BLOOD VOLUME 



59 



conditions being unchanged, the peripheral intra- 

 vascular pressure may remain imaltered, and the 

 blood volume will not necessarily be altered. On the 

 other hand, any increase of venous pressure will 

 increase the peripheral hydrostatic pressure and 

 cause loss of water i'rom the blood. The magnitude 

 of these blood volume changes is determined not 

 only by hydrostatic pressure changes but also by 

 simultaneous colloid-osmotic pressure changes. A 

 primary change of the colloid-osmotic pressure of the 

 blood is not known to occur under normal physiolog- 

 ical conditions. However, in certain pathological 

 conditions a decrease of the plasma protein concentra- 

 tion occurs which may influence the blood volume. 

 Upon a primary change in blood volume, the 

 same regulatory mechanisms may be expected to be 

 called into action, as in a change of the circulatory 

 rate. A sudden decrease of blood volume leads first 

 to a reduction of the amount of blood in the central 

 veins, decreasing the filling pressure of the right and 

 left heart. This may be expected to lead to a decrease 

 of stroke volume and cardiac output, and to falling 

 arterial pressure. However, these effects are largely 

 ofTset by an increase of the pulse rate and constriction 

 of the arterioles. This effect is not observed immedi- 

 ately after a small bleeding in recumbent humans. 

 However, a cardioacceleration could be demonstrated 

 in upright subjects shortly after deprivation of lo 

 per cent of their blood volume. In the recumbent 

 position the acceleration may occur i to 3 days after 

 bleeding {35). The pressure in the aorta is restored by 

 changes in vasomotor tone on the arterial side and, 

 as a consequence, the hydrostatic pressure in the 

 peripheral part of the vascular system falls. Water is 

 then drawn into the blood vessels from the extracel- 

 lular space, tending to restore the blood volume. This 

 compensation for blood loss occurs slowly, however. 

 Studies on man suggest that after withdrawal of 10 

 per cent of the blood volume full compensation for 

 the loss requires i to 2 weeks. 



Factors Influencing the Capacity of 

 the Vascular System 



The blood volume varies secondarih with changes 

 in the capacity of the vascular system, for instance 

 during pregnancy and resulting from pressure changes 

 on the venous side (57). Thus, the volume-pressure 

 relationships in the rest of the vascular system seem 

 to be restored to normal by a compensatory increase 

 of the total blood volume. 



Primary changes of vascular tone may also be 

 expected to occur and be compensated for by the 

 mechanisms which influence blood volume. Such 

 general variations of vascular tone may occur in 

 disturbances of adrenal function. Blood volume 

 variations related to changes in the ambient tempera- 

 ture should al.so be included here. Such volume 

 changes of the vascular system may be expected to 

 influence the blood volume through the same reflex 

 mechanisms as respond to primary variations of 

 blood volume. The change in blood distribution 

 brought about thereby influences the circulatory 

 equilibrium, and hence the cardiac and vasomotor 

 reflex mechanisms are called into action. Some 

 observations also suggest the presence of an active 

 regulation of the capacity of the vascular system — 

 probably mainly by a change in venous tone — which 

 to .some extent may compensate for primary changes 

 of the blood volume. Accordingly, bleeding experi- 

 ments in man seem to show that the efifects on the 

 circulation from changes of body position and from 

 work recede greatly the day after the venesection and 

 before the hemoglobin concentration indicates that 

 the plasma volume increase has made up for the 

 blood loss (35). The total blood volume seems also to 

 remain smaller than normal for a long time after a 

 large blood loss (55). Here variations in the quantities 

 of active vasoconstrictor substances in the blood may 

 be the mediator mechanism (4). The change of 

 vascular tone seems to occur gradualh , however; in 

 bleeding experiments in man it is not noticed during 

 the first hour after the withdrawal of blood. A seem- 

 ingly reflex venous tone change has also been observed 

 in experiments with venous congestion in dogs (2). 



Regulatory Mechanisms 



The intimate relationship between blood volume 

 and blood circulation discussed above suggests that 

 the blood volume may be largely controlled indirectly 

 by hemodynamic factors, and by the regulation of the 

 water balance. Here the different blood pressure- 

 regulating reflex mechanisms play a role, such as the 

 aortic and carotid sinus reflexes, the peripheral and 

 central vasomotor regulation of a nervous, hormonal, 

 and metabolic nature, and the hormonal regulation 

 of renal function. 



It has also been assumed that there would be special 

 blood volume-regulating reflex mechanisms. From 

 the central veins and the left (probably also the right) 

 auricle of the heart, impulses would be discharged 



