The Heart - The Cardiac Cycle.

1. Atrial systole, the ventricles are relaxed, the atria fill with blood, decreasing the volume and increasing the pressure. The high pressure makes the atrio-ventricular valves open, the blood flows into the ventricles the atria contract forcing the remaining blood out.

2. Ventricular systole, the ventricles contract and the atria relax. The higher pressure in the ventricles cause the atrio-ventricular valves to close to prevent back flow and the semi-lunar valves to open, blood flows into either the pulmonary artery or the Aorta.

3. Diastole, both the atria and ventricles relax, increasing their respective volumes, lowering the pressure inside the chambers. The high pressure in the pulmonary artery and aorta cause the semi-lunar valves to shut preventing back flow. The atria will fill with blood again because of the higher pressure in the Vena Cava and Pulmonary vein, the cycle starts again.

The contraction of…