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).


Fetal and maternal blood circulation systems

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 - Utero-placental circulation system

  1. Umbilical arteries
  2. Umbilical vein
  3. Fetal capillaries

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 circulation system

Spiral arteries
Uterine veins
Intervillous spaces
Basal plate

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.