Final answer:
The occurrence that initiates changes in the newborn circulatory system after birth is when the infant begins pulmonary ventilation.
Step-by-step explanation:
The occurrence that initiates the changes in the newborn circulatory system after birth is when the infant begins pulmonary ventilation. When the newborn takes its first breath, the lungs inflate and the circulatory system undergoes significant alterations, closing the three shunts that directed oxygenated blood away from the lungs and liver during fetal life. This breath also decreases blood pressure throughout the pulmonary system and causes the flow of blood to reverse temporarily through the foramen ovale, blocking the shunt with two flaps of tissue.