Placenta
Placenta · Low resistance Fetal 'lung'
The fetal gas-exchange organ: maternal blood in the intervillous space bathes fetal capillaries in the chorionic villi. A very low-resistance bed that receives roughly a third of combined ventricular output via the two umbilical arteries and returns it through the single umbilical vein.
Detail
- Function
- Gas exchange, nutrient and waste transfer, heat loss; the lungs do none of this before birth
- Oxygen gradient
- Maternal intervillous PO2 ~40-50 mmHg; fetal umbilical venous PO2 never exceeds ~30-35 mmHg because of venous-equilibrator flow and placental O2 consumption
- Fetal haemoglobin
- HbF (alpha2 gamma2) P50 ~19 mmHg vs HbA ~27 mmHg; gamma chains bind 2,3-BPG poorly, so HbF loads O2 at low PO2
- Double Bohr effect
- CO2 leaving fetal blood lowers maternal Hb affinity (unloading) while raising fetal Hb affinity (loading) at the same time
- Share of output
- ~30-40% of combined ventricular output at term (up to 40-45% in lamb studies)
- Resistance
- Lowest-resistance circuit in the fetus; clamping the cord removes it and systemic vascular resistance rises abruptly
When it goes wrong
Placental insufficiency (pre-eclampsia, smoking, chronic hypertension)
Asymmetric growth restriction with 'brain sparing': raised umbilical artery resistance, reduced middle cerebral artery pulsatility and increased ductus venosus shunting; a reversed ductus venosus a-wave is a pre-terminal sign
Acute placental abruption or cord compression
Late (abruption) or variable (cord) decelerations on CTG progressing to bradycardia; fetal metabolic acidaemia on cord arterial gas
Practise this structure
2 questions in the bank tagged Oxygen Transport.
A 34-year-old woman has had 8 months of heavy menstrual bleeding. She is breathless climbing one flight of stairs and tired all day. Her conjunctivae are pale.…
Foundations · middle twist
A laboratory technician is found unconscious in a poorly ventilated garage. Arterial blood shows pH 7.39, PaO2 13.0 kPa (reference 10.7–13.3 kPa), and PaCO2 5.1…
Mechanisms of disease · common