Aortic isthmus

The aortic isthmus is a constriction of variable size of the aortic arch just distal to the origin of the left subclavian artery at the site of the ductus arteriosus, behind the ligamentum arteriosum.

In the fetus, this portion of the aorta is partly constricted because of the lack of flow within the aortic sac and ascending aorta. It marks the partial separation of fetal blood flow derived from right and left ventricles.