A single pumping action of the heart is called a heartbeat. A healthy adult heart beats at a rate of 60–80 beats per minute at rest and at up to 200 beats per minute during strenuous exercise. One-way valves inside the heart prevent blood from being pumped in the wrong direction. The rhythmic “lub-dub” sound of the heart is due to the heart valves shutting tightly.
The heart cycle
A heartbeat has three phases. In diastole, the heart relaxes. During atrial systole, the atria contract, and in ventricular systole, the ventricles contract. The sinoatrial node (the heart’s pacemaker) regulates the timing of the phases by sending electrical impulses to the atria and ventricles.
Specialized muscle fibres in the walls of the heart conduct electrical impulses that regulate the heartbeat.
The heart muscle relaxes, and blood flows into the atria from the pulmonary veins and vena cava. Near the end of this phase, the sinoatrial node emits an electrical impulse.
The electrical impulse spreads through both atria. The impulse causes their muscular walls to contract and push blood into the ventricles. By the end of atrial systole, the impulse reaches the atrioventricular node, which is in the right atrium.
The impulse reaches the atrioventricular node, where it is momentarily delayed before it spreads throughout the walls of the ventricles. The impulse causes the ventricles to contract, pushing blood out into the aorta and the pulmonary arteries.
Heart valves consist of two or three cup-shaped cusps (flaps). The cusps consist mainly of collagen, a tough protein, and are covered in endocardium, a thin layer of tissue that lines the inside of the heart and joins the lining of the blood vessels.
Open heart valve
When a heart chamber contracts, the high pressure of the blood inside it pushes open the valve cusps, and blood flows through to the other side of the valve.
Closed heart valve
The pressure of blood on the other side of the valve rises and snaps shut the valve cusps. The closed valve prevents backflow.
Closed pulmonary valve
The pulmonary valve has three cusps with rounded undersides that are attached to the inner wall of the pulmonary artery.