ABSTRACT
Postnatally, blood flows serially through the pulmonary and systemic circulations; obstruction to inflow or outflow of either ventricle will thus affect the output of both ventricles. If obstruction is severe, blood supply to the body will be compromised. In the fetus, however, the presence of foramen ovale and ductus arteriosus shunts may permit a normal combined ventricular output to be maintained even if inflow or outflow of either ventricle is severely restricted (see Chapter 10). Thus, with tricuspid or pulmonary valve atresia, venous return is deflected through the foramen ovale to the left atrium and ventricle; blood flow to the lungs is provided from the aorta through the ductus arteriosus. In the presence of mitral or aortic valve obstruction, both pulmonary and systemic venous blood returns to the right atrium and is ejected by the right ventricle into the pulmonary artery; systemic and umbilical arterial blood is provided by flow through the ductus arteriosus. In those congenital malformations with severe obstructions, flow to one ventricle is reduced, while the output of the other ventricle is increased. One functioning ventricle in the fetus is capable of providing blood flow adequate for normal umbilical and fetal body requirements, as evidenced by normal body growth and absence of hypoxia in many of these fetuses.
