Fetal Circulation

In normal fetal circulation, there are four unique structures through which blood is shunted to ensure oxygen supply to the fetus. First, blood is oxygenated in the (1) placenta and returns to the fetus through the umbilical veins; a significant proportion of oxygenated blood (40-60%) bypasses the liver via the (2) ductus venosus and enters the right atrium. The blood returning via the umbilical veins joins the blood that returns from the lower half of the fetal body in the inferior vena cava.

The inferior vena cava flow to the heart is separated into two streams owing to the anatomic relationship between the inferior vena cava and the borders of the (3) fossa ovalis of the atrial septum. The most oxygenated blood returning via the umbilical veins is directed by the Eustachian valve and passes directly through the fossa ovalis into the left atrium and then into the left ventricle. This pattern of circulation ensures that most of the well-oxygenated umbilical venous blood returning from the placenta is supplied to the fetus coronary and cerebral circulations. Blood from both the inferior and superior vena cava empties into the right atrium.

During fetal development, the lungs are collapsed and pulmonary resistance is high. Oxygenated blood is provided by the placenta. Blood from the right atrium enters the right ventricle and is ejected into the pulmonary trunk; a large proportion of blood passes through the (4) ductus arteriosus to the descending aorta, with the remainder entering the pulmonary circulation. Blood in the left atrium comes from the pulmonary veins and right atrium (via the foramen ovale), then passes through the mitral valve to the left ventricle.

The left ventricle pumps blood out to the systemic circulation (body) via the aorta. Approximately three-fourths of the blood from the ascending aorta supplies the head and upper limbs, while the remaining one-fourth passes through the ductus arteriosus to the descending aorta to supply the lower body.

Deoxygenated blood is returned to the placenta via the umbilical arteries for reoxygenation. Waste is removed and the blood is then recycled to the fetus through the umbilical veins.

Postnatal Adaptation and Circulation

The first breath taken by a newborn expands the lungs with air. At the same time, this action increases pressure within the alveoli and in turn this causes vasodilation in peripheral pulmonary arteries, which leads to a decrease in pulmonary vascular resistance. As a consequence of this decreased resistance, blood from the right ventricle begins to pass through the pulmonary artery to the lungs instead of through the ductus arteriosus and foramen ovale. Increased blood flow to the lungs and back to the left atrium leads to an increase in left atrial pressure. This pressure pushes the thin septum primum back against the thick muscular septum secundum, functionally closing the foramen ovale and separating the two atria. The foramen ovale typically seals shut within 3 months to become the fossa ovalis. Note that it is estimated that up to 25% of adults do not have complete closures of their fossa ovalis, thus they elicit a patent foramen ovale.

Within 10-15 hours after birth, the increased arterial oxygen saturation that occurs with breathing causes the smooth muscle of the ductus arteriosus to constrict permanently. Fibrosis over the following days consolidates the closure, forming the ligamentum arteriosum.

Clamping of the umbilical cord severs the placental connection which converts the umbilical veins into the ligamentum teres. The distal ends of the bilateral umbilical arteries become the medial umbilical ligaments, while the more proximal ends remain as the superior vesicular arteries present throughout life. The ductus venosus that allowed oxygenated blood to bypass the liver sinusoids constricts to form the ligamentum venosum.