What happens to the pressure in the right atrium after an infant is born and begins pulmonary ventilation?

After an infant is born and begins pulmonary ventilation, the pressure in the right atrium declines. This occurs as the infant takes its first breaths, which leads to the inflation of the lungs. The initiation of breathing decreases the resistance to blood flow in the pulmonary vasculature, allowing more blood to circulate through the lungs for oxygenation.

As the pulmonary blood flow increases, the overall pressure in the right atrium decreases because the right side of the heart experiences less back pressure from the lungs. Consequently, the increased blood flow into the lungs reduces the pressure in the right atrium, facilitating the proper functioning of the cardiovascular system in the newborn. This transition is crucial for separating the fetal circulatory dynamics from the postnatal circulation, particularly in regard to the closure of fetal shunts, such as the foramen ovale and ductus arteriosus.