The heart and the cardiac cycle

Structure

Deoxygenated blood enters the right atrium from the vena cava and is pumped by the right ventricle to the lungs along the pulmonary artery. Oxygenated blood returns along the pulmonary vein to the left atrium and is pumped by the left ventricle into the aorta.

The left ventricle wall is thicker because it must generate higher pressure to push blood round the whole body.

• Atrioventricular valves (tricuspid on the right, bicuspid on the left) lie between atria and ventricles.
• Semilunar valves lie at the base of the aorta and pulmonary artery.

The cardiac cycle

1. Atrial systole: atria contract, pushing blood into the relaxed ventricles. AV valves are open.

2. Ventricular systole: ventricles contract. Ventricular pressure rises above atrial pressure, so the AV valves shut. When it rises above the pressure in the arteries, the semilunar valves open and blood is ejected.

3. Diastole: the heart muscle relaxes. Ventricular pressure falls below arterial pressure, so the semilunar valves shut. Blood flows into the atria and, once AV valves open, into the ventricles.

Valves follow pressure

Every valve opens when pressure behind it is higher than in front, and closes when pressure in front is higher. On a pressure graph, the points where the ventricle and atrium lines cross, or the ventricle and aorta lines cross, are the moments valves open or close.

Cycle length = 60 / heart rate. At 75 beats per minute, one cycle lasts 0.8 s. Dissecting a mammalian heart is a recommended practical.

Worked example

A heart beats 75 times per minute. How long is one cardiac cycle?

1. Cycle length = 60 / 75
2. = 0.8 s

Answer: 0.8 s

Check yourself

1. A heart beats 60 times per minute. How long is one cardiac cycle in seconds?
   Show answer

   1 s. 60 / 60 = 1 s.

2. One cardiac cycle lasts 0.75 s. What is the heart rate in beats per minute?
   Show answer

   80. 60 / 0.75 = 80 beats per minute.

3. Why do the atrioventricular valves close at the start of ventricular systole?
   • Pressure in the aorta becomes higher than pressure in the ventricles
   • The atria contract
   • Pressure in the ventricles becomes higher than pressure in the atria
   • The semilunar valves open first
   Show answer

   Pressure in the ventricles becomes higher than pressure in the atria. Higher pressure below the valve pushes it shut, preventing backflow into the atria.

4. When do the semilunar valves close?
   • When the atria contract
   • When ventricular pressure falls below pressure in the arteries
   • When ventricular pressure becomes higher than atrial pressure
   • When blood enters the vena cava
   Show answer

   When ventricular pressure falls below pressure in the arteries. As the ventricles relax, blood in the arteries would flow back, so the valves shut.

5. Why is the left ventricle wall thicker than the right?
   • It receives deoxygenated blood
   • It pumps blood to the lungs only
   • It contains more valves
   • It pumps blood round the whole body, which needs higher pressure
   Show answer

   It pumps blood round the whole body, which needs higher pressure. The systemic circuit needs higher pressure than the short pulmonary circuit.

6. Which vessel carries oxygenated blood into the left atrium?
   • Pulmonary artery
   • Pulmonary vein
   • Vena cava
   • Aorta
   Show answer

   Pulmonary vein. The pulmonary vein returns oxygenated blood from the lungs.

7. During diastole, which statement is correct?
   • The ventricles are contracting
   • The atria are contracting
   • Blood is ejected into the aorta
   • Heart muscle is relaxed and the chambers fill with blood
   Show answer

   Heart muscle is relaxed and the chambers fill with blood. Diastole is the relaxation and filling phase.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.