Blood fetal placental

Kuhnert PM, Kuhnert BR, Erhard P. 1981. Comparison of mercury levels in maternal blood, fetal cord blood and placental tissues. Am J Obstet Gynecol 133 209-213. 

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. 

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.

Discussion of Assumptions of the Model. The pattern of maternal to fetal placental flow affects the amount of 02 transferred. It seems unlikely that the simple concurrent system shown in Figure 3 accurately represents the placental capillaries in sheep, a species in which the exchange vessels interdigitate in a nonuniform and complex manner (30, 31). The flow patterns are also complicated in humans where maternal blood enters the intervillous spaces from the base and flows upward and outward past fetal capillary loops in placental villi. Physiological studies also fail to reveal a simple geometric flow pattern. For... 

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). 

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... 

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

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. 

The compensatory relation between DHA and n-6 DPA levels is clearer in placental transport between mothers and fetuses. Crawford et al. (19) showed gradual increases in DHA level from maternal plasma, fetal cord blood, fetal liver, to fetal brain. They designated the increase as a biomagnification of DHA (19). The same pronunciation of chi for blood, milk, knowledge, wisdom, and soul was used in the ancient... 

Placental transfer of trichloroethylene occurs in animals. Trichloroethylene inhaled by pregnant sheep and goats, at levels used to induce analgesia and anesthesia, is rapidly distributed into the fetal circulation, with peak levels occurring approximately 40-50 minutes after maternal exposure (Helliwell and Hutton 1950). The concentration of trichloroethylene in umbilical vein blood was comparable to that found in the maternal carotid artery. 

A small number of fetal cells cross the placental barrier and circulate in the mother s bloodstream. These can be isolated from a sample of the mother s blood using cell-sorting techniques, and DNA can be amplified by PCR for genetic diagnosis. Although still experimental, this technique offers the advantage that there is no risk of fetal loss as a result of the procedure. 

Ergot alkaloids contain lysergic acid (formula in A shows an amide). They act on uterine and vascular muscle. Ergo-metrine particularly stimulates the uterus. It readily induces a tonic contraction of the myometrium (tetanus uteri). This jeopardizes placental blood flow and fetal O2 supply. The semisynthetic derivative methylergometrine is therefore used only after delivery for uterine contractions that are too weak. 

The blood vessels of the fetus and mother are separated by a number of tissue layers that collectively constitute the placental barrier. Drugs that traverse this barrier will reach the fetal circulation. The placental barrier, like the blood-brain barrier, does not prevent transport... 

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