~ pumping action in a rhythmic sequence
~ atrial diastole - the atrium is relaxed, allowing blood from the body/lungs to fill the atrium
~ as the atria fill with blood, the pressure rises, forcing the tricuspid and mitral valves to open -> this allows blood to fill diastole ventricles
~ then the atria contracts (systole), filling the ventricles to capacity ** the atrial kick accounts for 30% of cardiac output
~ pressure in the atria and ventricles equalize and the tricuspid and mitral valves begin to close
~ then the ventricles contract (systole) causing ventricular pressure to rise and the aortic and pulmonic valves to open