98) .CONCERNING THE FETAL CIRCULATION

[Editor]

a) it is possible for vena caval blood to reach the aorta without going through the left ventricle or atrium
b) the po2 in the descending aorta is less than in the ductus arteriosus
c) the foramen ovale closes at birth because of increased left atrial pressure
d) at birth flow in the ductus arteriosus reverses because of decreased pulmonary resistance
e) umbilical arterial blood has a higher po2 than umbilical venous blood

[Editor]

T –through Ductus arteriosus
F Decending aorta also has better oxygenated blood from the L side of the heart
T
T
F Opposite

[Tonic]

Ductus arteriosus closes within 96 hrs due to increased pO2 and decreased PGE2 levels.

[drvp]

The question is about reversal of blood flow and not anatomic closure of the ductus arteriosus. The flow is reversed at birth due to increased systemic vascular pressure and decreased pulmonary vascular pressure. Functional closure occurs within 24 hrs in a majority of newborns due to constriction of the ductus, the stimuli being exposure to oxygenated blood and reduced PGE2. Histological closure occurs usually in about 3 weeks.