Which statement describes what occurs in an infant immediately after birth?

• A. Closure of the ductus arteriosus and adult respiration
• B. Normal hepatic function and bilirubin metabolism
• C. Adult rates of glomerular filtration by the kidneys
• D. Normal water homeostasis

Answer: (A)

Immediately after birth the newborn must switch from placental gas exchange to breathing air and establish separate pulmonary and systemic circulations. The ductus arteriosus, a fetal shunt between the pulmonary artery and the aorta, constricts in response to higher oxygen levels and reduced prostaglandins, leading to its functional closure and a new flow pattern where blood goes to the lungs for oxygenation. At the same time, the lungs expand and the infant begins to breathe in an adult-like manner, establishing independent respiration. Other systems, such as liver bilirubin metabolism, kidney filtration capacity, and fluid balance, are still maturing and do not sync to the immediate birth transition in the same moment.