A Comprehensive Review of the Physiology of Fetal Circulation and Normal Transition

Fetal circulation is a highly specialized system that sustains intrauterine life by ensuring efficient oxygen and nutrient delivery through placental support. This review explains the embryological history of development of the heart, starting with the formation and looping of the primitive cardiac tube and its differentiation to become mature cardiac chambers and outflow structures1. Coordinated vasculogenesis and angiogenesis form the vascular system, which forms the great arterial and venous pathways that supply hemodynamics in the fetus. Special circulatory shunts the ductus venosus, foramen ovale and ductus arteriosus allow preferential arteriovenous streaming of oxygenated blood to the brain and myocardium bypassing the high-resistance fetal lungs2. The combined output of the cardiac output is quantitatively distributed to emphasize the dominant contribution of the right ventricle and the placenta whereby 8% of output reaches the pulmonary circulation3. The review also outlines the profound cardiopulmonary adaptations at birth, such as lung aeration, the rapid decline in the pulmonary vascular resistance (PVR) and the rise in the systemic vascular resistance (SVR) following cord clamping and sequential closure of the fetal shunts4,5. The oxygen saturation increases gradually during the initial minutes of life, and at eight minutes of term, infants typically reach 90 percent saturation6. These mechanisms are essential in identifying the abnormal transition like persistent pulmonary hypertension and duct-dependent circulations7.