If one looks at heart activity more closely, different phases denoting a particular function can be distinguished in the heart chambers:

• Relaxation phase: the blood flows out of the veins into the two atria, at this point all the valves are closed. This is the resting phase. The first heart sound can be heard here. 
• Filling phase: the musculature of the heart chambers relaxes and there is low pressure in the heart chambers. The atrioventricular valves between atria and ventricles open, while the semilunar valves remain closed. The blood flows from the atria into the ventricles. This phase is called diastole.
• Tension phase: the musculature of the heart chambers contracts. Because the semilunar valves are still closed the pressure rises in the ventricles and the atrioventricular valves shut too. This produces the second heart sound.