Which statement best defines the cardiac cycle?

• A. The period during which the heart is at rest.
• B. The rate at which the heart beats.
• C. The part of the heartbeat when the heart chambers contract.
• D. The period between the start of one heartbeat and the beginning of the next one.

Answer: (D)

The cardiac cycle is the complete sequence of events that happens with each heartbeat, from the start of one heartbeat to the start of the next, and it includes both contraction and relaxation phases (atrial and ventricular systole followed by diastole). This makes the period from the beginning of one heartbeat to the beginning of the next the best definition, because it encompasses the entire cycle rather than just a part of it. The other ideas describe only a part of what happens: resting time is just diastole, heart rate is how fast the heart beats, and the contraction phase alone covers only systole, not the entire cycle.