KUHAS-PG-BASIC SCIENCE

Before birth, blood from the placenta, about 80% saturated with oxygen, returnsHto the fetus by way of the umbilical vein. On approaching the liver, most ofthis blood flows through the ductus venosus directly into the inferior vena cava, short-circuiting the liver. A smaller amount enters the liver sinusoids and mixeswith blood from the portal circulation). 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