Which of the following remains unchanged during strenous exercise?

Options

A. Stroke volume
B. Heart rate
C. Number of action potentials generated by pacemaker
D. Sequence of events in a cardiac cycle

Answer

To determine which of the following remains unchanged during strenuous exercise, we will analyze each option provided in the question.

1. **Stroke Volume**: This is the amount of blood pumped by each ventricle of the heart with each heartbeat. During strenuous exercise, the body's demand for oxygen increases, leading to an increase in stroke volume as the heart works harder to supply more blood to the muscles.

2. **Heart Rate**: This refers to the number of times the heart beats per minute. During strenuous exercise, the heart rate increases significantly to meet the increased oxygen demand of the body.

3. **Number of Action Potentials Generated by the Pacemaker**: The pacemaker of the heart, located in the sinoatrial (SA) node, generates electrical impulses that initiate heartbeats. During strenuous exercise, the frequency of these action potentials increases to elevate heart rate and stroke volume.

4. **Sequence of Events in the Cardiac Cycle**: The cardiac cycle consists of a series of events that occur during one heartbeat, including diastole and systole phases. While the rates of these events may change, the sequence in which they occur remains constant regardless of the intensity of exercise.

Based on this analysis, the correct answer is that the **sequence of events in the cardiac cycle** remains unchanged during strenuous exercise.

### Step-by-Step Solution:
1. **Identify the Options**: Analyze each option given in the question.
2. **Evaluate Stroke Volume**: Understand that stroke volume increases during strenuous exercise.
3. **Evaluate Heart Rate**: Recognize that heart rate also increases during strenuous exercise.
4. **Evaluate Action Potentials**: Acknowledge that the number of action potentials generated by the pacemaker increases during strenuous exercise.
5. **Evaluate Cardiac Cycle Sequence**: Conclude that the sequence of events in the cardiac cycle remains unchanged during strenuous exercise.
6. **Select the Correct Answer**: Identify that the correct answer is the sequence of events in the cardiac cycle.