[622 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



circulations. The "thinner" placentas start with 

 six cell layers between the maternal and fetal blood, 

 as in the epitheliochorial placenta, and there is a 

 progressive breakdown of tissue during development 

 which chiefly involves the three maternal layers. 

 Figure 2A and B shows that, even after the number 

 of cell layers has been determined in the human 

 placenta, there is a further reduction in depth of the 

 remaining tissue. The thickness of this placental 

 "barrier" will be one of the factors influencing the 

 rate of transfer of substances, such as the respiratory 

 gases, water, and electrolytes, dependent upon 

 simple diffusion for their transfer; but most of the 

 exchanges between the mother and fetus will take 

 place by active transport, and the composition of the 

 cytoplasm rather than the depth of the barrier is 

 likely to be the more important factor (63, 143); 

 the evidence suggests that placental tissue has a high 

 oxygen consumption, 10 ml per kg per min, probably 

 higher than the fetus itself (17, 111). Histochemical 

 techniques have identified the cytoplasmic content of 

 the cells with a variety of proteins, enzymes, lipids, 

 and carbohydrates (8, 190); the cytoplasmic structure 

 is transient, and changes as the functional capacity of 

 the fetal metabolic processes develop during gestation 

 (181). The varying structure of the barrier in different 



fig. 3. Arrangement of blood vessels in the placentome of 

 the sheep placenta. [Redrawn by Amoroso (8) after Barron.] 



species has been a constant target for speculation and 

 probably represents adaptations concerned with 

 differences in intermediary metabolism and the re- 

 quired rate of growth of the fetus. 



In the hemochorial placenta, in which the maternal 

 endothelium is absent, vesicles of maternal plasma 

 may be transported across the trophoblastic cells into 

 the fetal blood stream; these vesicles are formed by a 

 fusion of the microvilli of the syncytium, or pinocyto- 

 sis (128), and probably enable the transfer of whole 

 protein molecules, possibly those responsible for the 

 passive immunity of the fetus (22, 64). Other special 

 mechanisms occur in the pregnant uterus for trans- 

 ferring materials to the fetus; the fetal membranes in 

 the rabbit are able to transfer immune proteins, se- 

 creted by the uterine glands (46) and the endometrial 

 cups of the pregnant mare secrete gonadotrophin (9). 



Finally, the functional capacity of any placenta 

 will also depend upon the maternal and placental 

 blood flows. In most animals the number of chorionic 

 villi and the placental weight increase rapidly after 

 implantation and reach a maximum while the 

 fetus is differentiating and before the major increase 

 in weight gain (25). The opportunity for exchange 

 between the two circulations will be limited by the 

 efficiency of these chorionic villi, the disposition of the 

 maternal and fetal blood vessels in relation to each 

 other, and the blood flows on either side of the pla- 

 cental barrier. Bumm (48) suggested that a counter- 

 current flow mechanism might exist to facilitate ex- 

 change across the barrier in the human placenta, and 

 Mossman (137) demonstrated that suitable anatomical 

 arrangements of the blood vessels were, in fact, to be 

 found in the ground squirrel and in the rabbit. Figure 

 3 shows the probable direction of the two blood 

 streams in the sheep; fetal blood, passing through the 

 chorionic vessels has the opportunity of exchanging 

 with the maternal arterial blood before leaving in the 

 umbilical vein for the fetus. Similar arrangements 

 exist in all species with a labyrinthine placenta (8). 

 In the hemochorial placenta of the primate the prin- 

 ciple of countercurrent flow is insured functionally: 

 the maternal arteries enter the intervillous space 

 through funnel-shaped openings, and the blood is 

 projected up to the base of the chorionic villi to ex- 

 change with fetal blood leaving for the umbilical vein 

 (153). There has been much controversy over this 

 circulation through the years but recently elegant 

 radiological demonstrations by Borell et al. (38) in the 

 human, and by Ramsey (153) in the macaque 



