Oxygen delivery: the principal role of the circulation.

Autoregulation of blood flow to most individual organs is well known. The balance of oxygen supply relative to the rate of oxygen consumption ensures normal function. There is less reserve as regards oxygen supply than for any other necessary metabolite or waste product so oxygen supply is flow dependent. Reduced rate of supply compromises tissue oxygenation long before any other substance. The present report reiterates evidence from earlier studies demonstrating that the rate of oxygen delivery (DO2), for most individual tissues, is well sustained at a value bearing a ratio to oxygen consumption (VO2) which is specific for the organ concerned. For the brain DO2 is sustained at approximately three times the rate of oxygen consumption and for exercising skeletal muscle (below the anaerobic threshold), a ratio close to 1.5. The tissue-specific ratios are sustained in the face of alterations in local VO2 and lowered arterial oxygen content (CaO2). Tolerance varies between different organs. Hence, the role of the circulation is predominantly one of ensuring an adequate supply of oxygen. The precise values of the individual tissue DO2:VO2 ratios apply within physiological ranges which require further investigation.