Fetal Circulation and Congenital Heart Defects
Fetal Circulation
In the fetus gas and nutrient exchange occurs at the_________________, not the lungs. As well, the capillary rich organs such as the lungs and liver are not highly functional and therefore do not require large amounts of blood.
Fetal circulation has three modifications that allow the more efficient use of blood.
The first modification is called the _________________. This is a “hole” through the interatrial septum which acts to connect the right and left atria. The second adaptation is called the __________________. This is a shunt off the umbilical vein that connects the umbilical vein to the vena cava, allowing the bypass of the hepatic portal vein. The third is called the __________________. This is a shunt that connects the pulmonary artery and the aorta, allowing the circulation to bypass the lungs.
The flow of blood through the fetal circulation is as follows:
Staring at the placenta, blood flows through the umbilical _______________ into the fetus, then through the ductus venosus to the vena cava, into the right atrium, through the foramen ovale, into the left atrium, into the left ventricle to the aorta, and out of the fetus and back to the placenta through the umbilical arteries. Not all of the blood flows into the ductus venosus. Some enters the portal vein to supply the liver, and then rejoins the main circulation farther along the vena cava. At the right ventricle, some of the blood does not pass through the foramen ovale, but instead enters the pulmonary artery to supply the lungs, then rejoins the main circulation through the ductus arteriosus.
Neonatal Circulation
At birth the neonate no longer receives oxygen and nutrients from the placenta. Instead the neonate must pump blood to the lungs for it to be oxygenated. Four changes must occur in the circulation for the neonate to supply itself with oxygen. The first change is the closure of the umbilical blood vessels as they are no longer attached to the placenta. The release of ____________________ triggers the closing off of the umbilical vessels. The closure of the umbilical vessels causes an increase in vascular resistance and partially asphyxiates the neonate. This partial asphyxiation triggers the neonate’s ______________ reflex and increases the blood flow to the pulmonary artery.
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