The Cardiac Cycle – Exchange and Transport Ep 9

The cardiac cycle is the sequence of contraction and relaxation that the heart goes through continuously to keep blood pumping around the body. It’s fairly complicated, but broken down into steps it makes sense if you think about the direction blood travels through the heart. Two important words you need to know before we start are systole and diastole. If the atria or ventricles are in systole it means they are contracting, and if they are in diastole it means they are relaxing.

Let’s have a look at each stage of the cardiac cycle.

1. Ventricular diastole, atrial systole

We learnt in the last article that blood enters the heart through the vena cava and pulmonary vein, and ends up in the atria. So first of all the blood needs squeezing from the atria to the ventricles, and here’s how it happens:

  • The atria contract.
  • The chamber volume of the atria decreases so pressure increases.
  • The ventricles are relaxed.
  • Blood is forced through the open atrioventricular valves into the ventricles.
  • The pressure and chamber volume of the ventricles increase slightly as they fill.

2. Ventricular systole, atrial diastole

Now all the blood is in the ventricles; we’re moving in the right direction. Next we need to get the blood out of the ventricles through the aorta and pulmonary artery.

  • The atria are now relaxed, they’ve done their job.
  • The ventricles contract.
  • The chamber volume of the ventricles decreases so pressure increases.
  • The atrioventricular valves close due to the pressure in the ventricles becoming greater than the pressure in the atria (backflow to the atria is prevented).
  • Pressure in the ventricles is higher than in the arteries so the semi-lunar valves open.
  • Blood is forced through the semi-lunar valves out into the arteries.

3. Ventricular diastole, atrial systole

The blood is out, everything can relax now. But only for a moment, because the whole cycle needs to start again. The atria need refilling with blood.

  • The ventricles and atria are relaxed.
  • The pressure is higher in the arteries than in the ventricles so the semi-lunar valves close (backflow to the ventricles is prevented).
  • The atria begin to fill again as blood returns to the heart through the vena cava and pulmonary vein
  • Atrial pressure increases again and becomes higher than ventricular pressure, so the atrioventricular valves open.
  • A little blood flows passively into the ventricles through those valves.

And then we’re back to step 1. Where the blood travels is all to do with the relative pressure in all the different chambers and blood vessels, and whether the valves allow blood flow in a certain direction.

The cardiac cycle on a graph

All those changes in pressure and volume can be presented on a graph which you need to know how to interpret. It looks complicated, but does make sense if you link it to what is happening in the three stages.

Graph showing the changes in pressure and volume in the atria and ventricles during one cardiac cycle (heart beat). Numbers 1-3 correspond to the stages described above.

Don’t forget that the left ventricle has to contract with more force than the right ventricle, so on a graph comparing pressure in the two, the left ventricle would reach a higher peak.


It’s difficult to summarise such a detailed cycle. But if you learn the three main stages, hopefully you can visualise what is happening.

  1. Ventricles relax, atria contract.
  2. Ventricles contract, atria relax.
  3. Ventricles and atria relax.

Make sure to leave any questions in the comments!

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