Pulmonary circulation (fluid-filled lungs)
Great vessels · ~10-25% of CVO PVR high
Collapsed, fluid-filled lungs with hypoxic vasoconstriction offer very high resistance, so only a small share of pulmonary trunk blood passes through the pulmonary arteries. Their venous return is what slightly dilutes the left atrium.
Traced from the start
- Placenta
- Umbilical vein
- Ductus venosusHepatic sinusoids (portal and umbilical mixing)
- Right atrium to right ventricle and pulmonary trunk
- Pulmonary circulation (fluid-filled lungs)
Detail
- Share
- ~8-10% of combined output in classic lamb data; ~15-25% in the human fetus by late gestation
- Resistance
- Very high: hypoxia, fluid-filled alveoli and thick-walled muscular pulmonary arterioles; pulmonary artery pressure is at systemic level
- At birth
- Lung expansion and rising alveolar PO2 (with nitric oxide and prostacyclin) drop resistance by ~80% within minutes; pulmonary flow rises 8-10-fold and the rising left atrial pressure closes the foramen ovale
- Lung fluid
- Cleared by catecholamine-driven epithelial sodium channels during labour and absorbed after the first breaths
When it goes wrong
Elective caesarean section without labour
Less catecholamine surge and less lung-fluid clearance: transient tachypnoea of the newborn with a streaky, fluid-in-fissures chest X-ray, resolving within 24-72 h
Meconium aspiration, sepsis or congenital diaphragmatic hernia
Pulmonary resistance stays high (persistent pulmonary hypertension of the newborn): right-to-left shunting through duct and foramen, a pre/post-ductal saturation gap, treated with oxygen, ventilation and inhaled nitric oxide
Practise this structure
1 question in the bank tagged Ventilation Perfusion.