Cardiac Cycle

Diastole: blood returns from pulmonary vein and vena cava and fills the atria (pressure rises until it exceeds that of ventricles), AV valves open and blood flows to ventricles. atria and ventricles are relaxed which causes pressure to drop lower than that of aorta and pulmonary artery so semi-lunar valves close

Atrial Systole: contraction of atrial wall and relaxed ventricle wall forces blood from atria to ventricles

Ventricular Systole: after delay to allow ventricles to fill with blood they contract simultaneously (increases blood pressure and prevents backflow). with valves closed the pressure in ventricles rises further. once it exceeds that of pulmonary artery and aorta then blood is forced into these vessels (ventricles have thick walls - contract forcefully) this creates pressure needed to pump blood to whole body.