Final answer:
After birth, the clamping of the umbilical cord prompts a baby to breathe on its own, leading to significant respiratory and circulatory changes as the lungs take over gas exchange from the placenta.
Step-by-step explanation:
When the baby breathes and the umbilical cord is clamped, the newborn uses the lungs for gas exchange, a process where carbon dioxide is exchanged for oxygen. This significant event initiates several circulatory and respiratory adjustments. The clamping of the umbilical cord collapses the umbilical blood vessels, which had been the infant's source of oxygen and nutrition. Consequently, the baby must now rely on its own systems for survival. The first breath inflates the lungs to nearly full capacity, dramatically decreasing lung pressure and opening up the pulmonary alveoli. These changes allow blood to fill alveolar capillaries, and amniotic fluid in the lungs is either drained or absorbed rapidly.
Circulatory adjustments also occur as the umbilical vessels atrophy and become fibrotic remnants -- with only the proximal sections of the umbilical arteries continuing to function to supply blood to the upper part of the bladder. As the lungs take over the task of gas exchange, the closed shunts in the circulatory system reinvent the flow of blood, now directing oxygenated blood to the lungs and liver, as it is in normal postnatal life.