxygen is processed once it reaches the cells. 1) Macrocirculatory failure occurs when there is not enough oxygen supplied to meet the needs of the cells and is usually caused by hypotension, anemia, or hypoxemia, often in the setting of increased oxygen demand of the tissues/vital organs. 2) Microcirculatory failure occurs when there is global, or more often regionalized, shunting of blood, producing a disruption in the ability of oxygen to flow through the microcirculation to reach metabolically active tissues. Finally, 3) Mitochondria failure occurs when oxygenated blood is delivered to the tissues but the mitochondria can not process the oxygen. The ScvO2 measurement occurs after all three types of failure have occurred.