Symposium on Regulation of the Cardiovascular System in Health and Disease

IS THERE any evidence that the blood volume is regulated? Or, put more specifically, is there any evidence that signalling systems, nervous or hormonal, respond to changes in the optimal level of blood volume and bring into play mechanisms that restore the blood volume to optimal levels? By examining what is known of the factors controlling blood volume we may see if any answer can at present be given to this question. Table 1 shows that the average healthy adult man has about 2 liters by volume of red cells (in number about 2 X 1013) and 3 liters of plasma. In a given healthy adult these values remain fairly constant, rarely varying by more than 5 to 10 per cent about a mean value over periods of weeks or months. Table 1 also shows approximate estimates for the volume of the red marrow. It has beenl calculated that the red marrow contains about 7 x 1011 red cell precursors, or approximately 3 per cent of the number of the circulating red cells.2 Many of the precursor cells, however, are a good bit larger than the mature red cell, so that, at a guess, we may take the volumes of the red cell precursors in the bone marrow to be about 5 per cent of that of the eirculating red cells. This volume is only a small fraction of the red cell volume.