i6o6 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



(Retrohepatic 

 Region ) 



INFERIOR ° 



VENA CAVA 80 



PRESSURE 40 



(Retrouterine - 

 Region) 



80 



Case N. 889 



AMNIOTIC 



FLUID 

 PRESSURE 



FEMORAL 120-(-| 

 ARTERY 



PRESSURE 



80 



to\ 



fig. 19. Blocking of inferior (retrouterine) vena cava with strong uterine contraction. Note 

 fall of arterial blood pressure as venous pressure rises. [Permission of Caldeyro Barcia el al. (51). 

 See fig. 14.] 



The above differences in maternal hemodynamic 

 work are not related to the oxygen utilization of the 

 fetus, as shown by the same authors: 



Uterine oxygen consump- 

 tion per kilogram fetal 

 weight (ml/kg/min) 



Rabbit Human Ungulate 



8.3 7.4 8.9 



The syndesmochorial placenta in terms of uterine 

 work to supply a given weight of placenta appears 

 to be far less efficient than the hemochorial type of 

 placenta. 



On admittedly less sure grounds, observations on 

 human uteri have been made. Techniques of 

 sampling, numbers of subjects, assumptions regarding 

 uterine weights and other uncertainties all contribute 

 to the interpretation of the data, and a number of 

 workers have entered into this uncertain field. These 

 reports are discussed by Metcalfe et al. (152). In 

 their series, 13 single fetus pregnancies were studied. 

 They found the blood flow to be of the order of 500 

 ml per min and the O2 consumption of the uterus and 

 its contents, 25 ml per min. In one twin pregnancy, 

 both values were about doubled. It appears that the 

 fetal load, or drain upon the uterus, may be a kev 



factor in determining how much blood flows to the 

 uterus. 2 This is shown bv the following data: 



If this is for nonidentical twin pregnancies [not stated 

 by Metcalfe et al. (153)], the result is understandable: 

 there are two placentas to be supplied. If there were 

 but one placenta, the results with respect to blood 

 flow are less clear. Romney et al. (213) have added 

 data in the human also. 



With respect to the increase in uterine blood flow, 

 Ramsey et al. (188) have pointed out, it is about half 

 as great as the increase in maternal renal blood flow 

 during pregnancv. 



The problem of the utero-placental circulation as a 



2 Since this review was written, an important paper (H. 

 Wulf. Der Gasaustausch in der reifen Plazenta des Menschrn. 



Z. Geburlshilfe u. Gynakol. 152: 



-134, 1962) with extensive 



literature review on gas exchange in the placenta has been 

 published. While dealing mainly with gas exchange, it discusses 

 the causes (including anatomical and physiological) of utero- 

 umbilical oxygen and carbon dioxide tension gradients. 



