UTERINE BLOOD FLOW 



1609 



fig. 25. Pattern of distribution of radiopaque dye injected 

 into aorta by way of femora] artery in the monkey pregnant 

 for 1 1 1 days. Serial radiographs taken at 3 - ' 2 _ 4, 5, and 6 sec 

 after start of injection. Insert at lower left is of marked out 

 portion of picture above, enlarged 4 times. The arrows indicate 

 spurts of dye in the intervillous space. SA, spiral arteries of 

 endometrium; HA, hypogastric artery; (7.4, uterine artery; 

 RA, renal artery. [Permission of Ramsey f 1 88 ).] 



flow values determined by the electromagnetic flow- 

 meter and the nitrous oxide method were in good 

 agreement. Uterine blood flow increases from 50 ml 

 per min in the 10th week of gestation to 190 ml per 

 min at the 30th week. The flow of blood per unit of 

 uterine tissue was determined to be relatively constant 

 throughout pregnancy. The rate of increase in the 

 rate of blood flow and oxygen consumption of the 

 "uterus" exceeds that of the fetus; it is surmised that 

 the placenta absorbs the difference. 



A number of studies of placental blood flow in 

 women have been reported. Browne & Veal (44) were 

 the first to use the injection of \a- 4 for this purpose. 

 They injected it into the intervillous space of normo- 

 tensive and hypertensive women and estimated in the 

 former a flow of 600 ml per min; in the latter, about 

 200 ml per min. Similar differences were found in 

 the uptake of Na 24 in the myometrium by Johnson & 

 Clayton (1 18). Variations in growth of the conceptus 

 and associated changes in shape of the uterus affect 

 markedly uterine and placental blood flow [Browne 

 (43)]. After fetal death, the placenta cannot be 

 localized by the Na'- 4 method; placental blood flow 

 must decrease substantially. In later pregnancy, the 

 placental flow of the maternal blood exceeds by 

 three times the flow necessary to maintain the fetus. 

 The work of Browne (43) likewise suggests that as 

 maternal blood pressure diminishes in normal pa- 

 tients, placental blood flow increases by some en- 

 hancing mechanism, perhaps the A-V shunt of 

 Burwell. This is presumed to be a protective mecha- 

 nism, analogous to a renal shunt type of mechanism. 

 L^ The intervillous space pressure is equal to that of 

 amniotic fluid pressure or very close to it [Alvarez & 

 Caldeyro Barcia (3), Pryztowski (178), Hellman 

 et al. (102)]. Interestingly, fetal capillary blood 

 pressure in the placenta is considerably higher 

 [Reynolds (202)]. It is probably the association of 

 several factors that permits the escape of fetal blood 

 constituents into the intervillous space, and into 

 maternal blood. One is the high fetal capillary blood 

 pressure just noted. Another is the progressive thinning 

 of the trophoblast layer as the placenta ages, with 

 loss of the cytotrophoblast layer and with the capil- 

 laries coming to lie next to the thin syncytium. Still 

 another is the ever diminishing size of the villi as they 

 increase in number. Combined, these factors permit 

 some escape of fetal erythrocytes into the maternal 

 circulation [Naesland (159), Mengert et al. (151), 

 Bromberg et al. (42)]. Maternal erythrocytes do not 

 normally enter the fetal circulation [Mittelstrass & 

 Horst (155)]- How the exchange of water and other 

 substances occurs between the maternal circulation 

 where the pressure is low and the fetal circulation, 

 where the pressure is high, has been considered 

 theoretically by Wilkin (243, 244). Under certain 

 conditions, simple diffusion occurs; under others, the 

 process depends upon active transport mechanisms 

 [Huggett & Hammond (116)]. 



The connection between maternal uterine blood 

 and amniotic fluid is still a moot question, despite 

 intensive study. That there is a rapid and voluminous 



