As heart rate increases,

Question

BlackTom AI · Accepted Answer

Diastole shortens proportionately more than systole because the cardiac cycle’s longer filling phase is the one most compressed as rate rises.

1) Relevant knowledge points and concepts
- Cardiac cycle = systole (ventricular contraction/ejection) + diastole (ventricular relaxation/filling).
- At baseline heart rate there is a substantial diastolic interval (including rapid filling, diastasis, and atrial contraction).
- With increasing heart rate, total cycle length shortens (cycle length = 60,000 ms / HR in bpm). Systole has a physiological minimum required for contraction and ejection, while diastole (especially diastasis) is the phase most readily shortened or eliminated.
- Clinical consequence: reduced diastolic time decreases coronary perfusion time.

2) Step-by-step reasoning process
- Compute cycle lengths at two heart rates and partition into systole and diastole to compare proportional changes.
- Show numerically that diastole undergoes a much larger proportional reduction than systole.

3) Calculations with intermediate steps
Example: compare HR 60 bpm and HR 120 bpm.
- Cycle length at 60 bpm = 60,000 ms / 60 = 1000 ms.
- Typical systole at 60 bpm ≈ 330 ms → diastole = 1000 − 330 = 670 ms.
- Cycle length at 120 bpm = 60,000 / 120 = 500 ms.
- Systole shortens modestly (example ≈ 300 ms) because contraction/ejection time cannot fall below a limit.
- Diastole then = 500 − 300 = 200 ms.
- Percent changes: systole change = (300 − 330) / 330 = −9.1%; diastole change = (200 − 670) / 670 = −70.1%.

4) Why the correct answer is correct
- The numeric example shows diastole loses a far larger fraction of its duration than systole as HR increases. Physiologically, systole requires a minimum time for mechanical events and calcium cycling; diastole contains flexible intervals (especially diastasis) that are compressed or eliminated with tachycardia.

5) Why the other options are incorrect
- "Systole lengthens proportionately more than diastole" and "systole lengthens...": increasing HR shortens, not lengthens, the cycle phases.
- "Systole and diastole lengthen equally" / "shorten equally": they do not change equally; diastole shortens much more proportionally.
- "Systole shortens proportionately more than diastole": contradicts physiology and numeric examples.

6) Conclusion / confirmation
The correct choice is “diastole shortens proportionately more than systole,” which matches the provided answer.

As heart rate increases,Single choice

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As heart rate increases,

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