A myogenic pump
The heartbeat is myogenic — the signal to contract arises inside the heart muscle itself, not from a nerve. It starts at the sinoatrial node (SA node), the natural pacemaker in the wall of the right atrium. The electrical wave spreads across both atria, making them contract, then reaches the atrioventricular node (AV node). From there it travels down special fibres (the bundle of His and Purkinje fibres) to make the ventricles contract from the bottom upwards.
The cardiac cycle
One heartbeat is a cardiac cycle with three stages. During atrial systole the atria contract and push blood into the ventricles. During ventricular systole the ventricles contract, the atrioventricular valves snap shut ("lub"), and blood is forced into the arteries. During diastole the whole heart relaxes, the semilunar valves close ("dub"), and the chambers refill.
Example
The familiar "lub-dub" you hear through a stethoscope is the sound of heart valves closing: "lub" is the atrioventricular valves shutting during ventricular systole, and "dub" is the semilunar valves shutting at the start of diastole.
Valves and heart rate
Valves keep blood moving one way. The tricuspid valve lies between the right atrium and ventricle, and the bicuspid (mitral) valve lies on the left. Semilunar valves guard the exits into the pulmonary artery and aorta. A resting adult heart beats about 60–80 times a minute. The rate is adjusted by the nervous system and hormones such as adrenaline: it rises during exercise, fear or fever, and slows during rest. Each heartbeat sends a pressure wave along the arteries that can be felt as the pulse at the wrist or neck, so counting the pulse is a simple way to measure heart rate. A faster heart delivers oxygen more quickly to muscles that are working hard.
Key idea
The SA node sets the rhythm, valves keep blood one-directional, and nerves and adrenaline speed up or slow the heart to match the body's needs.