The heart is a four-chambered pump which moves a 5 to 7.5 liter volume of blood throughout the circulatory system. The process can be broken down into five distinct phases: 1) Passive filling, 2) Atrial systole, 3) Isovolumetric ventricular contraction, 4) Rapid ventricular ejection, and 5) Isovolumetric ventricular relaxation.
The first phase, 
passive filling occurs in early
diastole
when blood enters the left ventricle through the mitral valve,
causing expansion, while being replaced by pulmonary venous blood
which continues to enter the left atrium. The pressure in the
ventricle rises slowly as the ventricular volume increases. The
volume of inflow diminishes until diastasis in mid and late
diastole.
The second phase, 
atrial systole, begins as the active
contraction of the atrium abruptly stops the pulmonary venous and
systemic inflow. This rapidly pushes a small quantity of blood
into the ventricles. This "atrial kick" pushes the final amount
of blood into the already filled ventricles causing the pressure
to rise in both ventricles initiating mitral and tricuspid valve
closure.
Isovolumetric ventricular contraction occurs immediately
following mitral and tricuspid valve closure. This isovolumetric
contraction causes the pressure to rise in the left ventricle
until aortic pressure is achieved. A similar process occurs in
the right ventricle. However, the static pressure reached in the
right ventricle is not as great because pulmonary resistance is
lower than systemic resistance.
Once the pressure in the left ventricle exceeds the
diastolic pressure in the aorta, the aortic valve opens and 
rapid
ventricular ejection occurs. Once the aortic valve is open,
the
ventricle continues to contract rapidly until mid systole, after
which the ejection of blood lessens. A similar process occurs on
the right side of the heart.
The last phase, 
ventricular relaxation, causes the
aortic
valve to close as the ventricular pressure falls below the aortic
diastolic level. The pressure in the ventricle continues to fall
until it equals the pressure in the left atrium and falls
slightly below it. This causes the mitral valve to open and allow
passive filling to begin.