The heart is a surprisingly small bundle of muscles, nerves, valves and vessels. The heart beats on its own in response to the body’s oxygen demands during rest or activity. The normal neonatal heart is about the size of a plum and averages 125 beats per minute — up to 200 beats when the child is crying or excited. By adulthood, the heart is the size of a fist and works at a resting rate of 70 to 75 beats per minute.

A labelled diagram of the human heart.

The normal heart has four chambers. Each upper chamber, or atrium, serves as a reservoir to briefly hold blood and release it to the lower chambers, the ventricles. The right ventricle pumps blood to the lungs, where it is cleansed of carbon dioxide and picks up a fresh supply of oxygen. The left ventricle — much thicker than the right — has the tougher job of pumping the blood that returns from the lungs through the body.

Sometimes during fetal development, the heart’s chambers and vessels do not divide and grow properly. About one in every 100 babies is born with some type of heart defect. Cardiac testing helps determine the scope of these defects, many of which can be corrected with surgery, interventional catheterization methods or medical treatment.