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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



centrally, depending on the distention of these parts 

 of the vascular system (37). Since the blood volume 

 variations will first influence the filling of this low 

 pressure part of the vascular system, it can be expected 

 that the impulses of these vessel-wall receptors will 

 signal variations of blood volume under certain 

 conditions, and that the average frequency of impulses 

 discharged during a prolonged period of time will 

 reflect the degree of filling of the vascular system 

 (26, 38). The effector link in the reflex mechanism 

 would be the pituitary production of hormones for 

 regulation of diuresis. 



The evidence presented in support of this regulatory 

 principle is indirect, and in the author's opinion it 

 tends to show that the body's water balance can be 

 influenced in this way and, hence, indirectly the 

 blood volume, too. In combination with vasomotor 

 reflexes it is very probable, however, that diuresis- 

 regulating reflexes could influence the blood volume 

 and aid its adjustment to the circulation. 



The factors and mechanisms discussed above, which 

 might influence the blood volume, and under various 

 conditions have, in fact, been shown to do so, can be 

 expected to be involved in the regulation of the blood 

 volume, although none can be said to be the one 

 regulatory mechanism. A number of observations 

 also show that this matter of regulation is actually 

 much more complicated than can be intimated by a 

 schematic account (34, 51). Nor has it been pos- 

 sible to demonstrate the existence of a simple central 

 regulatory mechanism, the disturbance of which 

 would provoke extreme changes in blood volume 

 leading to plethora or circulatory failure. 



In the introduction it was pointed out that the 

 blood may be regarded as a tissue filling the vascular 

 system, and that the total blood volume is a factor of 

 paramount importance in the circulatory adjustment 

 to meet varying endogenous and exogenous demands. 

 Looking upon the blood as a tissue we can expect the 

 blood volume to be adjusted to the elementary func- 

 tions of the body, similarly to other organs and tissues, 

 such as tlte liver, kidneys, connective tissue, and 

 mucous membranes. There arc no evidences for an 

 adjustment of these organs and tissues homeostatically 

 by special central regulatory mechanisms. There is 

 no more reason to assume that the capacity of the 

 vascular system, and hence the blood \olume, should 

 be under any such influence. The innumerable 

 factors that constantly influence the various organs 

 and tissues in different ways determine the quantita- 

 tive relationships between them. In this way the 

 blood volume can aLso be expected to be adapted to 



the growth of the organism before inore differentiated 

 mechanisms exist. 



As a factor in the adjustment of the cardiac output 

 to varying external circumstances the blood volume 

 can, to some extent, be compared with the skeletal 

 mu.scles. These are adjusted to the demands of the 

 external circumstances apparently without specific 

 regulatory mechanisms. External conditions are the 

 stimuli that bring on adjustment of the size of skeletal 

 muscle and thus its functional ability, according to 

 the requirements. In the same way, the functional 

 demands upon the circulatory system may be supposed 

 to act as stimuli causing the size of the heart and the 

 vascular system to become adjusted to these require- 

 ments. Between the size and function of different 

 organs and tissues there are, presumably, optimal 

 relationships which determine the structure of the 

 organism under the influence of internal and external 

 circumstances. In the adjustment of the functions of 

 different organs, nervous reflexes play a regulatory 

 role in restoring optimal relationships after a tem- 

 porary disturbance. On the other hand, they do not 

 determine these optimal relations. These are deter- 

 mined by the plan of the organism and the properties 

 of the different tissue structures, which, in turn, 

 determine the particular characteristics of each animal 

 genus. Accordingly, the different reflex mechanisms 

 which influence the body's water balance and which 

 seem to be called into action at a change of pressure 

 in certain parts of the vascular system, as well as 

 some vasomotor reflexes influencing the hydrostatic 

 pressure in the peripheral vessels, may be regarded as 

 mechanisms which act in an adaptive manner to more 

 sudden changes of the blood volume, but which do not 

 themselves determine its size and its adjustment to in- 

 ternal and external circumstances. 



Regiilalinn of Hematopoietic Activity 



The assumption that the blood \olume is regulated 

 primarily by hematopoiesis has not been supported 

 by any evidence. An increase in hematopoiesis under 

 physiological and pathological conditions does not as 

 a rule lead to an increase in total blood \olume but 

 merely in blood cell \olume. Onh' in severe poly- 

 cythemia and in hypoplastic anemia is there a change 

 in the total blood xolume: in both conditions an 

 increase. Hematopoietic acti\ity thus regulates the 

 composition of the blood as regards blood cells and 

 plasma, but not the total blood volume. 



