The heart is a fist-sized muscular organ that contracts and relaxes around 70 times a minute. This keeps blood flowing around the lungs and body, transporting life-giving oxygen and nutrients.

Your heart is a muscular pump divided into two halves, left and right.

Each side of the heart is further divided into two chambers - an upper atrium and a lower ventricle.

Valves prevent backflow so that blood keeps traveling in the correct direction.

A patch of muscle acts as a natural pacemaker, generating an electric signal that makes the heart muscle cycle between contraction and relaxation.

The rhythmic squeezing of the heart pumps blood from the right side to the lungs and from the left side to the rest of the body.

Electrodes can record electric impulses within the heart to produce an electrocardiogram (EKG). Each heartbeat produces a characteristic trace on the EKG display. Its shape comprises 5 phases - P, Q, R, S, and T, each of which is a sign of a particular stage of the heartbeat cycle.

Electric activation of muscle cells makes the atria contract, pushing blood through valves into the ventricles and creating the P wave on the EKG.

The electric signal then passes down the thick, muscular wall between the left and right sides of the ventricles, creating the valley of the Q wave.

The electric message reaches the tip of the ventricles and spreads throughout them. The large R-wave occurs as the powerful ventricles reach peak contraction.

The S wave and flat ST segment occur as the ventricles contract and empty blood. The atrial muscle cells have recharged, ready for the next contraction.

The final T wave of the EKG trace occurs as the ventricular muscle cells recharge or repolarize. The heart rests as the muscle cells get ready for the next contraction.

The heart has a special part called the sinoatrial node, which acts like a pacemaker.

It's a muscle area in the upper right atrium that sends out electrical signals. These signals travel through the heart using special nerve fibers. The heart muscle cells are good at passing these signals quickly, so the heart beats in a specific order – first, the two atria, and then the two ventricles.

The natural pacemaker cells in the heart are a bit "leaky," meaning they let charged particles (ions) flow in and out.

This creates a regular electrical signal that makes the heart beat. The heart muscle cells have fibers that spread these electrical signals quickly to nearby muscle cells.