Which of the following would NOT promote transition from fetal circulation to a normal extrau-terine circulatory pattern?

Question

a. Closure of the foramen ovale
b. Constriction of the ductus arteriosus
c. Decreased pulmonary vascular resistance
d. Decreased systemic vascular resistance

Answer 1

ANS: D
Figure 9-9 summarizes the major cardiopulmonary changes that take place during the transition from the fluid-filled lung to an air-filled lung. As the lung expands with air and gas exchange starts within the lung, pulmonary blood PO2 increases, PCO2 decreases, and the pH rises. This results in pulmonary vasodilation, lower pulmonary vascular resistance, and constriction of the ductus arteriosus. This facilitates greater blood flow through the pulmonary circulation. Ductus arteriosus closure is further stimulated by the loss of maternal prostaglandins. The combination of increasing alveolar air content and constriction of the ductus arteriosus promotes progressive im-provement in the matching of ventilation and blood flow, which, in turn, increases the PO2 and decreases the PCO2 of blood leaving the lungs. Cessation of umbilical and placental blood flow, following the clamping of the umbilical cord, causes closure of the ductus venosus and a rapid rise in systemic vascular resistance. The combination of the above events establishes a normal ex-trauterine circulatory pattern.