The cardiac cycle includes all the events that occur during one complete ________.

• A. circulation of blood through the lungs
• B. contraction of the atria only
• C. heartbeat
• D. respiratory cycle

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the term 'cardiac cycle.' The cardiac cycle refers to all the events that occur during one complete heartbeat, including both systole (contraction) and diastole (relaxation) of the heart chambers.
2. Step 2: Break down the components of the cardiac cycle. It includes the contraction of the atria (atrial systole), the contraction of the ventricles (ventricular systole), and the relaxation phase (diastole) where the heart fills with blood.
3. Step 3: Compare the options provided in the problem. The cardiac cycle does not refer to the circulation of blood through the lungs (this is part of pulmonary circulation), nor does it refer to the contraction of the atria only (this is just one part of the cycle). It also does not relate to the respiratory cycle, which involves breathing.
4. Step 4: Identify the correct answer based on the definition of the cardiac cycle. The correct answer is 'heartbeat,' as the cardiac cycle encompasses all events during one complete heartbeat.
5. Step 5: Reinforce the concept by explaining that the cardiac cycle is essential for maintaining blood flow throughout the body and includes coordinated electrical and mechanical activities of the heart.