Expansion
The placenta is the organ of gas exchange, with three shunts bypassing liver and lungs
Expansion
The placenta is the organ of gas exchange, and the fetus lives at a umbilical venous oxygen tension of only about 4 kPa (30 mmHg).
Compensations that make this adequate:
- Fetal haemoglobin, with a higher oxygen affinity because it binds 2,3-DPG poorly
- Higher haemoglobin concentration, about 170 g/l
- High cardiac output relative to size
- Streaming: the most oxygenated blood from the umbilical vein passes through the ductus venosus and is preferentially directed across the foramen ovale to the left heart, supplying the brain and heart first
- The double Bohr effect: as carbon dioxide moves from fetus to mother, fetal blood becomes alkalotic (raising affinity) and maternal blood acidotic (lowering affinity), both favouring transfer
The three shunts, ductus venosus, foramen ovale and ductus arteriosus, bypass the liver and the fluid-filled lungs, so only about 10 per cent of the combined ventricular output reaches the lungs.
Both ventricles pump in parallel rather than in series, which is why the right ventricle is as thick as the left at birth.