In the cardiac cycle, atrial systole begins?

AIIMS M.Sc. Nsg Entrance exam-2026

Applied Physiology

Easy

Appeared in: AIIMS M.Sc. Nsg Entrance exam-2026

Explanation

Atrial systole is the contraction of the atria that occurs at the end of ventricular diastole.
This phase, known as the 'atrial kick,' contributes the final 10-20% of blood to the ventricles before they contract.
On an ECG, atrial systole begins after the P wave, which represents atrial depolarization.
This timing ensures the ventricles are completely filled to their end-diastolic volume, maximizing the stroke volume for the subsequent contraction.

Option A: During peak ventricular systole, the ventricles are contracting and ejecting blood. The atria are relaxed (in diastole) and filling with blood in preparation for the next cycle.
Option C: The closure of the semilunar valves (aortic and pulmonary) marks the end of ventricular systole and the beginning of ventricular diastole (specifically, the isovolumetric relaxation phase).
Option D: The start of ventricular diastole involves passive filling of the ventricles as the AV valves open and blood flows in due to the pressure gradient. Atrial systole is an active contraction that occurs later.

Visual 1: Wiggers Diagram. A diagram showing the relationship between the ECG, atrial pressure, ventricular pressure, ventricular volume, and heart sounds throughout the cardiac cycle. This visual would clearly place atrial systole (following the P wave) at the end of ventricular diastole, just before the sharp rise in ventricular pressure.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Timing of Atrial Systole in the Cardiac Cycle as background academic context rather than a clinical decision trigger.
The 'atrial kick' from atrial systole contributes about 10-20% of cardiac output at rest. This contribution becomes more critical at higher heart rates or in individuals with stiff ventricles (e.g., due to hypertension).
In conditions like Atrial Fibrillation (AFib), the atria do not contract effectively, leading to the loss of the atrial kick. This can reduce cardiac output and cause symptoms like fatigue, dizziness, and shortness of breath.
What if? If a patient has diastolic heart failure, the ventricles are stiff and do not relax properly. In this case, the atrial kick becomes even more important for adequate ventricular filling. The loss of this kick (e.g., if the patient develops AFib) can lead to a sudden and severe worsening of heart failure symptoms.

How to Approach the Question

First, identify the core concept of the question, which is the sequence of events in the cardiac cycle.
Recall the two main phases for the ventricles: diastole (relaxation and filling) and systole (contraction and ejection).
Think about the function of the atria: they receive blood and then pump it into the ventricles.
Logically, the atria must contract (atrial systole) to push blood into the ventricles before the ventricles contract (ventricular systole).
This places atrial systole during the ventricular filling phase (diastole). Specifically, it's the final event that 'tops off' the ventricles, so it occurs at the end of ventricular diastole.
Evaluate the options based on this sequence to find the one that correctly places atrial systole at the end of the ventricular filling/relaxation period.

Concept Tested & Keywords

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