Cardiac Cycle

A normal healthy heart beats approximately 75 times per minute. The heart contracts then begins to relax, during the relaxation period the ventricles begin to contract. This is known as systole and diastole which means heart contraction and relaxation respectively.

Cardiac cycle is the term used to refere to both these events, which complete a heart beat. The complete cardiac cycle is a term of events that occur during a three stage event.

1. Mid-to-late diastole

This is when the heart is in complete relaxation. The pressure in the heart is low. The semi lunar valves are closed and the atrio-ventricular (AV) valves are open. The atria contracts and forces any remaining blood in their chambers back into the ventricles.

2. Ventricular systole

This stage is when the pressure within the ventricles increase rapidly, closing the AV valves. When the intraventricular pressure is higher than that in the large arteries leaving the heart the semi lunar valves are forced open and blood rushes through them out of the ventricles. During ventricular systole the atria are relaxed and their chambers are filling with blood.

3. Early diastole

This is the end of diastole, the ventricles are relaxed, semi lunar valves snap shut to prevent back flow and for a moment the ventricles are completely closed chambers. During this time the intraventricular pressure drops. As it drops lower than the pressure in the atria, the AV valves are forced open again, begin to fill rapidly with blood and complete the cycle.
Essentials of Human Anatomy and Physiology(Elaine N Marieb)