Coarctation of the aorta is a narrowing of the aorta, the main blood vessel carrying oxygen-rich blood from the left ventricle of the heart to all of the organs of the body.

Coarctation occurs most commonly in a short segment of the aorta just beyond where the arteries to the head and arms take off, as the aorta arches inferiorly toward the chest and abdomen.

This portion of the aorta is called the "juxtaductal" aorta, or the part near where the ductus arteriosus attaches. It is also called the aortic isthmus.

The ductus arteriosus is a blood vessel that is normally present in a fetus and has special tissue in its wall that causes it to close in the first hours or days of life. Coarctation may be caused by the presence of extra ductal tissue extending into the adjacent aorta which results in aortic narrowing as the ductal tissue contracts.

In babies with coarctation, the aortic arch may also be small (hypoplastic). Coarctation may also occur with other cardiac defects, typically involving the left side of the heart. The defects most commonly seen with coarctation are bicuspid aortic valve and ventricular septal defect. Coarctation may also be seen as a part of more complex single ventricle heart defects.

Coarctation of the aorta is common in some patients with chromosomal abnormalities, such as Turner's syndrome.

In the presence of a coarctation, the left ventricle has to work harder, since it must generate a higher pressure than normal to force blood through the narrow segment of aorta to the lower part of the body.

If the narrowing is very severe, the ventricle may not be strong enough to perform this extra work, resulting in congestive heart failure or inadequate blood flow to the organs of the body.