THE FETAL AND NEONATAL CIRCI LATION 



[629 



their assumptions a) that the umbilical blood flow is a 

 constant fraction of the cardiac output during this 

 period of rapid growth, and h) that the umbilical 

 blood flow forms the same proportion of the cardiac 

 output in both the human and in the sheep fetus have, 

 to date, no foundation. 



ARTERIAL BLOOD PRESSURE 



Systemic Pressure 



The rate of increase in systemic arterial pressure 

 during gestation varies among the species and the 

 final values at term correspond most nearly to the 

 requirements of the newly born: for instance, in the 

 helpless newborn of the rat and rabbit the mean pres- 

 sure in the carotid artery is only 30 mm Hg after 21 

 and 31 days of gestation, respectively (49, 70); the 

 newborn kitten and puppy are also born with ar- 

 terial pressures of 30 mm Hg after 67 days (114) 

 while the active guinea pig is born with an arterial 

 pressure of 50 mm Hg after a similar time in utero. 

 Arterial pressures of 60 to 70 mm Hg are observed in 

 the newborn lamb and kid following 147 days ges- 

 tation (25), and in the newborn human babe after 

 an intrauterine life of twice this duration (196). 

 The rhesus monkey has a mean arterial pressure of 

 about 55 mm Hg at birth after 160 days gestation 

 (71). An increase in arterial pressure during intra- 

 uterine life must assist in increasing the umbilical 

 blood flow and the opportunity for exchange between 

 the mother and fetus; however, this is only one means 

 of meeting the increasing demands of growth, and 

 the potentialities of the placental and fetal tissues 

 vary among the species (98). 



The course of the rise in arterial blood pressure 

 during intrauterine life is shown in figure 8 for the 

 lamb. It is impossible to assess the relative parts 

 played by alterations in cardiac output and the de- 

 velopment of vasomotor tone in contributing to 

 these changes. After about 90 days gestation in the 

 lamb, when the arterial pressure rises more rapidly, 

 the heart rate continues to increase but no cardiac 

 output measurements are available; Barcroft's (25) 

 results in the goat suggest that there may be an 

 increase in cardiac output in relation to body weight 

 from go days onward and, as the umbilical blood 

 flow in the lamb decreases in relation to body weight 

 during the same period, the mean body blood flow is 

 probably increased. However, the cardiac output at 

 term is greater per kg of body weight than in the 



adult and the low arterial pressure may be accounted 

 for by a low peripheral resistance: as will be seen, 

 this low resistance is probably due to low tonic ac- 

 tivity of both nervous and chemical regulating mecha- 

 nisms. 



Pulmonary Artery Pressure 



In utero, before the lungs are inflated with air 

 there is no good reason why the pulmonarv vascular 

 resistance should be widely different from the vascu- 

 lar resistance elsewhere in the growing fetus. Ardran 

 et al. (13) find in the lamb that the pressure in the 

 left pulmonary artery is about 5 mm Hg higher than 

 that in the carotid artery, which suggests that the 

 vascular resistance in the lungs before birth is pos- 

 sibly slightly higher than the combined resistance of 

 the fetal tissues and the placenta; this has recently 

 been confirmed by Assali et al. (16). In keeping with 

 these observations are the findings that the thickness 

 of the walls of the two ventricles is approximately the 

 same during development, with a slight preponder- 

 ance of the right over the left, in the lamb and in the 

 human infant at birth (66). 



Development of the Cardiovascular Reflexes and the 

 Responses to Asphyxia and Hormones 



The anatomical pathways for the cardiovascular 

 reflexes are laid down early in development in both 

 the human fetus (44) and in the cat (103), but, as 

 predicted by Barcroft, though the machinery is 

 ready it may not be functional and the stage of 

 gestation at which the cardiovascular reflexes are 



60 



8O 



IOO 



I20 



140 



fig. 8. Systemic blood pressure of fetal lambs, under 

 dialurethane (o) or pentobarbitone (•) anesthesia. [From 

 Dawes (66).] 



