What physiological change occurs in the heart when a baby takes its first breath?

When a baby takes its first breath, several physiological changes occur in the circulatory system related to the transition from fetal to neonatal life. One significant change is that the pulmonary circulation begins to function as the lungs expand and fill with air. This leads to a decrease in resistance in the pulmonary vasculature because the air-filled lungs allow for better blood flow.

As a result of this decrease in resistance, more blood is directed through the pulmonary arteries to the lungs, which affects the pressures in both the right atrium (RA) and the left atrium (LA). With the increased blood flow to the lungs, the pressure in the right atrium decreases due to a reduction in blood returning from the inferior vena cava and the improved oxygenation dynamics.

Simultaneously, as oxygenated blood returns from the lungs, it enters the left atrium, which is now receiving more blood than before. This influx increases the pressure in the left atrium. Thus, the physiological change that occurs during this process is characterized by a decrease in right atrial pressure and an increase in left atrial pressure, corresponding to the transition to normal postnatal circulation.