A newborn is delivered and takes its first breaths. The paediatric team explains the circulatory transition to a student. Which single event most directly causes the fall in pulmonary vascular resistance at birth?

Answer: B. Lung expansion with increased alveolar oxygen tension

Circulatory changes at birth

At birth the circulation must convert from a parallel fetal arrangement to a series (adult) arrangement. The trigger is the first breath.

Sequence of events

1. The first breath generates negative intrathoracic pressure, drawing in about 80 ml of air to establish functional residual capacity.
2. Lung expansion and increased alveolar oxygen tension cause a fall in pulmonary vascular resistance, allowing blood to flow from the right ventricle into the lungs.
3. Cord clamping stops placental flow, so systemic vascular resistance rises.
4. The rise in SVR reverses flow in the ductus arteriosus; exposure to oxygenated blood and reduced prostaglandin E2 stimulates ductal constriction (usually closed by 24 hours).
5. Increased pulmonary venous return raises left atrial pressure above right atrial pressure, closing the foramen ovale.

Why the first breath is primary

The fall in pulmonary vascular resistance is the initiating haemodynamic change; the downstream shunt closures follow from the new pressure relationships.