BLOOD VOLUME 



55 



same time, however, the hydrostatic pressure in 

 most of the peripheral vessels changes. This may 

 directly influence the exchange of water between the 

 blood and the extracellular fluid space and cause 

 temporary changes counteracting any compensatory 

 increase of blood \olume, and restoring the normal 

 volume-pressure relationships. The suggestion that 

 such a compensatory mechanism pla\s some role 

 under normal conditions is supported by the fact 

 that the blood \olume tends to decrease in the first 

 weeks of confinement to bed (19). Here the reduced 

 physical activity may, however, be a contributory 

 factor. 



Under pathological conditions, on the other hand, 

 particularly cardiac decompensation, consideraiile 

 pressure elevations occur on the venous side that may 

 lead to an increase of the blood volume. During 

 treatment of cardiac insufficiency the blood volume 

 decreases, seemingly in proportion to clinical improve- 

 ment [for references, see (12)]. 



In patients with heart disease without decompensa- 

 tion, the blood volume is not increased as compared 

 with that of healthy persons. On the other hand, in 

 patients with signs of raised pressure in the pulmonary 

 circulation without decompensation, there seems to 

 be an increase of blood volume in comparison with 

 patients who have cardiac symptoms and enlarge- 

 ment of the heart without a rise in pressure in the 

 pulmonary circulation (56). The blood volume varia- 

 tions in cardiac decompensation and during treatment 

 thereof result mainly from variations of plasma 

 volume, although hematopoietic acti\ity also seems 

 to be influenced. The latter may be explained by the 

 fact that the relative anemia caused by the increase 

 of plasma volume stimulates hematopoiesis. It has 

 often been suggested that this is due to a "hypoxic 

 effect" on the bone marrow. But this interpretation 

 has gained no direct support and the hypothesis may 

 be said to be superfluous, since the effect on hemato- 

 poietic activity can be explained by well-known 

 factors. 



Increased Cardiac Output 



An increased load on the circulation without a 

 simultaneous change in metabolic rate occurs in a 

 case of arteriovenous fistula. The blood volume tends 

 to increase, judging from observations in clinical 

 cases, before and after operation on the fistula (53) 

 as well as from experimentally provoked fistulas in 

 dogs (24), the variations being noted particularly in 

 the plasma volume. 



Pregnancy 



A great number of studies have shown that the 

 blood volume increases during pregnancy [for refer- 

 ences, see (27)]. The increase occurs fairly continu- 

 ously up to the 35th week, and thereafter the blood 

 volume remains constant or decreases slightly. Pri- 

 marily, the plasma volume seems to increase and, if 

 hematopoiesis is inhibited by iron deficiency, a rela- 

 tive anemia develops despite the fact that the total 

 hemoglobin concentration and red cell volume may 

 be increased (52). After delivery the blood volume is 

 quickly restored to normal both as a result of the 

 bleeding at delivery and al.so of a plasma volume 

 decrease. 



EXTERN.\L F.ACTORS THAT AFFECT BLOOD VOLUME 



Season 



Minor seasonal variations of blood \olume were 

 observed in a series of five women in the author's 

 laboratory (56). The observations covered several 

 consecutive years and the average difference between 

 the yearly maximum (spring-early summer) and 

 yearly minimum (autumn-winter) was statistically 

 significant. These variations seemed to show a positive 

 correlation with the length of daylight and to be 

 associated with the frequent observation that the 

 concentration of reticulocytes increases in early 

 spring. Observations in studies on seasonal variations 

 in the hemoglobin concentration might also be 

 interpreted in accordance with the above observations 

 on blood volume. These show, in fact, a peak value 

 in the winter (usually in February) and a minimum 

 value in the autumn (usually September or October) 

 with the onset of the decrease between January and 

 April (16). Assuming that the volume of the vascular 

 system increases with the increase in the length of 

 daylight, we may expect primarily an increase of 

 plasma volume and, thus, a decrease of the hemo- 

 globin concentration which stimulates hematopoiesis. 

 And, conversely, with the diminishing length of 

 daylight in late summer and in the autumn the volume 

 of the vascular system, and thus the plasma volume, 

 should decrease with a tendency to an increase in he- 

 moglobin concentration. Observations on changes in 

 the tone of the skin vessels seem to point in the same 

 direction (5). Whether such seasonal variations occur 

 more generally is not known, but, if so, they should 

 be slight in view of the fact that the blood volume 



