1634 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



when the arterial pressure falls following the injec- 

 tion of hypotensive drugs such as acetylcholine and 

 hexamethonium into the fetal circulation (74) and 

 following severe hvpoxaemia of 10 to 1 r , min duration 

 (6 9 )- 



Oxygen Requirements and Environment of the Fetus 



Primarily dependent upon the maternal placental 

 circulation, the fetal heart provides an umbilical 

 blood flow which, under normal conditions, main- 

 tains a steady oxygen consumption of 4 to 6 ml per 

 kg body weight per min in both the lamb during the 

 last half of gestation ( 1 ) and in the human fetus of 9 

 to 28 weeks gestation (17): this represents an oxygen 

 consumption in relation to weight comparable with 

 the adult and the constancy is remarkable in view of 

 the changing oxygen utilization of the various organs, 

 and their varying weights in relation to each other, 

 during development. Huckabee et al. ( 1 1 1 ) point 

 out that without a knowledge of the anaerobic metab- 

 olism of the fetus it is impossible to obtain an accurate 

 estimate of the energy requirements of growth from 

 the quantity of oxygen consumed alone; however, 

 there is no good evidence for anaerobic metabolism 

 in the normal fetus for blood lactate levels are com- 

 parable with the adult (71). Huckabee et al. also 

 point out that the metabolic rate of the fetus, if it 

 were known, is not synonymous with the metabolic 

 rate required for the life and growth of the fetus and 

 the metabolic needs of the placenta must be included. 

 These observers found in the goat, as did Assali et al. 

 (17) in the human, that the oxygen consumption 

 of the pregnant uterus was about 10 ml per kg per 

 min; the calculations were made lrom uterine blood 

 flow and A-V O2 differences. But, while Assali et al. 

 consider the placenta to have a greater oxygen 

 consumption than the fetal tissues, Huckabee et al., 

 from uterine oxygen utilization measurements after 

 fetal death, suggest that this may be the reverse; 

 the latter estimate of fetal oxygen consumption, as 

 approximately 10 ml per kg per min, would agree 

 with determinations of the minimal oxygen consump- 

 tion of the newborn lamb. However, Dawes and 

 Mott have shown that such a high oxygen consump- 

 tion is characteristic of the newborn only and is 

 attained at different ages in the different species; 

 further, they have demonstrated that this increase in 

 oxygen consumption is not dependent upon the raised 

 arterial oxygen saturation following the establishment 

 of respiration for it does not occur in immature lambs 



delivered by Cesarean section and artificially ven- 

 tilated (72). 



What is the oxygen environment of the fetal tissues 

 in utero? Recently, Misrahy et al. (135) have measured 

 the oxygen availability (a0 2 ) in fetal brain and kid- 

 ney, in a number of species under Nembutal anes- 

 thesia. Nondiffusion limited polarographic elec- 

 trodes, too n in diameter, with a circumferential 

 recording surface, 2 mm in width, were inserted into 

 the tissues, with little disturbance of the uterine wall; 

 the aO; ranged between 18 per cent and 30 per cent 

 of the diffusion current in air, corresponding to 30 

 to 45 mm Hg O2 and was similar to the maternal 

 tissue oxygen tensions measured in the same manner. 

 Misrahy el al. consider these readings to represent 

 the rate of oxygen transport between the capillaries 

 and the active cells. The tension of oxygen in the 

 arterial blood of the fetus is, however, probably con- 

 siderably lower than that in the maternal blood. It is 

 difficult to assess the values for arterial oxygen satura- 

 tion in utero for when the uterus is opened and um- 

 bilical vein samples collected the placental circulation 

 is impaired to an unknown extent. Westin (187) has 

 shown, by hysterophotography, that the oxygen 

 saturation of the fetal blood is probably high in 14 

 to 18 week human fetuses for the skin is pink and the 

 umbilical vein arterial in color in utero, and Dawes 

 and his colleagues ( 1 , 39) have observed arterial 

 oxygen saturations as high as 74 per cent in the lamb 

 near term. In the human at term, blood collected from 

 the choriodecidual space by placental puncture has 

 been reported to have a mean pO> of 38 mm Hg; 

 the p()j in umbilical vein blood is probably 10 mm 

 lower (31). These results in the human should, how- 

 ever, be regarded with reserve for there is no means 

 of knowing whether the sample of blood obtained 

 comes from the choriodecidual space or a uterine 

 vein. 



How is equality of oxygen availability to the fetal 

 and adult tissues attained in spite of the low arterial 

 oxygen saturation in the former? The mechanisms 

 appear to be, for the most part, similar to the adult 

 response to low arterial oxygen saturations. First, 

 the fetal blood has a greater affinity for oxygen than 

 the maternal blood; this is a property of the fetal 

 hemoglobin and its environment in the red cells 

 which enables fetal blood to leave the placenta with a 

 greater oxygen saturation than the maternal blood 

 at low oxygen tensions. The factors involved in the 

 transfer of 0> and CO» between the maternal and 

 fetal circulations are clearly outlined by Barron & 

 Meschia (30) and Bartels et al. (31). Second, there is a 



