Hemodynamics of the Circulatory Transition

The transition of a fetus to a newborn is perhaps the greatest physiological challenge that we must all overcome if we are to survive after birth. Before birth, the future airways of the lungs are liquid-filled, and the lungs play no role in gas exchange (1). Instead, gas exchange occurs across the placenta and the majority of right ventricular output by-passes the lungs and passes through the ductus arteriosus (DA) to enter the descending aorta (Figure 11.1) (2). As much of this blood is directed through the placenta, the right ventricle provides the majority of blood flow through the organ of gas exchange in the fetus, just as it does in the adult (lung) (3). Similarly, due to the presence of the ductus venosus (DV) and foramen ovale (FO), highly oxygenated umbilical venous blood passes directly into the left atrium and left ventricle (2). Thus, the left ventricle in the fetus receives highly oxygenated blood from the organ of gas exchange, just as occurs in newborns and adults (Figure 11.1), and provides the majority of preload for the left ventricle in the fetus, just like it does in the adult (3–5). As the lungs must take over the role of the gas exchange at birth, the circulation has to undergo a massive reorganisation so that the lungs can: (i) Become the sole recipient of blood exiting the right ventricle and (ii) become the sole provider of preload (venous return) for left ventricular output. It is very important not to overlook this second role as it is critical for cardiac function after birth.