Final answer:
A complete contraction and relaxation of the heart is called a cardiac cycle, which includes diastole (relaxation period) and systole (contraction period) for both atria and ventricles.
Step-by-step explanation:
One complete contraction and subsequent relaxation of the heart is called one cardiac cycle. The cardiac cycle consists of two main phases: diastole and systole. Diastole is the period when the heart muscle relaxes and the chambers fill with blood, while systole is the period of contraction when the heart pumps blood into circulation. Both the atria and ventricles undergo these phases, ensuring efficient blood flow through the body. For example, during atrial diastole, blood flows into the heart while all chambers are relaxed. Then, during atrial systole, the atria contract, pushing blood into the ventricles which then enter ventricular systole to push blood out of the heart.