Maternal placental blood flow

Effect of Varying Maternal Placental Blood Flow. Figure 11 shows the dependence of end-capillary p02 on maternal placental flow, Qm. At large flow rates the p02 of end-capillary blood approaches that of the maternal artery. At lower rates of flow the equilibrated po2 value decreases because less 02 is available for exchange. The rate of 02 transfer follows a trend similar to that of end-capillary po2, except that increases in above 300-400 ml/min result in little additional 02 transfer because the rise in mean maternal p02 adds little 02 content to the flat part of the fetal oxyhemoglobin saturation curve. 

Cyclic Changes on Maternal Velocity. During labor the contractions of the uterus create a rhythmical increase and a decrease of the amniotic pressure. Although no specifically determined relationship between amniotic pressure and maternal placental blood flow rate has been shown, it is well known that the maternal blood flow rate is retarded in... 

Although furosemide has embryotoxic properties in some animal species, it has been widely used in pregnant women without any adverse effects. Nevertheless, it should be used with great caution, since hypovolemia can lead to reduced uterine and placental blood flow. Careful monitoring of fetal heart action is necessary. Furosemide passes the placenta and increases fetal urine production. It can also increase acid concentrations in maternal serum, fetal serum, and amniotic fluid, thus masking a useful index for the development of pre-eclampsia (24). Its use in pregnant women should therefore be restricted to the treatment of cardiac failure. 

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. 

Normal values for the various determinants of 02 transfer are necessary for quantitative analysis of the exchange process. Some values— e.g., those for the maternal and fetal arterial 02 tensions, 02 capacities, and 02 affinities—are fairly well defined. Others—e.g., the diffusing capacity and maternal and fetal placental blood flows—are less well determined. 

Consumption. Our model predicts that the maternal and fetal end-capillary p02 difference would be less than 1 mm Hg. Experimentally the uterine vein to umbilical vein (V-v) po2 difference is from 10-15 mm Hg (41,42). Possible explanations for the large V-v difference are placental 02 consumption (22), uneven distribution of maternal and fetal placental blood flows (27), and vascular shunts (25, 26). 

Effects of Changing Maternal and Fetal Placental Blood Flows Together. Figure 13 shows the effects of varying maternal and fetal blood... 

Figure 3. Diagrammatic representation of maternal and fetal placental exchange vessels with uniform dimensions and concurrent flows. As the blood flows along the capillaries, Ot diffuses in a one-dimensional plane from maternal to fetal blood.

Effect of Varying Fetal Arterial 02 Tension. Placental exchange has usually been considered limited by either maternal and fetal blood flows or by diffusion. The present analysis suggests umbilical arterial p02 (Pf) is a third and very important factor, based on the observation... 

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. 

Distributed parameter, nonlinear, partial differential equations were soloed to describe oxygen transport from maternal to fetal bloody which flows in microscopic channels within the human placenta. Steady-state solutions were obtained to show the effects of variations in several physiologically important parameters. Results reported previously indicate that maternal contractions during labor are accompanied by a partially reduced or a possible total occlusion of maternal blood flow rate in some or all portions of the placenta. Using the mathematical modely an unsteady-state study analyzed the effect of a time-dependent maternal blood flow rate on placental oxygen transport during labor. Parameter studies included severity of contractions and periodicity of flow. The effects of axial diffusion on placental transport under the conditions of reduced maternal blood flow were investigated. 

The assumption that maternal blood flows in a cylindrical annulus of capillary size bounded by the surrounding terminal villi requires that the terminal villi be fixed in position. The actual position of the terminal villi and the space between them is highly variable. Many of the spaces between villi are of capillary size. At the point of entry of maternal blood, unattached villi are swept aside, and the space between them greatly enlarged. Also, there are several anatomical features which alter the size of the maternal stream—pools and lakes occur in the areas around the chorion or basalis or margin or septa, as well as placental caverns... 

In 19 healthy parturients undergoing elective cesarean section who were to phenylephrine or ephedrine as a prophylactic infusion supplemented with minor boluses if systolic arterial pressure fell by more than 10 mmHg, both the vasopressors restored maternal arterial pressure effectively [25 J. Ephedrine had no significant effects on Doppler velocimetry but phenylephrine infusion significantly increased the blood flow velocity waveform indices in the uterine and placental arcuate arteries and reduced vascular resistance significantly in the fetal renal arteries. However, healthy... 

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