An electrocardiogram (electro refers to electricity, cardio to the heart and gram to a curve) is used to determine the stages of heart contraction.

Take a look at the image and the colored middle part of the curve with numbers 1-6. There is electrical activity in the yellow-coloured parts of the heart.

In steps 1 and 2, the electrical impulses occur in the atria. As a result, the atria contract and the blood flows to the ventricles.

The electrical impulse is transmitted to the muscles of the ventricular walls. The ventricles contract (step 3), and the blood flows into the aorta and pulmonary arteries.

In step 5, the ventricles relax. In step 6, all muscle cells inside the heart are at rest.