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Placental Circulation: Maternal-Fetal Exchange Without Blood Mixing

Placental Circulation: The Interface of Two Circulations

The placenta is a hemochorial organ where maternal blood directly bathes fetal chorionic villi, separated only by the syncytiotrophoblast layer. There is no direct mixing of maternal and fetal blood.

Maternal Circulation

  1. Uterine arteries → radial arteries → spiral arteries (remodeled by trophoblasts: low resistance, high flow).
  2. Spiral arteries eject blood into intervillous space (not capillaries) at ~500–700 mL/min at term.
  3. Blood bathes villi → exchanges → drains via uterine veins.

Fetal Circulation

  1. Umbilical arteries (2) → carry deoxygenated, nutrient-depleted fetal blood to placenta.
  2. Branch into chorionic arteries → arterioles → chorionic villi capillaries (exchange site).
  3. Venules → chorionic veins → single umbilical vein → oxygenated, nutrient-rich blood to fetus.
  4. Umbilical vein → ductus venosus (bypasses liver) → IVC → heart.

The Placental Barrier (Maternal-Fetal Interface)

Layers (from maternal to fetal blood):

  1. Syncytiotrophoblast (multinucleated, no intercellular junctions — true barrier)
  2. Cytotrophoblast (early pregnancy; disappears by term)
  3. Basement membrane
  4. Villous stroma (fibroblasts, Hofbauer cells, vessels)
  5. Fetal capillary endothelium

Total thickness: ~2–5 µm at term (vasculosyncytial membrane).

Exchange Mechanisms

SubstanceDirectionMechanism
O2Maternal → FetalDiffusion (ΔPO2 ~30 mmHg). Fetal Hb (HbF) has higher O2 affinity (left-shifted curve).
CO2Fetal → MaternalDiffusion (ΔPCO2 ~5 mmHg). Bohr effect enhances.
GlucoseMaternal → FetalFacilitated diffusion (GLUT1, GLUT3). Insulin-independent.
Amino acidsMaternal → FetalActive transport (system A, L, y+). Concentrated in fetus.
Fatty acidsMaternal → FetalProtein-mediated (FATP, FABPpm). Essential for brain.
ElectrolytesBidirectionalActive transport, channels.
IgG antibodiesMaternal → FetalActive transport via FcRn receptor (peaks 3rd trimester). Passive immunity.
Waste (urea, creatinine)Fetal → MaternalDiffusion.

Clinical Correlates

  • Placental insufficiency: Failed spiral artery remodeling → reduced perfusion → IUGR, preeclampsia.
  • Drug transfer: Lipophilic, low MW, non-ionized, non-protein-bound cross easily. Most antibiotics, opioids cross; heparin, insulin do not.
  • Infections: TORCH pathogens cross via transcytosis or breaks (syphilis, CMV, Zika, HIV).

The placenta is a selective, active barrier—not a passive sieve—orchestrating fetal development.

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