Aerobic activity: Aerobic activity is any kind of movement that makes your muscles use oxygen. Swimming, dancing, and soccer are all types of aerobic activity.

Anesthesia: Method that causes sleepiness and prevents pain during surgery.

Angina: People with angina feel a pain in the chest that means the heart isn't getting enough oxygen.

Angioplasty: This operation opens a blocked blood vessel by using a balloon-like device at an artery's narrowest point. The doctor also may insert a stent, which is a tiny tube that props the vessel open and makes sure blood flows freely.

Aorta: The aorta is the major blood vessel that carries blood away from the heart to the rest of the body.

Aortic stenosis: In aortic stenosis, the aortic valve is stiffened and has a narrowed opening. It does not open properly, which increases strain on the heart because the left ventricle has to pump harder to send blood out to the body.

Aortic valve: The aortic valve is one of two valves in charge of controlling the flow of blood as it leaves the heart. The other is the pulmonary valve. These valves work to keep the blood flowing forward. They open up to let the blood move ahead, then close quickly to keep the blood from flowing backward.

Arrhythmia: An arrhythmia is an abnormal heartbeat. It can cause the heart to pump too fast, too slow, or irregularly, which may lead to shortness of breath, dizziness, and chest pain.

Arteries and veins: If you've ever seen a road map, you probably saw many roads going here, there, and everywhere. Your body has a system that sends blood to and from your body parts. It's called the circulatory system and the roads are called arteries and veins. Arteries, which usually look red, carry blood away from the heart. Veins, which usually look blue, return blood to the heart.

Arteriosclerosis: Arteriosclerosis means the arteries become thickened and less flexible.

Atrial septal defect (ASD): ASD is a hole in the heart wall (called the septum) that separates the left atrium and the right atrium.

Atrioventricular canal defect: This defect is caused by a poorly formed central area of the heart. Typically there is a large hole between the upper chambers of the heart (the atria) and, often, an additional hole between the lower chambers of the heart (the ventricles). Instead of two separate valves allowing flow into the heart, there is one large common valve that might be quite malformed.

Atrium: The two upper chambers of the heart are called the atria. They are the chambers that fill with the blood returning to the heart from the body and lungs. The heart has a left atrium and a right atrium.