When ventricle begins to relax, very little blood is ejected (protodiastole) When ejection is over, and the ventricle continues to relax, the high pressure in the aorta causes momentary backflow of blood into the ventricle, which causes the semilunar valves to close: aortic pressure. Diastolic = 80mmhg (doesn"t fall to 0 because of the resistance) As blood enters the aorta as the ventricle contracts, pressure rises (depends not only on blood volume, but also on peripheral vascular resistance) Incisura: small hump in aortic pressure curve. Momentary backflow of blood, causing closure of the aortic valve. The pressure relationships are the same in the pulmonary artery, but the pressure are about 1/6. The sounds are caused by the closing of the heart valves (much more rapid than opening of the valves), which cause vibrations of the surrounding fluid: phonocardiogram. Ventricular pressure rises, and atrial pressure is low. Cardiac cycle: ratio of stroke volume to end diastolic volume = ejection fraction.