58 



HANDBOOK OF PHYSIOLOGY ^-^ CIRCULATION I 



Blood Transfusion 



When blood is administered, the blood volume 

 seems to be reduced by a compensatory decrease of 

 plasma \olume. The blood volume can thus remain 

 constant within certain limits after repeated transfu- 

 sions. The hemoglobin concentration increases along 

 with the increase of blood cell volume. However, 

 more systematic investigations into this question 

 have been made only on patients who received pre- 

 operative blood transfusions, since they were anemic 

 or suspected of having a small blood volume (25). 



Plasma Expander Infusions 



Administration of high molecular substances, as 

 plasma volume expanders, may cause an increase of 

 the total blood volume (12). Infused dextran 

 (Macrodex) has been found to increase the blood 

 volume by as much as 40 per cent in humans (9). 

 With such infusions the plasma volume seems to 

 increase with the increase of the colloid-osmotic 

 pressure and, thus, the concentration of dextran in 

 the blood. The molecular size plays an important 

 role here. Infusion of high molecular dextran solutions 

 does not cause a compensatory diuresis, as infusion 

 of low molecular dextran does in animals which have 

 not been bled (56). The blood volume decreases to 

 the ordinary level at the same rate as the elimination 

 of dextran from the blood. 



REGULATION OF BLOOD VOLUME 



Regulation of the Body^s Ji'ater Balance 



Plasma is a part of the extracellular fluid, the volume 

 of which is related to the intracellular fluid \olume 

 and, hence, to the body's total water content. The 

 blood volume is therefore dependent upon the water 

 balance of the body, on the one hand, and upon the 

 distribution of water between the blood vessels and 

 the extracellular fluid space, on the other. A discus- 

 sion of regulatory mechanisms influencing the blood 

 volume must therefore be based on the regulation of 

 the water balance (34, 51)- 



A detailed description of the normal regulation of 

 the bodv water content falls outside the .scope of this 

 section. It will only be pointed out here that the 

 normal water balance depends primarily upon two 

 factors: /) steady supply of water, and 2) the ability 

 of the kidneys to remove adequate amounts of water 



in the control of the constancy of the salt content of the 

 blood. Any alteration of water intake or of renal 

 output can upset the water balance in a negative 

 or a positive direction and thus lead to a change in 

 blood volume as well. Here the central and humoral 

 regulation of water intake and diuresis, respectively, 

 may play a part, as has been shown experimentally. 

 Under exceptional circumstances a negative water 

 balance may be present, for instance during hard 

 work in the heat, when the blood \olume may be 

 greatly reduced. Under pathological conditions the 

 water balance is often disturbed, as in excessive 

 vomiting, diarrhea, circulatory insufficiency, and 

 renal failure. In the former two conditions the blood 

 volume may, as expected, be reduced; and in the 

 latter two it may be increased. Normally, however, 

 the water content of the body varies only slightly and 

 the blood volume changes that thus occur seem to be 

 very small. Moderate \ariations of the body's total 

 water content, as by the intake of i to 2 liters of 

 water or experimentally induced diuresis, need not 

 cause any measurable changes of blood volume (34). 

 This suggests the presence of an active regulation of 

 water distribution in the extracellular space in such 

 a way that, within certain limits, the blood volume 

 is kept constant. 



Factors Influencing Distribution of Water Between 

 Blood Vessels and Extracellular Space 



The continuous exchange of water between the 

 blood and the extracellular space means that a 

 certain balance of distribution is brought about, 

 conditioned primarily by hydrostatic and colloid- 

 osmotic pressures on each side of the capillary wall. 

 Of these forces, the intravascular hydrostatic pressure 

 ma>' undergo quick changes and may be thought to 

 influence the blood volume \ia nervous and hormonal 

 factors. The reflexes thought to be in\olved here are 

 primarily concerned with the regulation of the blood 

 circulation; thus regulation of blood volume may be 

 expected to be intimately related to vasomotor 

 regulation. 



An increase of the blood flow through the body is 

 as a rule dependent upon an increase of the hydro- 

 static pressure in the peripheral vessels, resulting in 

 a decrease of blood \olume, as during muscular 

 work. ."X decrease of the cardiac output, as during 

 sleep, may be expected to have the opposite effect. 

 During increased arterial pressure as a result of con- 

 striction of arterioles, with cardiac output and other 



