Final answer:
Increased cardiac output after birth is mainly due to decreased pulmonary vascular resistance, allowing more blood to flow into the lungs, and the increased systemic vascular resistance following the initiation of breathing and the closure of fetal shunts.
Step-by-step explanation:
The mechanism of increased cardiac output (CO) immediately after birth involves several factors that are pivotal for the newborn's adaptation from fetal to neonatal circulation. The most critical factor is the decreased pulmonary vascular resistance following the infant's first breath. This first breath inflates the lungs, dramatically decreasing pulmonary resistance and pressure which, in turn, allows for more blood to flow into the pulmonary arteries. As a consequence, blood flow through the foramen ovale into the left atrium diminishes, increasing left atrial pressure and thereby promoting the closure of the foramen ovale. Additionally, the increased oxygen concentration from the initiation of breathing triggers the constriction and eventual closure of the ductus arteriosus. As the pulmonary circulation becomes functional and the umbilical cord is cut, there's also an increase in systemic vascular resistance, redirecting the blood flow through the neonatal circulatory system. These changes collectively result in an increased return of oxygenated blood to the left side of the heart, effectively increasing the cardiac output.