Oxygen transportation in the blood

The most important determinants of the overall O2 delivery to tissues are the cardiac output and the arteriovenous O2 content difference. The latter is influenced mainly by the haemoglobin concentration, arterial haemoglobin O2 saturation and venous haemoglobin O2 saturation. Also the O2 tension has a minor contribution. The venous haemoglobin O2 saturation decreases, without a concomitant, potentially detrimental decrease in the venous blood O2 tension, when the haemoglobin O2 affinity decreases as a consequence of e.g. decreased pH or increased PCO2 (Bohr effect) increased temperature and increased red cell 2,3-DPG. This effect allows greater O2 extraction from the blood by tissues. The changes in the haemoglobin O2 affinity are compensated in physiological conditions by changes in the cardiac output and in the venous O2 tension. If, however, in a situation of limited tissue O2 supply these mechanisms are used up or severely compromised the haemoglobin O2 affinity becomes an important determinant of the O2 delivery to tissues.