56 



HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION 



shows great constancy in determinations made at 

 intervals of a few months. 



Climate 



Several, partly contradictory, investigations have 

 been published relative to variations of blood volume 

 in connection with climatic changes (12, 34). An 

 analysis of these data seems to show that the blood 

 volume tends to increase slightly in a warm climate 

 and to decrease in a cold climate. This is also con- 

 sistent with the changes observed in experimental 

 studies (6). The variations represent only a small per- 

 centage, however. Since other external factors, such 

 as physical activity, influence the blood volume, the 

 effect of temperature may be masked by the effect 

 of reduced physical activity, for instance in the 

 inhabitants of tropical or subtropical regions. The 

 reverse may be expected in people living in colder 

 regions. Among the Eskimos, it has been found that 

 those working outdoors have larger blood volumes 

 than those living indoors during the coldest part of the 

 year (13). 



Barometric Pressure 



The effect of prolonged hypoxia, as at low baro- 

 metric pressure, has been dealt with in the section on 

 "Arterial Oxygen Content." Several studies have 

 been made on the blood volume of people and animals 

 living at high altitudes (12, 43, 58). At low baro- 

 metric pressure the anoxemia is offset first by an 

 increase of the hemoglobin concentration, and not 

 until marked polycythemia develops is there an 

 additional compensatory increase of the total blood 

 volume. It has been observed both in man and in 

 animal experiments that in a change from sea level 

 to high altitudes or from high to low atmospheric 

 pressure there occurs first a hemoconcentration, thus 

 a decrease of blood volume. 



CIRCUMSTANTIAL FACTORS THAT AFFECT 

 BLOOD VOLUME 



Xutrition 



In excessive nutritional disturbances with changes 

 in the electrolyte-water balance, resulting directly 

 and indirectly from protein deficiency, the body's 

 power for regulation may be affected with consequent 

 changes of blood volume. In cases of malnutrition 



the blood volume seems to decrease at the same rate 

 as the serum-proteins (64). Hematopoiesis in some de- 

 ficient states is influenced more than is the blood 

 volume, and in such cases the proportion of blood 

 cells to plasma \olume is altered. 



Physical Activity 



Many data support the view that the blood volume 

 varies with physical activity, and this question has 

 been partly dealt with in the foregoing paragraphs. 

 The supporting evidence for such variation can be 

 summarized as follows: 



a) Animal species which show high physical activity 

 and performance have essentially larger blood volumes 

 per kg body weight or per calorie of basal metabolic 

 rate than have less physically active animals. 



b) Individuals who are physically active (e.g., 

 athletes) have a larger blood volume than those who 

 are inactive but comparable in other respects. 



c) Blood volume increases during periods of physical 

 training. 



In earlier studies, in which Welcker's technique 

 was used, considerable differences were noted in 

 blood volume per kg body weight among different 

 animal species (21, 22). More recent investigations 

 with the dilution technique have, to some extent, 

 confirmed these observations (see table i). In more 

 physically active animals, such as dogs and horses, the 

 blood volume per kg body weight is up to twice that 

 of less physically active animals, such as tame rabbits, 

 rats, sheep, and cows. The most striking difference is 

 found when rabbits are compared with hares. The 

 hare has been reported to have more than twice the 

 blood volume and about three times the amount of 

 hemoglobin as the tame rabbit (22). 



As the blood volume \aries directly with the body 

 weight and the basal metabolic rate with the two- 

 thirds to three-fourths power of the body weight, 

 the blood volume calculated per unit of basal 

 energy metabolism of small and large animals can- 

 not be directly compared. However, as can be 

 seen from table i, there are great differences in 

 blood volume per unit of basal metabolic rate among 

 animals that cannot be explained by the diflferences 

 in basal energy metabolism per unit of tissue mass. 

 Thus, the dog has greater blood volume per calorie 

 per day than larger animals, like sheep, cows, and 

 ordinarily physically trained men and women. The 

 "warm-blooded" horses (thoroughbred and other 

 racing breeds) also have larger blood volumes per 



