ABSTRACT

NORMAL PHYSIOLOGY In health, 90% of the total body oxygen utilization is by aerobic respiration for the generation of ATP. Oxygen delivery (DO2), the product of cardiac output, haemoglobin and oxygen saturation, is about 1000 ml/min in the normal resting adult. About 250 ml of this is utilized for aerobic respiration and other oxygen-consuming processes (VO2); that is, DO2 is well in excess of the body’s VO2. In response to increased demand, as in exercise, oxygen delivery is enhanced by both an increase in cardiac output and an increased extraction of oxygen from haemoglobin. The increase in extraction is mainly a result of nutrient capillary recruitment but also of a reduced haemoglobin affinity for oxygen in the presence of hypoxaemia (right shift of the oxyhaemoglobin dissociation curve). Once the oxygen supply falls below demand (DO2crit) (Figure 11.1), oxygen consumption becomes dependent on delivery (physiological supply dependency), and it is believed the tissues switch to anaerobic respiration to make up any energy shortfall.1 Lactic acid accumulates and the tissues build up an ‘oxygen debt’.