THE FETAL AND NEONATAL CIRCULATION 



1633 



Soft rubber 

 seal 



To volume 

 recorder 



Perspex curtain Perspex window 

 hanging into /for direct 



saline to / observation of 



make fluid seal / umbilical vessels 



Pneumatic 

 bag for 

 compression 

 of cord 



Umbilical 

 cord 



Electric lamp 



Tinned brass tank 

 fig. 12. Section through a fetal plethysmograph at the 

 point of entry of the umbilical cord. The umbilical cord lies on 

 a gently curved perspex strip. [From A. D. M. Greenfield. A 

 foetal plethysmograph. J. Phyiiol., London 108: 158 (Fig. 2), 

 1949] 



fig. 13. Umbilical blood flow in the lamb, per kg body 



weight during gestation. [Data of K. E. Cooper el al. (O ) 



G. H. Acheson el al. (• ).] The thin continuous curved line 



indicates the weight increase per cent per day. [From Acheson 

 el al. (1).] 



than in the sheep and the pressure gradient between 

 artery and vein is likely to be smaller and, assuming 

 that the vascular resistances are similar, this will 

 account for the lower placental blood flow. When 

 these umbilical blood flow rates are compared with 

 growth curves it is observed that 5.5 liters of blood 

 are required to lay down 1 .0 g of fetal tissue in the 

 sheep, as compared with only 1.3 liters in the guinea 

 pig, (98). Using an electromagnetic flowmeter Assali 

 et al. (17) found the umbilical arterial flow in nine 

 human fetuses of 12 to 28 weeks gestation to range 

 between 94 and 127 ml per kg per min. It is remark- 

 able that vessels so contractile as those in the cord 

 have yielded, on the whole, reproducible results. 

 Each worker has been most aware of the experimen- 



tal errors involved in his measurements. Dawes & 

 Mott (72) also point out that the venous occlusion 

 plethysmograph has a disadvantage in the present 

 application, for when the umbilical vein is temporarily 

 occluded the return to the heart must be reduced; 

 they found that a velodyne flowmeter, providing a 

 direct measure of flow, inserted into the vein in the 

 abdomen gave results which were higher than those 

 obtained by the plethysmograph. 



The umbilical blood flow may be increased at the 

 end of term in the lamb by reducing the fetal ar- 

 terial oxygen saturation (39). This is probably mainly 

 due to the rise in arterial pressure caused by the re- 

 sponse of the fetal vasomotor center to the altered 

 chemical composition of the blood. Reynolds & Paul 

 (160) found that no rise in umbilical venous pressure 

 accompanied the rise in umbilical arterial pressure 

 and suggested that the tone of sphincter of the ductus 

 venosus was decreased in response to the increased 

 umbilical venous flow. The injection of adrenaline 

 into the femoral or jugular vein of the fetus causes an 

 increase of umbilical blood flow which is proportional 

 to the rise in arterial blood pressure (74) (fig. 14). 

 Isolated umbilical vessels are very sensitive to the 

 vasoconstrictor action of adrenaline and these results 

 suggest that the hormone is destroyed before it reaches 

 the umbilical vessels; no figures are available to 

 show how the hormone influences the resistance in 

 the placenta. The umbilical blood flow is reduced 



100 



80 



60 



40 



20 



§5 



o 



- o 

 o 



.0 

 h E 



"o 



V 



O 



•o 



O o° 



• cfo 



o 



Increase of systemic blood pressure (%) 

 l l I i i i l_ 



20 40 60 



fig. 14. Increase in umbilical blood flow following the in- 

 jection of adrenaline (•) or noradrenaline (O) in the mature 

 fetal lamb. [From Dawes el al. (74).] 



