1626 



I! Will',' II >k Ml I • I I -i Ml il I >,,\ 



CIRCULATION II 



100 



FORAMEN 

 OVALE 



BODY 



L HEART 



70 



130 



130 



R HEART 



100 



■200- 



4-70- 



v m i/- 



DUCTUS 

 ARTERIOSUS 



fig. 7. To show that both sides of the fetal heart work in 

 parallel; the approximate volume of blood flow through the 

 principal vessels, in the lamb, is indicated in ml/kg/min. 

 (From G. S. Dawes. Changes in the circulation at birth. Brit. 

 Med. Bull. 17: 149, 1 961.) 



difference in saturation in favor of the carotid artery 

 in the sheep (25). Everett and Johnson injected 

 labeled phosphorus into the superior or the inferior 

 vena cava and, from its partition in the left and right 

 atria, were also in favor of the Sabatier hypothesis 

 (91). Evidence which suggests that the upper half 

 of the body may require a better oxygen supply is 

 provided by Spratt who added metabolic inhibitors 

 to the developing chick embryo in vitro; he concluded 

 that the developing nervous system depended pri- 

 marily upon oxidative metabolism, in contrast with 

 the heart which depended chiefly upon anaerobic 

 glycolysis (177). It was also shown, by tissue slice 

 technique in the sheep that the requirements of the 

 brain per gram of tissue increased during the last 

 third of gestation but that the proportional oxygen 

 uptake of the brain per kg of body weight remained 

 constant, at about five times the adult value (53). 



Eranko and Karvonen, however, could find no 

 difference in the number of hemopoietic foci between 

 the lower and upper limb bone marrow of fetuses 

 which might be expected if the oxygen tensions of 

 the two bloods were different (90). Dawes and his 

 colleagues observed that the oxygen content of the 

 carotid artery only exceeded that of the umbilical 

 artery by 6 per cent, when both were sampled simul- 

 taneously, in the lamb (76) and the monkey fetus 

 (71); greater differences were observed following 

 hemorrhage, constriction of the umbilical cord, or 

 hypoxia, especially in young fetuses (68). These ob- 

 servers also approached the problem more quan- 

 titatively by estimating, simultaneously, the oxygen 

 content of the blood in the two venae cavae and, 

 after the two streams have mixed, in the pulmonary 



trunk in the sheep at term. Similar analyses were 

 applied to the three other positions in the fetal cir- 

 culation where blood of differing oxygen content 

 meet, namely the upper part of the inferior vena 

 cava, the left atrium and the junction of the ductus 

 arteriosus with the descending aorta. From these 

 measurements they were also able, by making certain 

 assumptions, to calculate the blood flow in all the 

 principal vessels as a fraction of the cardiac output 

 (fig. 7) : they concluded that the similarity of the oxy- 

 gen content of the blood supplying the upper and 

 lower extremities could easily be accounted for. 

 Measurements of the regional blood flows and oxygen 

 utilizations are needed to prove the hypothesis that 

 the course of the circulation in the fetus is designed to 

 ensure the supply of the most arterial blood to the 

 brain and coronary circulation. 



The blood flow through the various fetal organs 

 and through the placenta will vary both quantita- 

 tively and relatively to one another during growth, 

 and this theme has been well developed by Barcroft 

 (25) and Barron (28). What are the relative propor- 

 tions of the cardiac output which perfuse the fetus 

 and the placenta? What is the magnitude of the pul- 

 monary blood flow during development? Barcroft & 

 Kennedy (24) found the relative distribution of the 

 blood between the fetus and the fetal placental cir- 

 culation in the sheep to change during growth in 

 such a manner that when the embryo was young, the 

 greater part of its blood volume was in the placenta; 

 halfway through gestation, when the placenta had 

 reached its full size, the position was reversed and the 

 amount of blood in the placenta remained constant 

 while that in the fetus increased. The anatomical 

 limit having once been set, the rate of turnover of the 

 blood in the placenta will become increasingly im- 

 portant and the fetal heart does not "keepe holiday" 

 (William Harvey) but has an increasing responsibil- 

 ity to meet the demands of growth; the increase in 

 cardiac output and vasomotor tone will ensure the 

 gradual rise in arterial pressure upon which the 

 umbilical blood flow will depend. Barcroft (25) esti- 

 mated that at least 50 per cent of the combined car- 

 diac output perfused the placenta in the goat and 

 in the sheep near term, and Dawes el al. (76) cal- 

 culated a figure of 57 per cent. How this proportion 

 changes during gestation is not known. Cineradio- 

 graphic observations in the sheep (26) and human 

 infant (129) suggest that the blood flow through the 

 fetal lungs is a small proportion of the combined 

 cardiac output during intrauterine life. Since the 

 development of blood vessels is dependent upon 



