What is the term used to describe the alternating contraction of the atria and relaxation of the ventricles?

The term that accurately describes the alternating contraction of the atria and relaxation of the ventricles is the cardiac cycle. The cardiac cycle encompasses all the events that occur during one complete heartbeat, including the contraction (systole) and relaxation (diastole) phases of both the atria and ventricles. During this cycle, the atria contract first to fill the ventricles with blood, followed by the ventricles contracting to pump blood out of the heart. Understanding the cardiac cycle is fundamental in studying heart function, as it highlights the coordinated actions of different heart chambers.

In contrast, heart rhythm refers to the pattern of electrical impulses in the heart, which controls the heartbeat but does not specifically describe the mechanical events of contraction and relaxation. Cardiac output measures the volume of blood the heart pumps per minute, linking directly to heart efficiency but not detailing the contraction-relaxation cycle itself. Atrial fibrillation is an irregular heart rhythm that occurs when the atria experience disorganized electrical signals, leading to an ineffective contraction, which is not reflective of the normal alternating contractions described in the question.