What happens when the baby takes its first breath and the umbilical cord is clamped?

• A. Alveolar fluid is absorbed; pulmonary vessels constrict
• B. Alveolar fluid remains; pulmonary vessels dilate
• C. Alveolar fluid is absorbed; pulmonary vessels dilate
• D. Alveolar fluid remains; pulmonary vessels constrict

Answer: (C)

When the baby takes its first breath, the lungs switch from being fluid-filled to air-filled. The air entering the alveoli helps to clear the lung fluid, which is rapidly absorbed into the surrounding tissue and blood via epithelial ion transport processes; this leaves the alveoli ready for gas exchange. At the same time, the rise in oxygen tension in the lungs causes the smooth muscle in the pulmonary arteries to relax, dramatically lowering pulmonary vascular resistance and increasing pulmonary blood flow to oxygenate the blood. Clamping the umbilical cord removes placental circulation, which shifts circulation toward the lungs, but the key immediate changes for breathing are clear alveoli from fluid and dilation of the pulmonary vessels driven by higher oxygen levels.