Describe the placental circulation.

Maternal blood reaches the cotyledons via 80-100 spiral arterioles that pierce the decidual plate into the intervillous spaces (containing ~150 mL of blood), draining via endometrial veins.

Fetal blood flows through the villi; the villous surface area is 4-14 m^2, and after the 4th month the capillary wall lies in direct contact with the syncytium, minimising the diffusion distance.

Clinical relevance. The spiral-artery-fed intervillous circulation is the system that fails in pre-eclampsia and placental FGR.

More Detail

Placental circulation

The placenta brings maternal and fetal circulations into close apposition for efficient exchange without mixing.

Maternal side

• Cotyledons receive blood through 80-100 spiral arterioles.
• These pierce the decidual plate and spurt blood into the intervillous spaces, which contain about 150 mL of maternal blood.
• Blood drains via the endometrial (uterine) veins.

Fetal side and exchange surface

• Fetal blood circulates through the chorionic villi bathed in maternal blood.
• The villous surface area is 4-14 m^2 - an enormous area for exchange.
• After the 4th month, the fetal capillary wall lies in direct contact with the syncytium, thinning the membrane and minimising diffusion distance.

Significance

This high-flow, low-resistance, large-surface system supports the metabolic needs of the growing fetus; failure of spiral artery remodelling impairs it (pre-eclampsia/FGR).