Okay, let's tackle this question about the duration of atrial systole. First, I need to recall the cardiac cycle phases. Atrial systole is the period when the atria contract to push blood into the ventricles. I remember that the cardiac cycle is divided into atrial and ventricular systole and diastole. The total cycle time depends on the heart rate, but in a normal adult with a heart rate of about 75 bpm, the cycle is around 0.8 seconds.

Atrial systole itself is part of the early phase. I think the atrial contraction contributes about 20-30% of ventricular filling. Now, how long does this take? From what I remember, the atria start to contract after the ventricles have started filling passively. The duration of atrial systole is shorter than ventricular systole. Let me think about the timeline. The total cardiac cycle is 0.8 seconds. The ventricular systole is about 0.3 seconds, and diastole is 0.5 seconds. Atrial systole occurs during the last part of ventricular diastole. Wait, no—atrial systole is part of the diastolic phase. So, atrial systole duration is around 0.1 seconds. But I need to confirm this.

Looking up some references, the atrial systole typically lasts about 0.1 to 0.15 seconds. So in the options, if one of them is 0.1 seconds, that's the correct answer. Let's check the options. The user didn't list the options, but the correct answer is provided as 0.1 seconds. So the core concept here is the cardiac cycle phases and their durations. The key point is that atrial systole is a brief phase contributing to ventricular filling.

Now, the wrong options might include longer durations like 0.3 or 0.5 seconds, which are more related to ventricular systole or diastole. Another wrong option could be 0.2 seconds, which is a common misconception. The clinical pearl here is that atrial systole is short, and it's important for the final push of blood into the ventricles before the AV valves close at the start of ventricular systole. This is crucial for understanding heart function and conditions like atrial fibrillation where atrial contraction is lost.

**Core Concept**
Atrial systole is the phase of the cardiac cycle during which the atria contract, contributing to ventricular filling. Its duration is closely tied to the overall cardiac cycle timing, which is inversely proportional to heart rate. In a normal adult at 75 bpm (cycle duration ~0.8 seconds), atrial systole lasts approximately **0.1 seconds**.

**Why the Correct Answer is Right**
Atrial systole begins just before ventricular systole and lasts ~0.1 seconds. It follows atrial depolarization (P wave on ECG) and ends as the AV valves close. This brief contraction adds ~20-30% of ventricular filling volume, ensuring optimal preload. The timing is critical for synchronizing atrial and ventricular activity, regulated by the SA and AV node conduction delays. In a normal heart, atrial systole overlaps with the end of ventricular diast

✓ Correct Answer: D. 0.11 sec