The Cardiac Cycle

blood from systemic circulation flows into atria

blood flows passively through open AV valves into ventricle (semilunar valves closed)

70% of ventricular filling occurs before atria depolarize

atrial depolarization 

atria contract, pressure increases forcing AV valves to stay open

blood propelled into ventricles (30%)

blood in ventricles called end diastolic volume (EDV)

atria relax 

ventricles depolarize

ventricles contract, pressure rises

when ventricular pressure exceeds pressure in large arteries (pulmonary artery, aorta) semi lunar valves open

blood expelled from ventricles, pressure in aorta is 120mHg

ventricles relax, small amount of blood remaining in ventricles - end systolic volume

ventricular pressure drops - backflow of blood - closing semi lunar valves

when pressire in atria exceeds pressure in ventricles, AV valves open and cycle begins again