Before birth, blood from the placenta, about 80% saturated with oxygen, returns to the fetus by way of the umbilical vein. On approaching the liver, most of this blood flows through the ductus venosus directly into the inferior vena cava, short-circuiting the liver. A smaller amount enters the liver sinusoids and mixes with blood from the portal circulation (Fig. 11.47). A sphincter mechanism in the ductus venosus, close to the entrance of the umbilical vein, regulates flow of umbilical blood through the liver sinusoids. This sphincter closes when a uterine contraction renders the venous return too high, preventing a sudden overloading of the heart.
After a short course in the inferior vena cava, where placental blood mixes with deoxygenated blood returning from the lower limbs, it enters the right atrium. Here it is guided toward the oval foramen by the valve of the inferior vena cava, and most of the blood passes directly into the left atrium. A small amount is prevented from doing so by the lower edge of the septum secun-dum, the crista dividens, and remains in the right atrium. Here it mixes with desaturated blood returning from the head and arms by way of the superior vena cava.
From the left atrium, where it mixes with a small amount of desaturated blood returning from the lungs, blood enters the left ventricle and ascending aorta. Since the coronary and carotid arteries are the first branches of the ascending aorta, the heart musculature and the brain are supplied with well-oxygenated blood. Desaturated blood from the superior vena cava flows by way of the right ventricle into the pulmonary trunk. During fetal life, resistance in the
Ductus arteriosus.. Superior vena cava Pulmonary vein —.
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