Mechanisms of placental blood low must allow blood to leave syncytiotrophoblast, rather than remaining in contact with villi long enough for adequate exchange. For this, maternal blood enters through the basal plate and is driven high up toward the chorionic plate by arterial pressure before laterally dispersing (Fig. 5-15). After bathing the external microvillous surface of chorionic villi, maternal blood drains back through venous oriices in the basal plate and enters uterine veins. Thus, maternal blood traverses the placenta randomly without preformed channels. The previously described trophoblast invasion of the spiral arteries creates low-resistance vessels that can accommodate massive increase in uterine perfusion during gestation. Generally, spiral arteries are perpendicular to, but veins are parallel to, the uterine wall. his arrangement aids closure of veins during a uterine contraction and prevents the exit of maternal blood from the intervillous space. The number of arterial openings into the intervillous space is gradually reduced by cytotrophoblastic invasion. here are about 120 spiral arterial entries into the intervillous space at term (Brosens, 1963). hese discharge blood in spurts that bathes the adjacent villi (Borell, 1958). After the 30th week, a prominent venous plexus lies between the decidua basalis and myometrium and helps develop the cleavage plane needed for placental separation after delivery.
Both inlow and outlow are curtailed during uterine contractions. BIeker and associates (1975) used serial sonography during normal labor and found that placental length, thickness, and surface area grew during contractions. They attributed this to distention of the intervillous space by impairment of venous outlow compared with arterial inlow. During contractions, therefore, a somewhat larger volume of blood is available for exchange even though the rate of low is decreased. Similarly, Doppler velocimetry has shown that diastolic low veloci ty in spiral arteries is diminished during uterine contractions. Thus, principal factors regulating intervillous space blood low are arterial blood pressure, intrauterine pressure, uterine contraction pattern, and factors that act specifically on arterial walls.
• Breas in the Placental"Barrier" he placenta does not maintain absolute integrity of the fetal and maternal circulations. here are numerous examples of
FIGURE 5-15 Schematic drawing of a section through a full-term placenta. Maternal blood flows into the intervillous spaces in funnel-shaped spurts. Exchanges occur with fetal blood as maternal blood flows around the villi. Infiowing arterial blood pushes venous blood into the endometrial veins, which are scattered over the entire surface of the decidua basalis. Note also that the umbilical arteries carry deoxygenated fetal blood to the placenta and that the umbilical vein carries oxygenated blood to the fetus. Placental lobes are separated from each other by placental (decidual) septa.
traicking cells between mother and fetus in both directions.
This situation is best exempliied clinically by erythrocyte
D-antigen alloimmunization (Chap. 15, p. 301). Fetal cell admixtures likely are small in most cases, although rarely the fetus exsanguinates into the maternal circulation.