Factors influencing oxygen availability.

This review gives an account of some of the factors affecting the transport of oxygen from the lungs until its final discharge in the tissues. The sequence of physiological and metabolic processes to be described may be considered collectively as the 'coarse adjustment' of oxygen transport. The 'fine adjustment' will be discussed in the succeeding chapter which surveys the local factors influencing distribution of oxygen from the microcirculation. Three terms which will doubtless be used more than once during this Symposium are worthy of definition. 'Oxygen availability' consists of two main components: blood flow and oxygen content (which is the product of haemoglobin concentration and oxygen saturation). By means of the Fick equation it can be shown that 'oxygen consumption' equates with the product of blood flow and arteriovenous difference for oxygen. Finally, 'oxygen debt' is the term used to describe the ability of certain tissues, notably muscle, to survive for short periods in the absence of oxygen. This factor may be expressed quantitatively as the product of the reduction in oxygen consumption during a period of hypoxia or ischaemia and its duration.