Roughly the size of a clenched fist, the heart pumps blood throughout the body. The circulating blood supplies oxygen and nutrients to the tissues while eliminating carbon dioxide and other waste through the kidneys, liver, and lungs.
The heart executes its task through the combined effort of three components – the structure, electrical system and circulatory system.
The heart has four chambers - the right and left atria, and the right and left ventricles. A muscle wall called the septum divides the two sides of the heart; the right side of the heart drives blood to the lungs while the left side pumps it to the rest of the body.
The heart circulates blood through two circuits: the pulmonary circuit and the systemic circuit.
Blood from the body is collected in the right atrium and is pushed into the right ventricle with a small beat of the upper chamber of the heart. In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart through the pulmonary artery and moves to the lungs. It then returns as oxygenated blood to the left atrium of the heart through the pulmonary vein.
In the systemic circuit, oxygenated blood leaves the heart through the left ventricle to the aorta. From there, the blood enters the arteries and capillaries, supplying oxygen to the body's tissues. Deoxygenated blood returns through the veins to the venae cavae, re-entering the heart's right atrium.
Thus, the heart functions like a two-stage electric pump that circulates blood through the body.
The sino-atrial node is a special cell present in the right atrium. This node generates the first electrical impulse that coordinates the heart to beat in a regular fashion. The SA node is our body’s natural pacemaker; this pacemaker function initiates the electrical impulse which then flows into the atrial walls to the atrio-ventricular node (AV node). This node acts as a junction box between the atrium and the ventricle.
Due to this electric signal, the muscle cells in both atria contract at the same time. The electrical signal at the AV node waits for about one to two tenths of a second to allow the ventricles to fill up with the blood pumped from the atria. The signal then passes through electric bundles in the ventricle walls to allow these chambers to contract, again in a coordinated way and pump blood to the lungs and body.
The valves prevent backflow, keeping the blood flow unidirectional through the heart. The ‘heartbeat’ is in fact an electric beat produced by the SA node around 60-100 times a minute. This normal rhythm of the heart is called as 'sinus rhythm' and can be felt as an external pulse.
The electrical signal or electrical impulse produced by the heart can be recorded by placing electrodes on the chest. This is called an electrocardiogram (ECG or EKG).