BLOOD VOLUME 



57 



calorie per day than the '"cold-blooded" typed 

 (working breeds). 



Men who participate in sports that require vigorous 

 effort, like running, skiing, and cycling, have been 

 found generally to have larger blood volumes per kg 

 body weight than those ordinarily physically active; 

 more also than those athletes who take part in more 

 static muscular training, like wrestling and weight- 

 lifting (see table 2). Women trained in sports inxohing 

 great physical effort, like skiing, also ha\e been found 

 to have essentially larger blood volumes than ordi- 

 narily found (46). Determinations of the blood volume 

 of blind women whose handicap has forced them to 

 lead physically inactive lives showed lower values 

 than those in the axerage woman. This is also the 

 case when the blood volume is calculated on the 

 basis of body height, as mentioned above. 



In experiments on dogs it has been found that 

 physical exercise increases the blood volume (18). 

 The same has been observed in man, in a series of 

 neurotic patients, and in a series of healthy subjects 

 who were put through systematic exercises (40, 41). 

 Even a short period of intensive exercise with daily 

 skiing has been found to cause a significant increase 

 of blood volume. 



The larger blood volume per kg body weight in 

 more physically active animal species and humans, 

 and the increa.se of blood volume during physical 

 training could i)c attributable, as has been pointed 

 out above, to an increase of the metabolically more 

 active bod\' mass at the expense of metabolically 

 inactive tissue. However, the blood \olume/basal 

 metabolic ratio is larger in physically active animals 

 than in the inactive but otherwise comparable 

 animals. A study of the proportion of blood \olume to 

 body height shows the same difference among groups 

 of people with varying physical activity, as the blind, 

 ordinarily active individuals, and athletes. It seems 

 probable tlierefore that larger blood volumes in 

 physically active individuals are associated with an 

 adjustment of the vascular system to meet the circula- 

 tory requirements determined by external conditions. 



Stress 



The blood volume under conditions of work and 

 hypoxia is dealt with elsewhere in this chapter. .Studies 

 of the blood volume under other stress conditions 

 seem to have given ambiguous results. During an 

 acute stress reaction with raised arterial pressure and 

 increased adrenaline secretion some reduction of the 



blood volume should be expected. But such a reduc- 

 tion has not been unequivocally demonstrated in 

 animal experiments (34). Chronic stress conditions 

 can be expected to be attended by minor changes in 

 blood volume in association with disturbances in the 

 function of the thyroid and the adrenals (see above). 



EXPERIMENT.'^L SITUATIONS THAT .EFFECT 

 BLOOD VOLUME 



Body Position 



In short-term experiments it has been shown that 

 the blood \olume decreases in the erect position and 

 increases with a change to the recumbent position 

 (8, 66). The observation is explained by the changes in 

 the hydrostatic capillary pressure resulting from 

 changes of posture (see below). After prolonged 

 positional changes a compensation seems to occur 

 (67). Besides the hxclrostatic pressure changes, the 

 reduced recumbent physical activity may be a con- 

 tributory factor here. 



Phvsical Work 



During muscular work the blood volume decreases 

 by a reduction of the plasma volume [for references, 

 see (39)]. The decrease occurs principally in the 

 first I o to 15 min after onset of work and seems to be 

 related to the intensity of work. The effect is attribut- 

 able to an increase of pressure in the capillaries on 

 exertion. This acute response is not to be confused 

 with the opposite long-term effect of habituation to 

 muscular work. 



Bleeding 



Normally, the decrease of blood volume after a 

 hemorrhage is compensated for by an increase in 

 plasma volume. This occurs by a shift of fluid from 

 the extracellular space (14, 17). The degree and rate 

 of compensation depend on the fluid balance and on 

 the extent to which the lost interstitial fluid is replaced 

 by intake of water. A 10 per cent blood loss in human 

 experiments was not fully compensated even a week 

 or more after the venesection (35, 50). After profuse 

 bleeding the compensation seems to be incomplete 

 and the blood volume remains smaller than normal 

 for some time (55). 



