---
title: "Atrial Systole | Anatomy and Physiology I"
description: "Atrial systole is the contraction of the atria that tops off ventricular filling before ventricular systole in Anatomy and Physiology I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/atrial-systole"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 19"
---

# Atrial Systole | Anatomy and Physiology I

## Definition

Atrial systole is the contraction of the atria that pushes the last bit of blood into the ventricles. In Anatomy and Physiology I, it happens near the end of ventricular filling in the cardiac cycle.

## What It Is

Atrial systole is the phase of the cardiac cycle when the atria contract and push blood into the ventricles. In Anatomy and Physiology I, this is the final part of ventricular filling, so it happens right before the ventricles contract.

By the time atrial systole starts, most blood has already moved passively from the atria into the ventricles during atrial diastole. The atria then give the ventricles a final boost, often called the atrial kick. That extra push is not the whole story of ventricular filling, but it adds a meaningful amount of blood to the ventricles before the next heartbeat.

This matters because the ventricles do not start with the same amount of blood every beat. The amount left inside after filling is the end-diastolic volume, and atrial systole helps raise that volume. A larger end-diastolic volume usually gives the ventricles more to eject during ventricular systole, which is one reason the timing of atrial contraction matters so much.

The sequence also depends on valve behavior. During atrial systole, the atrioventricular valves are open, so blood can move from the atria into the ventricles. The semilunar valves stay closed until ventricular pressure rises enough to open them during ventricular systole. If the timing is off, blood flow becomes less efficient and the heart loses part of its normal filling pattern.

A common misconception is that atrial systole does most of the filling. It does not. Most ventricular filling happens passively, and atrial systole adds the final 20 to 30 percent. That is why a healthy person can sometimes tolerate small changes in atrial function, but conditions like atrial fibrillation can reduce how well the ventricles fill, especially when the heart is working harder.

## Why It Matters

Atrial systole matters because it sits at the handoff point between filling and pumping. If you can trace this step, you can make sense of the whole cardiac cycle instead of memorizing it as a list of phases.

It connects directly to ventricular systole, since the amount of blood the ventricle starts with affects how much it can pump out on the next beat. That link shows up again when you study end-diastolic volume and the Frank-Starling law, where greater filling stretches the ventricle and increases force of contraction.

It also helps you explain why valve timing matters. Atrioventricular valves must be open during atrial systole, then close before ventricular pressure rises too much. When you read a heart sound, interpret a diagram, or follow a pressure-volume loop, atrial systole gives you a reference point for where the cycle is.

In a lab or case study, this term can help you reason through what happens when the atria do not contract effectively. Even if blood still moves into the ventricles passively, the heart may lose that final fill, which can lower stroke volume and cardiac output.

## Connections

### [Ventricular Systole](/anatomy-physiology/key-terms/ventricular-systole)

Atrial systole comes right before ventricular systole. The atria finish filling the ventricles first, then the ventricles contract and eject blood to the lungs or body. If you mix these up, the whole timing of the cardiac cycle gets harder to follow, especially when you are tracing valve openings and closings.

### [Atrial Diastole](/anatomy-physiology/key-terms/atrial-diastole)

Atrial diastole is the relaxation phase of the atria, which happens while blood is flowing into them from the veins. Atrial systole follows that relaxation and turns the atria from filling chambers into pumping chambers. The two phases work as a pair, so one prepares the heart for the other.

### [Atrioventricular Valves](/anatomy-physiology/key-terms/atrioventricular-valves)

During atrial systole, the atrioventricular valves are open, letting blood move from atria to ventricles. Their position matters because atrial contraction only helps filling if blood has somewhere to go. If these valves were closed, atrial systole would not increase ventricular filling the way it should.

### [End-Diastolic Volume](/anatomy-physiology/key-terms/end-diastolic-volume)

Atrial systole increases end-diastolic volume by topping off the ventricles before they contract. That extra volume affects how much blood the ventricle can eject on the next beat. When you see EDV on a pressure-volume loop or in a cardiac cycle question, atrial systole is one of the reasons it rises.

## On the AP Exam

A quiz or lab question may ask you to place atrial systole in the correct order of the cardiac cycle, label it on a diagram, or explain what happens to ventricular filling during this phase. You might also get a scenario about atrial fibrillation and be asked why stroke volume drops when atrial contraction is less effective. The move is to connect atrial systole to end-diastolic volume, atrioventricular valve opening, and the start of ventricular systole. If you can explain that the atria add a final fill before the ventricles contract, you are usually on the right track.

## Atrial Systole vs Atrial Diastole

Atrial systole is the contraction phase, while atrial diastole is the relaxation phase. They happen at different points in the cardiac cycle, and they do opposite jobs. Atrial diastole is when the atria fill with blood, and atrial systole is when they push blood into the ventricles.

## Key Takeaways

- Atrial systole is the contraction of the atria that tops off ventricular filling before the ventricles contract.
- It adds the final portion of blood to the ventricles, usually about 20 to 30 percent of the total filling for that beat.
- This phase happens when the atrioventricular valves are open, so blood can move from the atria into the ventricles.
- Atrial systole raises end-diastolic volume, which affects how much blood the ventricles can eject during ventricular systole.
- If atrial contraction is weak or uncoordinated, ventricular filling and cardiac output can drop.

## FAQs

### What is atrial systole in Anatomy and Physiology I?

Atrial systole is the contraction of the atria that pushes blood into the ventricles. It happens near the end of ventricular filling in the cardiac cycle. In A&P, you usually study it as the phase that tops off the ventricles before they contract.

### Is atrial systole the same as atrial contraction?

Yes, atrial systole is the contraction phase of the atria. The term "systole" means contraction, so the phrase describes the atria actively squeezing blood forward. It is different from atrial diastole, when the atria relax and fill.

### How much blood does atrial systole add to ventricular filling?

Atrial systole usually contributes about 20 to 30 percent of the blood that enters the ventricles during a cardiac cycle. Most filling happens passively before that, but the atrial contraction gives the ventricles a final boost. That extra volume helps support the next ventricular contraction.

### What happens if atrial systole does not work well?

If the atria do not contract effectively, the ventricles may fill less completely. That can lower end-diastolic volume and reduce stroke volume, especially during exercise or in people with heart rhythm problems like atrial fibrillation. Blood still fills the ventricles passively, but the heart loses that final push.

## Related Study Guides

- [19.3 Cardiac Cycle ](/anatomy-physiology/unit-19/3-cardiac-cycle/study-guide/YvRk8a3OeQm0aWtw)

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