ABSTRACT
This chapter discusses the mechanisms preparing the fetus to be born, the transition at birth, and the successful adaptation to the air-breathing world. It reviews the respiratory system, the respiratory drive and chemoreceptor role, and the circulatory system including fetal circulation and its changes at birth. The fetal circulation is characterized by high pulmonary vascular resistance (PVR), low systemic vascular resistance (SVR), presence of an additional low resistance vascular bed, and right-to-left shunting via the foramen ovale and ductus arteriosus (DA). Absence of the DA is associated with a high incidence of fetal anomalies and adverse outcomes, including associated malformations, chromosomal aberrations, in utero heart failure, and absence of the portal vein. At birth, with the onset of breathing, PVR dramatically decreases and pulmonary blood flow increases such that the entire right ventricular output goes to the lung, as they assume the function of gas exchange.
