The cardiac cycle

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The cardiac cycleSpec 1.8

In short

The cardiac cycle is the sequence of events in one heartbeat: atrial systole, ventricular systole and cardiac diastole. Contraction of a chamber raises its pressure and forces blood into the next chamber or artery. Valves open and close passively as pressure differences change, so blood flows in only one direction.

Systole is contraction. Diastole is relaxation. The cycle repeats about 70 times a minute at rest. Valves have no muscle: they open when pressure behind them is higher than in front, and shut when pressure in front is higher.

Atrial systole: the atria contract. Atrial pressure is higher than ventricular pressure, so the atrioventricular valves are open and the last of the blood is pushed into the ventricles. The semilunar valves are shut.
Ventricular systole: the ventricles contract. Ventricular pressure rises above atrial pressure, so the atrioventricular valves shut (the first heart sound). When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is forced out into the arteries.
Cardiac diastole: the ventricles relax and pressure falls. Arterial pressure is now higher, so the semilunar valves shut (the second heart sound). Blood from the vena cava and pulmonary veins flows into the relaxed atria. When ventricular pressure falls below atrial pressure, the atrioventricular valves open and blood flows into the ventricles.

Summary of the cardiac cycle
Stage	Atria	Ventricles	Atrioventricular valves	Semilunar valves
Atrial systole	Contract	Relaxed, filling	Open	Closed
Ventricular systole	Relaxed, filling	Contract	Closed	Open (once pressure exceeds arterial pressure)
Cardiac diastole	Relaxed, filling	Relaxed, filling	Open	Closed

Graph of pressure in kPa against time over 0.8 s for the left atrium, left ventricle and aorta, divided into atrial systole, ventricular systole and diastole, with the points where the atrioventricular valve closes and opens and the aortic valve opens and closes. — Each valve opens or closes when the pressure on one side becomes higher than on the other.

Length of the cardiac cycle

A person’s resting heart rate is 75 beats per minute. Calculate the time for one cardiac cycle.

There are 60 seconds in a minute
Time for one cycle = 60 ÷ heart rate = 60 ÷ 75

Answer: 0.8 s

Exam tip:

When asked why a valve opens or closes, always compare the pressures on each side of it, and say which is higher. For example: 'ventricular pressure is higher than atrial pressure, so the atrioventricular valve shuts'.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

Why do large animals need a heart and circulation?

Large animals have a small surface area to volume ratio and long diffusion distances, and active ones have high metabolic rates, so diffusion alone is too slow. A heart pumps blood in bulk (mass transport) through vessels, delivering oxygen and nutrients to all cells quickly and removing waste.

How are arteries, veins and capillaries different?

Arteries have thick, elastic, muscular walls to withstand high pressure. Veins have thin walls, wide lumens and valves to return blood at low pressure. Capillaries have walls one cell thick and a very narrow lumen, giving a short diffusion distance for exchange between blood and cells.

What happens in the cardiac cycle?

The cardiac cycle has three stages. In atrial systole the atria contract and fill the ventricles. In ventricular systole the ventricles contract, the atrioventricular valves shut and the semilunar valves open to force blood out. In cardiac diastole the heart relaxes and the atria refill.

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