**Core Concept:** The duration of a cardiac cycle is the time it takes for a single heartbeat to occur, which includes the time taken for the heart to fill with blood (diastole) and eject blood (systole). The heart rate (HR) is the number of heartbeats per minute.

**Why the Correct Answer is Right:** In humans, the normal heart rate is approximately 60-100 beats per minute (bpm). The duration of a cardiac cycle can be calculated using the formula:

Duration of a cardiac cycle = HR (beats per minute) × 1 minute

For a HR of 72 bpm, the calculation would be:

Duration of a cardiac cycle = 72 bpm × 60 seconds/minute = 4,320 seconds

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the duration of a cardiac cycle is not directly proportional to HR. The cardiac cycle remains constant at around 0.8 seconds regardless of HR.

B. This option is incorrect as the duration of a cardiac cycle does not depend on the heart's efficiency or the force of contraction.

C. This option is incorrect because the duration of a cardiac cycle is not dependent on the diastolic or systolic phases of the cardiac cycle. The cardiac cycle remains constant at around 0.8 seconds.

D. This option is incorrect because the duration of a cardiac cycle is not determined by the heart rate. The cardiac cycle remains constant at around 0.8 seconds regardless of HR.

**Clinical Pearl:** Understanding the core concept of the cardiac cycle and its duration helps in assessing the heart rate and interpreting heart rate variability in clinical practice. For example, a prolonged cardiac cycle might indicate bradycardia or heart failure, while a shortened cardiac cycle might indicate tachycardia or atrial fibrillation.