Fetal and transitional circulation

Fetal and transitional circulation

Fetal and transitional circulation: Since the fetal lungs are not functional, the respiratory gas exchange occurs in the placenta. Blood flow to the lungs is very little in the fetal circulation and the pulmonary vascular resistance is very high. Blood from the placenta with oxygenation leaves by the umbilical veins and gets shunted by the ductus venosus in the liver. Thus the blood reaches the inferior vena cava and into the right atrium. From the right atrium, most of the oxygenated blood is shunted across the foramen ovale into the left atrium. Blood from the left atrium reaches the left ventricle, with a minimal contribution from the pulmonary veins with scanty blood flow. Thus the oxygenated blood is pumped out of the left ventricle into the aorta and hence to the brain and upper extremities. Blood from the superior vena cava reaching the right atrium is streamed towards the right ventricle and into the pulmonary artery. Most of this deoxygenated blood gets shunted across the ductus arteriosus into the descending aorta as there is no role for the lungs in oxygenation and hence very little pulmonary blood flow. The descending aorta thus has less saturated blood which reaches the placenta through the umbilical artery. The lower extremities also get the less saturated blood. But the fetal circulation ensures that the developing brain gets a higher saturation of oxygen. Fetal hemoglobin has a higher oxygen binding affinity than the adult hemoglobin and oxygen dissociation curves are different.

Transition from fetal circulation after birth

Several changes occur after birth during the transition from the fetal circulation. The umbilical cord is tied and the umbilical arteries and vein become non functional. Pulmonary vascular resistance drops markedly from the first breath and continues to fall for weeks as the musculature of the pulmonary vessel regress. Hence pulmonary blood flow increases markedly. Right atrial pressure decreases and left atrial pressure increases, leading to closure of the valvular foramen ovale which permits only blood flow from the right atrium to the left atrium. The increase in the oxygen levels acts as a strong stimulus for the closure of the ductus arteriosus. Ductus venosus also closes. As a consequence of the decrease in pulmonary vascular resistance and fall in right sided pressures, right ventricle starts regressing. The levels of fetal hemoglobin also gradually drops.