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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



50 70 90 110 130 150 



fig. 10. Pressures in umbilical veins and arteries at successive 

 fetal ages in the lamb. [From Barcroft (25).] 



ductus venosus may regulate this pressure and thus 

 both placental and hepatic blood flows in this species. 



From figure 10 it can be seen that the pressure drop 

 across the placenta is about 40 mm Hg in the lamb at 

 term and, in comparison, the pressure drop in the 

 systemic circulation is of the order of 60 mm Hg; since 

 about 60 per cent of the cardiac output goes to the 

 placenta the resistance in the fetal placental circula- 

 tion is about half that of the fetal systemic circulation. 

 The resistance in the fetal liver is still lower than in the 

 placenta for the greater part of the umbilical blood 

 flow traverses this organ with a pressure fall of only 

 20 mm Hg. No arteriolar regulating mechanism has 

 been described in the chorionic villi but B0e (37) has 

 demonstrated, in the human placenta, the existence 

 of a shunt mechanism within the villous circulation 

 which may possibly open up during asphyxia and 

 increase the fetal placental reserve. The walls of the 

 capillaries in the chorionic villi have neither smooth 

 muscle nor a nerve supply, but in teased specimens 

 their endothelium has been observed to undergo 

 spontaneous rhythmic movements and to be con- 

 stricted by histamine and acetylcholine (179). 



When the placenta has reached its maximum 

 weight in the lamb, at 80 days gestation, the pressure 

 drop across the placenta is 25 mm Hg; during the last 

 third of intrauterine life this pressure drop only in- 

 creases by a further 15 mm Hg while the umbilical 



140 



50 

 MM HG. 

 AORTIC -UV PRESSURE 



fig. 1 1 . Pressure How curves for the fetal placental circula- 

 tion at 90, 115, and 140 days gestation in the sheep. [From 

 Dawes (69).] 



blood flow is increasing tenfold. From pressure-flow 

 measurements in the umbilical circulation (fig. 11) 

 Dawes concludes that the increase in flow is chiefly 

 brought about by a decrease in placental vascular 

 resistance; at the end of term no further decrease in 

 resistance occurs and the increasing flow is dependent 

 on the rising pressure gradient (69). 



Umbilical Blood Flow 



Umbilical blood flow has been measured in the 

 sheep fetus the most frequently and by a variety of 

 methods. Cooper el at. (56) used the venous occlusion 

 plethysmograph (see fig. 12) and found that the blood 

 flow per kg of fetal weight ranged from 250 ml per 

 min at 60 days gestation to about 1 30 ml per min at 

 term, 147 days. The actual flows and their decrease 

 in relation to body weight are in good agreement 

 with the later observations of Acheson el al. (1) using 

 the same technique but a different breed of sheep 

 (fig. 13). Reynolds et al. (158) made a few measure- 

 ments of the blood flow in the umbilical artery 

 in the lamb by cineangiography and concluded that 

 there was no reduction in relation to body weight at 

 the end of term; this suggests that the fall in blood 

 flow using the plethysmograph may be an artifact 

 due to the greater sensitivity of the umbilical vessels 

 at this time. In the guinea pig the venous occlusion 

 plethysmograph gave values of 45 to 108 ml per kg 

 per min, with no tendency to change as the fetal 

 weight increased (172): the arterial pressure is lower 





