At birth, the first breath triggers lung expansion. Which of the following cardiovascular changes occurs primarily as a result of the rise in pulmonary blood flow and the fall in pulmonary vascular resistance?

• A Left atrial pressure rises above right atrial pressure, functionally closing the foramen ovale ✓
• B The ductus arteriosus closes immediately due to mechanical distension by increased pulmonary blood flow
• C Umbilical vein dilation increases systemic venous return, raising right atrial pressure to close the foramen ovale
• D Prostaglandin F2α (PGF2α) released from the lungs during first breath constricts the ductus arteriosus directly

Explanation

At the first breath, alveolar oxygen rises, pulmonary vascular resistance falls dramatically, and pulmonary blood flow increases substantially. This large return to the left atrium raises left atrial pressure above right atrial pressure, pushing the septum primum against the fossa ovalis and functionally closing the foramen ovale. The ductus arteriosus (option B) closes primarily in response to rising arterial oxygen tension (not mechanical distension), which reduces local prostaglandin E2 synthesis and triggers smooth muscle contraction; this process takes hours to days. Functional closure occurs within hours, anatomical closure over 2–3 weeks. Option D is incorrect — it is the fall in PGE2 (not rise in PGF2α) that closes the DA.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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