The important point about cardiac function is how much blood is pumped out but the blood moves because of pressure differences, so the pressures within the chambers of the heart will be considered first. The diagram below shows the pressures in the left side of the heart and the aorta and the volume of blood in the left ventricle during a normal cardiac cycle. The left side of the heart will be considered first. Early in diastole, blood is flowing from the pulmonary veins, through the left atrium and the mitral valves, which are open, into the ventricle. Atrial systole increases the pressure in the atrium and ventricle but the rise is small because there is little muscle in the walls of the atrium and some of the pressure rise is transmitted back along the pulmonary veins.
Atrial systole is followed by ventricular systole.
As ventricular systole begins the pressure in the ventricle rises and, when ventricular pressure becomes greater than atrial pressure, the mitral valve closes. The pressure in the aorta is around 80mmHg, so the aortic valve is closed as well, and the ventricle becomes a closed chamber containing a fixed volume of blood, upon which the contracting muscle fibres are pressing. When the pressure in the left ventricle exceeds 80mmHg, it has become greater than the pressure in the aorta so the aortic valves open and blood leaves the ventricle, but the force which the ventricle exerts is so high that the pressure in the ventricle and root of the aorta rises to 120mmHg. The closure of the aortic valves is marked on the aortic pressure trace by an irregularity called the dicrotic notch. After the aortic valves have closed, the ventricle is a closed chamber, because the mitral valve is still closed, and the muscle fibres in its wall are relaxing, so the pressure drops rapidly during this phase of isovolumetric, or isometric, relaxation which lasts for around 80ms.