54 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



decrease, as in extreme anemia (31, 36), or may 

 increase as in extreme polycythemia (7, 63). In 

 anemia it is to be expected that the blood volume 

 will tend to increase when the effect of anemia must 

 be offset by an increase of the cardiac output. In 

 polycythemia the blood volume increase can be 

 assumed to occur when the increased viscosity of the 

 blood leads to a redistribution of the blood between 

 the central and peripheral sections of the vascular 

 system. In polycythemia, conditioned by hypoxia, it 

 may be that the hypoxia also causes a demand for 

 increased cardiac output, so that the blood \ olume is 

 adjusted accordingly (see below). 



Arterial Oxygen Content 



As will be further discussed the blood volume 

 increases in prolonged hypoxia in decompression- 

 chamber experiments which lead to maximal polycy- 

 themia, and in which the arterial oxygen content is 

 reduced (58, 61). The same is the case in anemia. On 

 the other hand, in hypoxia of more acute onset, in 

 which the hematopoiesis has not made up for the 

 reduction of arterial oxygen content, the blood 

 volume seems to decrease (12, 62), perhaps because 

 of changes of the circulation (see below). 



Basal Metabolic Rate 



As regards the relationship of blood volume to 

 basal metabolic rate, no studies dealing directly with 

 this question seem to have been published. Indirectly, 

 investigations into the variations of the blood volume 

 in hypothyroidism and hyperthyroidism in man (32) 

 and in animal experiments (47) favor the idea that the 

 blood volume can vary with the basal metabolic rate. 

 This view is also indirectly supported by observations 

 on ground squirrels during dormancy, which showed 

 that the blood volume decreased to less than half, 

 mainly by a decrease of plasma volume (59). 



The total red cell volume is more markedly affected 

 by thyroid activity. In hypothyroidism the cell 

 volume is smaller than normal. This may, however, 

 be explained by the effect of thyroxin on the forma- 

 tion of red cells as well as on cell formation generally, 

 and cannot be taken as evidence of a direct influence 

 of changes in the metabolic rate on hematopoiesis. 



Hormone Levels 



The total blood volume seems to vary rather slightly 

 with the variation of endocrine activity. Hypophy- 



sectomy seems to produce a small reduction of the 

 total blood volume (28), whereas adrenalectomy 

 increases it (30). If AC'.TH is given to hypophysec- 

 tomized rats and hydrocortisone to adrenalectomized 

 rats, the blood volume tends to increase in relation to 

 body weight. The growth hormone does not seem to 

 alter the blood volume in relation to body weight 

 (28). After thyroidectomy (47), as in hypothyroidism 

 in man (32), the blood volume tends to decrease; 

 whereas hyperthyroidism and the administration of 

 thyroid hormone cause an increase of the blood 

 volume. Because these different hormones greatly 

 influence the deposition of fat and cell metabolism 

 generally, it is difficult to draw any definite conclusion 

 about their effect on the vascular system. Administra- 

 tion of these hormones to animals has not been shown 

 to have any significant effect on their blood volume. 

 On the other hand, the blood volume can be indirectly 

 influenced by aldosterone via the renal function. 

 Increased aldosterone production apparently leads 

 to an increase of blood volume. 



Castration of male and female rats does not seem 

 to have any constant and definite effect on blood 

 volume in relation to body weight. By giving testos- 

 terone to normal rats, on the other hand, it seems 

 possible to provoke a small increase of the blood 

 volume absolutely and in relation to body weight 



(29)- 



The endocrine organs influence hematopoiesis, and 

 hence the red cell volume, more markedly than the 

 total blood volume (23). Hypophysectomy, adrenalec- 

 tomy, and thyroidectomy lead to a decrease of the 

 red cell volume. This is explained by the influence of 

 these organs on cellular proliferation and protein 

 metabolism generally. But administration of ACTH, 

 growth hormone, hydrocortisone, and thyroxin to 

 normal animals does not produce an increase of 

 red cell volume. Testosterone, on the other hand, 

 seems to have a stimulating effect on the bone marrow, 

 whereas estrone inhibits hematopoiesis (29). 



Intravascular Pressure 



As a result of intrax ascular pressure \ariations, the 

 volume may change greatly on the venous side of the 

 vascular system. This leads to a disturbance in the 

 distribution of blood with altered pressure/volume 

 ratios in other parts of the vascular system. If it 

 persists, the change seems to be offset by an increase 

 in total blood volume. Under normal conditions, 

 only changes in body position produce marked 

 changes in the pressure-\olume relationship. At the 



