UTERINE BLOOD FLOW 



l6l I 



pregnant uteri have also been made. Evidence shows 

 that the blood enters the intervillous space of the 

 placenta in spurts and diffuses into relatively localized 

 areas where, circulating about the placental villi, 

 it leaves the spaces by nearby veins. However, simul- 

 taneous blood samples from different parts of the 

 intervillous space yield the same blood oxygen levels 

 [McGaughey et a!. (148)]. There is no appreciable 

 Spanner type of circulation toward the chorial plate 

 and then to the margin of the placenta where it is 

 carried off through a marginal sinus. The latter 

 structure does not, in fact, exist. There are veins that 

 drain various parts of the margin of the placenta but 

 veins also drain the basal plate and the septa as well. 



The number of vessels supplying the placenta has 

 received recent attention. The number of arteries 

 emptying into the placenta in late pregnancy per unit 

 area is less than at an earlier time [Boyd (41)]. The 

 number of arterial openings into the human inter- 

 villous space is about 300 for 25,000 mm- at term and 

 about 1 20 for 6000 mm 2 in the fourth month. More- 

 over, the lumens of the arterioles are much reduced in 

 size by an accumulation of intimal tissue near the 

 orifice [Ramsey (184)]. In elephantulus, each pla- 

 centa is supplied by three small arterioles [van der 

 Horst (114)]. The number of veins draining the 

 placenta is about double that of the arteries [Ramsey 

 (182, 186)]. Radiographic (serial) studies of the 

 entry of blood into the placenta are reported in 

 monkeys [Ramsey et al. (188)] and women [Borell et 

 al. (38, 40), Fernstrom (79), Hormann (111), Hart- 

 nett (97)]. 



The question of the pathway of maternal bood 

 flow vis-a-vis the fetal blood flow in the hemochorial 

 placenta has received consideration. Barcroft & Bar- 

 ron (18, 19), Wimsatt (248), and Mossmann (158) 

 incline to the view that incoming maternal arterial 

 blood (oxygenated) encounters incoming fetal blood 

 (reduced) and, running parallel to the point at which 

 the streams part, the maternal blood gives up oxygen 

 to the fetal blood along the way. Noer (161) has 

 shown this to be true in an artificial model when 

 acid ions, dyes, and dextrose are used. To apply the 

 principle of countercurrent flow to the hemochorial 

 placenta, as Spanner (224, 225) has done, is in error, 

 as a number of observations show [Stieve (226), 

 Ramsey (183, 185), Fernstrom (79), Borell et al. (39), 

 Hilleman (107), Kladetzky-Haubrich (129)]. It does 

 not apply in the labyrinthine placenta of the rat 

 [Hamilton & Boyd (92), B0e (32, 33)] or hamster 

 [Adams & Hilleman (1)], or in the placenta of the 

 sow [Amoroso (4)]. 



Extensive studies of the arrangements of fetal 

 cotyledons and of the blood vessels within them have 

 been made. The gross vascular arrangement in the 

 hemochorial placenta shows the cotyledon to be a 

 tuft, arising from a single stem artery. It sends an- 

 choring branches to the basal plate. Free villi are 

 given off from the anchoring villi and from recurrent 

 free villi that pass toward the chorionic plate from 

 the anchoring sites [Wilkin (244), Crawford (54)]. 

 Gross relations are described by Falkiner (75), Earn 

 & Nicholson (68), Crawford (56), ten Berge (232), 

 Thoyer-Rogart & Harris (235), Vernete & Esteba- 

 Caballera (236), Lemtis (135), Danesino (60, 61), 

 and La Have (133). One author claims the fetal 

 vessels are densest near the decidual plate [ten Berge 

 (232)], but this is denied by Beker & van Steenis (29) 



Vasculature of human chorionic villi 

 (after BfJe) 



Superficial capillary network 



Paravascular network 



fig. 28. Schematic representation of major and minute 

 vessels in villus of human placenta. [Permission of B0e (33).] 



