ABC of oxygen: Oxygen transport---1. Basic principles

Mammalian life and the bioenergetic processes that maintain cellular integrity depend on a continuous supply of oxygen to sustain aerobic metabolism. Reduced oxygen delivery and failure of cellular use of oxygen occur in various circumstances and if not recognised result in organ dysfunction and death. Prevention, early identification, and correction of tissue hypoxia are essential skills. An understanding of the key steps in oxygen transport within the body is essential to avoid tissue hypoxia. Red blood cells entering a capillary Although oxygen is the substrate that cells use in the greatest quantity and on which aerobic metabolism and cell integrity depend, the tissues have no storage system for oxygen. They rely on a continuous supply at a rate that precisely matches changing metabolic requirements. If this supply fails, even for a few minutes, tissue hypoxaemia may develop resulting in anaerobic metabolism and production of lactate. ### Key steps in oxygen cascade Oxygen transport from atmosphere to mitochondria. Values given in parenthesis are for a healthy 70 kg adult breathing air (FiO2 0.21) at standard barometric pressure View this table: Partial pressures of oxygen at sea level (101 kPa) Oxygen transport from environmental air to the mitochondria of individual cells occurs as a series of steps. The heart, lungs, and circulation extract oxygen from the atmosphere and generate a flow of oxygenated blood to the tissues to maintain aerobic metabolism. The system must be energy efficient (avoiding unnecessary cardiorespiratory work), match oxygen distribution with metabolic demand, and allow efficient oxygen transport across the extravascular tissue matrix. At the tissue level, cells must extract oxygen from the extracellular environment and use it efficiently in cellular metabolic processes. Arterial oxygen tension (PaO2 …