The Cardiac Cycle.

The heart fills with blood and the muscle in the atrial wall contracts. This stage is known as Atrial Systole. The pressure that is developed in this contraction is not very big because the muscle walls of the atria are thin however is sufficient to push the blood in the atria down through the atrio-ventricular valves into the ventricles. They have semilunar valves that prevent backflow so the blood from the atria does not go back into the pulmonary veins or venae cavae.

At 0.1 seconds after the atria has contracted Ventricular Systole happens where the ventricles contract. The thick muscular walls squeeze inwards on the blood increasing the pressure and pushing it out of the heart. When the pressure becomes bigger than the pressure in the atria it pushes the atrio-ventricular valves shut. Preventing the back flow of blood into the atria. Instead blood goes to the aorta and the pulmonary artery opening the semilunar valves in the vessel as it happens.  

Ventricular Diastole-lasts about 0.3 seconds the muscle relaxes and then the pressure in the ventricles drop. The high pressure in the blood that has been pushed into the arteries would flow back into the ventricles, but for the existence of the semilunar valves that snap shut as the blood fills their cusps making a dup sound. During diastole the whole of the heart muscle relaxes, blood from the veins flow into the atria. The blood is at a low pressure but because of the thin atria walls it is easily distended giving little resistance to the flow.  A little amount of the blood trickles down into the ventricles and through the atrio- ventricular valves. The atrial muscle contracts to push blood forcefully down into the ventricles then repeats the cycle again.

Normally, the electrical impulse moves through the heart's conduction system, and the heart contracts. Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so the blood empties into the ventricles before the ventricles contract.

• Under some conditions, almost all heart tissue is capable of starting a heartbeat, or becoming the "pacemaker," just like the sinus node. An arrhythmia (abnormal heartbeat) may occur when:  The heart's natural pacemaker (the sinus node) develops an abnormal rate or rhythm.
• The normal conduction pathway is interrupted.

Another part of the heart takes over as pacemaker.