The ductus arteriosus connects which two arteries?

• A. Superior vena cava and right atrium
• B. Pulmonary veins and left atrium
• C. Pulmonary trunk and aorta
• D. Aorta and left ventricle

Answer: (C)

In fetal life, the ductus arteriosus acts as a bypass between the pulmonary circulation and the systemic circulation by connecting the pulmonary trunk to the aorta. Blood leaving the right ventricle goes into the pulmonary trunk, but much of it is shunted through the ductus arteriosus into the aorta, because the lungs aren’t used for gas exchange yet. After birth, the lungs expand, and the rise in oxygen plus falling prostaglandin levels cause the ductus arteriosus to constrict and eventually form the ligamentum arteriosum. The other connections describe different parts of circulation (systemic venous return, pulmonary venous return, or normal arterial outflow) and do not form the fetal shunt between the pulmonary trunk and the aorta.