How the heart normally works

The heart is divided into four hollow chambers, two on the right and two on the left. To pump blood throughout the body, the heart uses its left and right sides for different tasks.

The right side of the heart moves blood to the lungs through vessels called pulmonary arteries. In the lungs, blood picks up oxygen then returns to the heart's left side through the pulmonary veins. The left side of the heart then pumps the blood through the aorta and out to the rest of the body.

Why do heart defects develop?

Doctors know that heart defects present at birth (congenital) arise from errors early in the heart's development, but there's often no clear cause. Genetics and environmental factors may play a role.

How the heart works with an atrial septal defect

An atrial septal defect (ASD) allows freshly oxygenated blood to flow from the left upper chamber of the heart (left atrium) into the right upper chamber of the heart (right atrium). There, it mixes with deoxygenated blood and is pumped to the lungs, even though it's already refreshed with oxygen.

If the atrial septal defect is large, this extra blood volume can overfill the lungs and overwork the right side of the heart. If not treated, the right side of the heart eventually enlarges and weakens. If this process continues, the blood pressure in your lungs may increase as well, leading to pulmonary hypertension.

Atrial septal defects can be several types, including:

• Secundum. This is the most common type of ASD, and occurs in the middle of the wall between the atria (atrial septum).
• Primum. This defect occurs in the lower part of the atrial septum, and may occur with other congenital heart problems.
• Sinus venosus. This rare defect usually occurs in the upper part of the atrial septum.
• Coronary sinus. In this rare defect, part of the wall between the coronary sinus — which is part of the vein system of the heart � and the left atrium is missing.