The heart is an extraordinary organ that, when it is well, alternates in a highly precise way a phase of muscle relaxation - called diastole - to a phase of muscular contraction - called systole.
During diastole, cardiac cavities - ie the atria and ventricles - widen and fill with blood. During the systole, instead, the same cavities contract and become empty of blood.
Thus described, the cardiac cycle - this is the name that takes the alternation between diastole and systole - appears very simple. In reality, however, the situation is slightly more complex than just reported. Let's see why.
The systole can be divided into two stages: the atrial systole, which corresponds to the contraction of the only atria and serves to transfer blood into the ventricles, and the ventricular systole, which corresponds to the contraction of only the ventricles and serves to pump blood into the blood vessels.
Like systole, diastole also consists of two moments: the atrial diastole, which is the re-expansion of the atria before a new atrial systole, and the ventricular diastole, which is the re-expansion of the ventricles before a new ventricular systole.
Therefore, systole and diastole overlap over time, starting when one has already been partially performed.On the other hand, if they were two separate events - which first occurs one and then another occurs - the heart would not be able to guarantee the right amount of blood to the tissues they need.
