10.3 Placental blood circulation

• Fetal and maternal blood circulation systems
  • The fetal circulation system
  • The maternal circulation system
• The placental barrier

Fetal and maternal blood circulation systems

The placental circulation brings into close relationship two circulation systems: the maternal and the fetal. The supply of blood to the placenta is influenced by various factors, especially by the arterial blood pressure, uterine contractions, tobacco abuse, medications and hormones. Placental blood flow is increased at term and amounts to 500 ml/min (80% of the uterine perfusion).

The fetal circulation system

The villus capillaries are branches of the umbilical vessels. Fetal blood comes via the two Aa .umbilicales in the villi and leaves the placenta through a single navel vein, the vena umbilicalis. Their supply amounts to approximately 40% of the fetal heart blood volume per minute.
The blood pressure in the arteria umbilicalis amounts to 50 mmHg and the blood flows through finer vessels that cross through the chorionic plate to the capillaries in the villi where the arterial blood pressure falls to 30 mmHg. In the umbilical vein the pressure is 20 mm Hg. The pressure in the fetal vessels and their villus branches always lies over that of the intervillous space. This protects the fetal vessels from collapse (interactive diagram).

Fig. 33
Fetal blood flows via the Aa. umbilicales, where the blood pressure amounts to 50 mm Hg, through finer vessels in the chorionic plate, in order to finally reach the capillaries in the villi, where the pressure sinks to 30 mm Hg.

Note: Due to their low oxygen content, the fetal Aa. umbilicales are shown in blue and the oxygen-rich veins in red.

The maternal circulation system

During the pregnancy the uterine circulation constantly adapts in order to be adequate for the growing metabolic needs of the embryo. Via the spiral arteries (80 -100 mm Hg) that come from the uterine arteries (Aa. uterinae), maternal blood gets into the intervillous spaces in a region delimited by the anchoring villi. Subsequently the blood leaves the intervillous spaces via the uterine veins that are arranged in the periphery of the intervillous space.
The flow of the placental blood amounts to 600 cm3/min and the pressure in the spiral arteries to 70 mm Hg. In the intervillous spaces the pressure falls to only 10 mm Hg .The blood in the intervillous space is exchanged 2-3 times per minute.

Fig. 34
Maternal blood arrives at the intervillous space via arteries that open directly into the intervillous space. At the placental level, it thus finds itself temporarily outside the vessel network.

The placental barrier

The placental barrier is composed of structures that separate the maternal and the fetal blood .The makeup of the placental barrier changes over the course of the pregnancy.

Fig. 35
The villus has an intact syncytio-
and cytotrophoblast layer. In the villus interior there are mesenchymal cells with macrophages and fetal capillaries.

Fig. 36
In the middle third of the pregnancy the capillaries migrate to the villus surface. The cytotrophoblast layer disappears slowly and the syncytiotrophoblast layer becomes thinner.

During the 6th month the nuclei of the syncytiotrophoblast group together in the so-called proliferation knots. The other zones of the syncythiothrophoblast lack nuclei and are adjacent to the capillaries (exchange zones).

Fig. 37
At its thinnest part, the placental barrier is reduced to the nucleus-free syncytiotrophoblast, the merged basal membrane and the endothelium.