Initiation of Cardiac Cycle

Pacemacker (Sinotrial Node) sends out electrical signals.

Stage 1: Artial Systole

The electrical signal (from SA node) cause 2 atrium to contract at the same time.

AV valves open so the blood can pass (semilunar valves closed).

AV node (Lowever right atrium) keep on sending electrical signals from SA node.

Stage 2: Ventricular Systole

Electrical signals pass on ventricles, 2 ventricles contract at the same time.

Av valves and Semilunar valves closed, Semilunar valves open when pressure is higher than aorta.

Blood leaves Ventricles to aorta & plumonary artery

Stage 3: Artial & Ventricular Diastole

Heart muscle relaxes, pressure in heart decreases.

Av valves open, blood refill into ventricles (about 70% of the volume).