The arterial duct refers to a blood vessel connecting the main pulmonary artery and the aorta, which is normally present during fetal development. It allows blood from the mother's placenta to seep into the pulmonary artery and then enter the aorta to supply the entire body's vasculature.

After the birth of an infant, this blood vessel naturally closes, typically within 3 months, and most people will have it closed by 1 year. If it has not closed after 1 year, it is referred to as patent ductus arteriosus.

Because the arterial duct can connect the aorta and the main pulmonary artery, blood from the systemic circulation in the aorta may directly return to the pulmonary artery at this time. If the pulmonary artery blood flow significantly increases, it can lead to pulmonary arterial hypertension and might also cause right ventricular hypertrophy. Increased cardiac workload can lead to right-sided heart failure and may also cause left-sided heart failure. When severe left-sided heart failure occurs, a right-to-left shunt can develop, which allows pulmonary artery blood to directly enter the aorta, causing venous blood to be delivered directly into the systemic circulation. At this point, differential cyanosis occurs, characterized by bluish discoloration of the upper limbs and mild blueness in the lower limbs.