992 



SPERM, OVA, AND PREGNANCY 



play an important role in the salt and 

 water metabolism of the gravid female. 

 Furthermore, it is noted that the cow 

 exhibits a hypervolemia but no anemia 

 during pregnancy (Reynolds, 1953) and 

 that a similar condition is produced in the 

 rabbit by the treatment with various com- 

 binations of these steroids. Hence the inter- 

 action of estrogen and progesterone may be 

 responsible for this species difference. 



Inasmuch as no antidiuretic hormone 

 (ADH) could be detected in any of the 

 plasma samples, it is apparent that the 

 plasma titers of ADH did not rise above 

 10 fiV. per ml. during the experimental 

 period. However, in view of the increased 

 ability of the blood to inactivate ADH dur- 

 ing pregnancy (McCartney, Vallach and 

 Pottinger, 1952; Croxatto, Vera and Bar- 

 nafi, 1953), there may be an increased 

 rate of turnover of ADH during gestation. 

 Consequently the data obtained in this 

 study neither substantiate nor eliminate 

 ADH as an etiologic agent in the anemia of 

 pregnancy. 



A number of investigators have attributed 

 the hypervolemia of pregnancy to structural 

 changes in the circulatory system. Burwell 

 (1938) observed a marked similarity be- 

 tween the circulatory changes observed in 

 pregnancy and those observed in a patient 

 with an arteriovenous fistula. He noted that 

 in both conditions there is an increase in 

 the blood volume, cardiac output, pulse 

 rate, pulse pressure, and an increased venous 

 pressure near the opening of the fistula. He 

 concluded that the changes in the circula- 

 tion of the pregnant woman are caused by 

 an arteriovenous leak through the placenta 

 and the obstruction of the venous return 

 by the enlarged uterus. Bickers (1942) cor- 

 related the intensity of the edema of the 

 right or left leg with the location of the 

 placenta in the uterus. The edema was ob- 

 served to be consistently greater on the same 

 side as the location of the placenta whereas 

 the edema of the legs was equal when im- 

 plantation occurred on the anterior or pos- 

 terior wall of the uterus. However, when 

 the uterus was lifted off the interior vena 

 cava, there was no precipitous drop in the 

 venous pressure in the femoral vein. Thus 

 this study supports the arteriovenous shunt 

 theory of Burwell. 



One objection to this theory is that it i 



does not account for the decrease in the i 



blood volume during the 10th lunar month j 



of pregnancy. However, it has been reported i 



that during the latter part of pregnancy 

 there is an increase in the resistance to the 

 flow of blood through the placenta. This in- 

 crease is due to the increased number of 

 villi and to the anastomizing of the villi in 

 the placenta. Since an increase in the pe- 

 ripheral resistance to blood flow results in 

 a hemoconcentration, this would account for 

 the decrease in the blood volume during 

 the last lunar month of gestation (Kline, 

 1951; McGaughey, 1952). 



Other objections to the arteriovenous 

 shunt theory were reported by Kellar ( 1950) 

 who found that blood flow through the 

 placenta is sluggish rather than rapid as 

 in an arteriovenous shunt. He also observed 

 that the uterine venous blood is not ex- 

 ceedingly rich in oxygen as is the venous 

 return of an arteriovenous aneuryism and 

 lie concluded that, although the uterus is 

 an area of decreased resistance to blood 

 flow, the effect is not entirely due to the 

 placenta. He suggested that thyroxine may 

 be partially responsible for the expansion of 

 the blood volume since mild thyrotoxicosis 

 is common in pregnancy. This concept is 

 supported by the observation that there 

 is a tendency for vasodilation in the upper 

 extremities during the latter months of ges- 

 tation (Burt, 1950). Furthermore, the basal 

 metabolic rate increases during this period 

 (Sandiford and Wheeler, 1924; Rowe and 

 Boyd, 1932). It is to be noted, however, 

 that the changes in the blood flow in the 

 extremities are closely correlated with the 

 cardiovascular changes occurring during 

 pregnancy but not with the changes in the 

 hormonal levels in the blood (Herbert, 

 Banner and Wakim, 1954 ) . 



Since there is no disproportionate in- 

 crease in the thiocyanate space of the rabbit 

 during pregnancy, the increase in the blood 

 volume can best be explained on the basis 

 of cardiovascular changes during the latter 

 part of gestation. However, in view of the 

 previous discussion, it is extremely im- 

 probable that this hypervolemia is induced 

 by any one factor. Rather, it is more prob- 

 able that the condition is produced by a 

 multiplicity of factors. On the basis of the 



