The heart is a large muscular organ in the chest which pumps blood to the lungs and the rest of the body. It contains four chambers, two large and two small. Each chamber contains a one-way valve which only allow the blood to flow in the proper direction if they are functioning properly. The chambers in rhythm to force the blood through arteries. The two major arteries leading from the heart are the pulmonary artery, which brings deoxygenated blood to the lungs to be reoxygenated, and the aorta, the largest artery in the body, which takes oxygenated blood to the rest of the body.

The heart requires a steady flow of blood and the first arteries to branch off from the aorta are the coronary artery, which supplies blood to the muscles of the heart.

The period of time between successive large contractions of the heart is called a 'beat' and the normal pattern of the contraction of the various muscles in the heart is called Sinus rhythm. In humans, the heart in a healthy person awake and at rests averages between 70-80 beats per minute. However, it can go as low as 50 when a person is asleep, and can rise to 170 or even higher during intense physical activity. A patient's heart rate is an important indicator of their overall health and is often the first thing a doctor checks in time of distress.

When a person's heart rate goes too high, or the rhythm is interrupted for any reason, the heart muscle will often fibrilate, a condition where the muscles fire randomly. This prevents the patient's heart from pumping blood and can lead to cardiac arrest. As such, when this happens, a patient's heart must be defibrillated, often by applying an electric shock which restores the regular rhythm.

In some patients, the patient cannot maintain a regular heart rhythm. In such cases, a pacemaker must be installed, which applies a small shock at a regular interval to keep the heartbeat stable. However, patients with pacemakers cannot change their heart rate and, as such, must avoid intense physical activity.