8.3.2

The Cardiac Cycle

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The Cardiac Cycle

The flow of blood from the lungs to the heart and around the body is called the cardiac cycle. The cardiac cycle is the continuous relaxing and contracting of the heart. The steps involved are:

1) Atrial contraction

1) Atrial contraction

  • Blood from the lungs flows into the left atrium and blood from the body flows into the right atrium simultaneously.
  • The atria contract, increasing the pressure in the atria.
  • The blood in the atria is forced into the ventricles.
  • The ventricles are relaxed and fill with blood.
2) Ventricular contraction

2) Ventricular contraction

  • The atria relax and the ventricles start to contract.
  • Contraction of the ventricles causes the pressure inside the ventricles to increase.
  • The pressure shuts the atrioventricular valves so that blood does not flow back into the atria.
  • The blood in the ventricles is forced out of the ventricles and out of the heart through the pulmonary artery or the aorta.
3) Relaxation

3) Relaxation

  • The blood in the pulmonary artery and the aorta is at high pressure. -The pressure shuts the semi-lunar valves so that blood does not flow back into the ventricles.
  • Both the ventricles and the atria relax and the atrioventricular valves reopen.
  • Blood flows into the ventricles and the atria from the pulmonary vein and vena cava.
4) Repeat

4) Repeat

  • The cycle continues.

Interpreting Data

At each stage in the cardiac cycle, the pressure and volume of the atria and ventricles changes considerably. These changes can be presented as a change over time.

Pressure in the atria

Pressure in the atria

  • When the atria contract, the pressure in the atria increases.
  • When the atria relax and the ventricles contract, the pressure in the atria decreases.
  • When both the atria and the ventricles relax, there is a slight increase as the atria fill with blood again.
Pressure in the ventricles

Pressure in the ventricles

  • When the atria contract, the pressure in the ventricles is relatively low. There is a slight increase in pressure as the ventricles fill with blood.
  • When the ventricles contract, the pressure increases dramatically. The pressure increases considerably more than when the atria contract.
  • When both the atria and the ventricles relax, there is a slight increase as the ventricles fill with blood again.
Volume in the atria

Volume in the atria

  • When the atria contract, the volume in the atria decreases.
  • When the atria relax and the ventricles contract, the volume in the atria increases again.
  • When both the atria and the ventricles relax, there is a slight decrease when blood flows into the ventricles from the atria.
Volume in the ventricles

Volume in the ventricles

  • When the atria contract, the volume in the ventricles increases slightly as they fill with blood.
  • When the ventricles contract, the volume decreases dramatically. The volume decreases considerably more than when the atria contract.
  • When both the atria and the ventricles relax, the volume increases as the ventricles expand again.
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Cell Structure

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Practice questions on The Cardiac Cycle

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Stages in the Cardiac CyclePut in order
  2. 2
    Where does blood flow when the atria contract?Multiple choice
  3. 3
    Steps in the cardiac cycle:Fill in the list
  4. 4
    What causes the significant decrease in volume in the ventricles?Multiple choice
  5. 5
    Which of the following statements about the cardiac cycle are true?True / false
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Structure of the Heart

Coordination of Heart Action