At birth, when the neonate takes its first breath, the series of cardiovascular changes that convert fetal to neonatal circulation occur rapidly. Which sequence correctly describes the closure of fetal cardiovascular shunts?

• A Functional closure: foramen ovale first (within minutes, as left atrial pressure exceeds right); ductus arteriosus next (within hours to days, due to O2-mediated prostaglandin reduction); ductus venosus last (closure begins with cord clamping, complete within days) ✓
• B Functional closure: ductus arteriosus first (immediately at birth due to O2 exposure); foramen ovale next (within 24 hours); ductus venosus last (months later)
• C Simultaneous functional closure of all three shunts within seconds of birth, triggered by the rise in arterial PO2 acting on smooth muscle cells
• D Ductus venosus closes first (cord clamping eliminates flow); foramen ovale closes second; ductus arteriosus closes last (months after birth in term infants)

Explanation

At birth: (1) Lung expansion drops pulmonary vascular resistance sharply, increasing pulmonary venous return to the left atrium; this raises left atrial pressure above right atrial pressure, pressing the septum primum against the septum secundum and functionally closing the foramen ovale (within minutes). (2) The ductus arteriosus constricts over hours to days as arterial PO2 rises; high O2 inhibits prostaglandin E2 and promotes endothelin-mediated smooth muscle contraction in the ductal wall; functional closure by 10–15 hours, anatomic closure by 2–3 weeks. (3) Cord clamping immediately ceases umbilical venous flow through the ductus venosus; muscular sphincter closes within 1–3 hours; anatomic closure over weeks. Option D reverses the order of foramen ovale and ductus venosus.

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

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