Blood-volume: a commentary.

Under physiological conditions and on average, the total volume of blood in the circulation is constant. The total blood volume (BV) and the separate volumes of plasma and erythrocytes vary according to climatic conditions, in pregnancy and in the presence of disease. Such changes can have clinical significance although they are rarely revealed in common clinical practice because of a lack of availability of accurate, cheap and simple techniques for the measurement of BV. Diminution of BV may occur under intensive-care regimes and is life-threatening; acute exposure to a hot climate leads to an expansion of plasma volume and a corresponding fall in the circulating haemoglobin level. The same changes occur in normal pregnancy and are, perhaps, exaggerated in the tropics. Failure of expansion of PV in pregnancy has adverse effects on the foetus. New investigations in Singapore of BV in health, disease and pregnancy seem to be desirable now that people, including pregnant women, move frequently between a hot climate and a cold environment provided by air-conditioning.