What does systole refer to in the cardiac cycle?

Systole refers specifically to the phase of the cardiac cycle during which the heart muscles contract. This contraction is crucial for pumping blood out of the heart and into the circulation. During this phase, both the atria and the ventricles squeeze to propel blood; the atria contract first to fill the ventricles, followed by the ventricles contracting to send blood to the lungs and the rest of the body. This process helps to maintain adequate blood flow and pressure throughout the circulatory system.

The other options represent different phases or aspects of the cardiac cycle. For instance, relaxation of the heart occurs during diastole, which is the phase following systole. Filling of the chambers primarily happens during diastole, not systole. Blood volume increase can occur due to various factors but is not directly defined within the phases of contraction and relaxation during the cardiac cycle. By understanding that systole denotes contraction, one can appreciate its vital role in ensuring effective heart function and circulation.