Umbilical flow rate

Effects of Varying Umbilical Flow Rate. The fetal circulation of the cotyledon was perfused at flow rates from 0.22-2.6 ml/min in sheep and from 0.05-2.3 ml/min in rabbits. About ten different flow rates were studied randomly in each of five ewes and five rabbits (47). 

In the placenta a volume of oxygen sufficient for fetal needs must diffuse across the membranes from maternal to fetal blood during the short time the two circulations are in close contact. This oxygen transfer is a function of several factors which include uterine and umbilical arterial 02 partial pressures, maternal and fetal placental blood flow rates, the 02 capacity and 02 affinity of maternal and fetal hemoglobin, the diffusing capacity of the placenta, the amount of C02 exchanged, and the vascular arrangement of maternal to fetal vessels. 

Effect of Varying Fetal Placental Blood Flow. The effects of changes in umbilical flow, Qf, are shown in Figure 12. As umbilical flow increases, it can carry more 02, and the end-capillary p02 decreases until finally at infinitely rapid flow rates the po2 approaches umbilical arterial p02. A small end-capillary po2 difference becomes apparent at high values of Qf because of a diffusional limitation. The 02 transfer rate varies almost linearly with Qf over the range from 100-400 ml/min. Above... 

We have tried to circumvent these problems by isolating a cotyledon of the placenta and perfusing its umbilical circulation in situ with blood of known and accurately controlled flow rates and 02 tensions (47). We have also varied maternal arterial Oo tension by administering various concentrations of inspired 02. 

Role of Various Factors in Placental O2 Transfer. These experiments characterize the dependence of 02 transfer and umbilical venous po2 on maternal arterial p02> fetal placental flow rate, and fetal inflowing po2 on O2 exchange in a single cotyledon of the sheep placenta and on fetal placental flow in the rabbit placenta. Each factor was studied individually while the fetal placental circulation was isolated and perfused in situ. The present findings do not apply for an intact fetus whose blood recirculates between peripheral tissues and the placenta because compensations would tend to maintain 02 transfer equal to fetal 02 consumption in this latter instance. The present data take account of changes in only a single variable. 

If it is true that the rate of umbilical blood flow remains constant in spite of spontaneous fluctuations in 02 need and delivery, then there are important consequences for fetal homeostasis as recently discussed by Faber (50). A constant umbilical blood flow would assure that intravascular, hydrostatic pressures would remain constant in fetal placental capillaries, and the balance of maternal-fetal hydrostatic forces determining transplacental water movement would be maintained. The fetus would not gain or lose water as might otherwise happen if umbilical flow and pressure were to vary in response to different fetal 02 needs. The fetus could avoid becoming dehydrated during periods of increased 02 transport. 

After it was determined that changing the linear velocity of the maternal or fetal blood stream did not affect the equilibration pressure, it was possible to determine the maternal to fetal volumetric flow rate ratio that would result in an equilibration pressure which matched experimentally determined values of umbilical vein and artery oxygen partial pressures (37). As the maternal to fetal volumetric flow rate ratio is increased, the partial pressure of oxygen at which equilibration... 

Fetal Volumetric Flow Rate. During pregnancy the umbilical cord of the fetus may become partially or totally occluded if it becomes knotted or compressed against the body of the fetus. A study was made... 

Figure 20. Relation of outflowing (umbilical venous) p02 to the rate of blood flow. Data from five rabbit placentas perfused in situ through an umbilical artery with blood (Hb = 10.7 gram/100 ml) with an average p02 of 10 mm Hg. Maternal arterial p0 averaged 87 mm Hg. The solid curve shows results predicted by a mathematical model using experimental values and assuming a total maternal flow 1 ml/min and diffusing capacity [0.04 ml/(min X mm Hg)].

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