Final answer:
The critical physiologic change for a newborn after birth is starting and maintaining breathing, which leads to the closing of fetal shunts in the circulatory system and ensures proper oxygenation of the blood by the lungs.
Step-by-step explanation:
The most critical physiologic change required of the newborn after birth is the initiation and maintenance of respirations. This pivotal action triggers the closure of fetal shunts in the circulatory system, a process that reallocates blood flow to the liver and lungs. Once the lungs inflate, the circulatory system undergoes dramatic changes: the fetal shunts close, ensuring that all blood passing through the pulmonary circuit gets oxygenated by the functional neonatal lungs. The first breath causes a drop in blood pressure in the right side of the heart and lungs, prompting a reverse flow through the foramen ovale, leading to the eventual closure of the shunt. Heating the body is also a significant adjustment for the newborn, utilizing nonshivering thermogenesis by breaking down brown adipose tissue.