---
title: The cardiac cycle
description: What happens in the heart during a single beat — systole and diastole, the phases of the cycle, how the valves work, the heart sounds and how it all lines up with the ECG.
type: anatomy
aliases:
  - systole and diastole
  - phases of the cardiac cycle
  - heartbeat cycle
  - Wiggers diagram
  - heart sounds
sources:
  - tier: reference
    org: statpearls
    title: Physiology, Cardiac Cycle
    year: 2022
    url: https://www.ncbi.nlm.nih.gov/books/NBK459327/
  - tier: textbook
    title: Anatomy and Physiology 2e (OpenStax), Chapter 19
    year: 2022
    isbn: 9781711494067
    url: https://openstax.org/books/anatomy-and-physiology-2e/pages/19-3-cardiac-cycle
sources_checked: 2026-10-11
review: null
---

The cardiac cycle is everything that happens in the heart during one beat: it begins with contraction of the atria and ends with relaxation of the ventricles.<Cite n={2} /> Over the cycle, the pressure in the heart's chambers rises and falls, and these pressure changes move blood from one chamber to the next, while the valves set its direction.<Cite n={1} /> The period of contraction is called systole, and the period of relaxation, when the chambers fill with blood, is called diastole.<Cite n={2} />

<EcgMonitor preset="normal">

The Wiggers diagram shows the ECG and left ventricular pressure and volume on a shared time scale over one cycle.<Cite n={1} />

</EcgMonitor>

## How it is built

<Patient>

Blood always flows from where the pressure is higher to where it is lower.<Cite n={2} /> That is why the heart's chambers work in turn: first both atria contract together, and then, after a short pause at the atrioventricular node, the ventricles.<Cite n={1} /> The same sequence happens on the left and right sides of the heart, only at much lower pressures in the right ventricle.<Cite n={1} /> There is more about the chambers in "[Atria and ventricles](../heart-chambers/)" and about the valves in "[Heart valves](../heart-valves/)".

Systole and diastole are usually timed by the ventricles and their valves.<Cite n={1} /> Ventricular systole begins when the mitral valve (on the right, the tricuspid valve) closes and ends when the aortic valve (on the right, the pulmonary valve) closes.<Cite n={1} /> Diastole lasts from the closure of the aortic valve to the closure of the mitral valve.<Cite n={1} />

The "heart clock" on this page divides the cycle into five phases:

- atrial contraction;
- isovolumic contraction;
- ejection;
- isovolumic relaxation;
- ventricular filling.

Isovolumic contraction and ejection make up ventricular systole; isovolumic relaxation and filling make up ventricular diastole.<Cite n={2} /> Atrial contraction falls at the very end of ventricular diastole.<Cite n={2} /> The atria contract before the ventricles and finish contracting before ventricular systole begins.<Cite n={2} />

</Patient>

<Student>

OpenStax divides ventricular systole into two phases, isovolumic contraction and ejection, and ventricular diastole into an early phase (isovolumic ventricular relaxation) and a late phase, at the end of which the atria contract.<Cite n={2} /> Diastole lasts longer than systole.<Cite n={2} /> In late ventricular diastole the atrioventricular valves are open.<Cite n={2} />

</Student>

## How it works

<Patient>

**Ventricular filling.** The mitral and tricuspid valves are open, and blood flows freely from the atria into the ventricles.<Cite n={2} /> This is how the ventricles receive about 70–80% of the blood they will hold when contraction begins.<Cite n={2} />

**Atrial contraction.** The atria contract after they are electrically activated, which shows on the ECG as the P wave.<Cite n={2} /> This contraction (the "atrial kick") adds the remaining 20–30% of filling.<Cite n={2} />

**Isovolumic contraction.** The ventricles start to contract, the pressure inside them rises, and blood pushes back on the leaflets of the mitral and tricuspid valves, which close.<Cite n={2} /> The closure of these valves makes the first heart sound.<Cite n={[1, 2]} /> The aortic valve is still closed, so no blood leaves yet and the volume of the ventricle does not change.<Cite n={[1, 2]} />

**Ejection.** When the pressure in the ventricles rises above the pressure in the pulmonary artery and the aorta, the pulmonary and aortic valves open and blood is pumped out into the vessels.<Cite n={[1, 2]} /> With each contraction, a healthy ventricle pumps out more than half of the blood it held.<Cite n={[1, 2]} />

**Isovolumic relaxation.** The ventricles relax and the pressure inside them falls.<Cite n={2} /> When it drops below the pressure in the aorta and pulmonary artery, blood flows back towards the heart and closes the aortic and pulmonary valves.<Cite n={2} /> Their closure makes the second heart sound.<Cite n={[1, 2]} /> The mitral valve has not opened yet, so the volume of the ventricle again does not change.<Cite n={[1, 2]} /> When the pressure in the ventricle falls below the pressure in the atrium, the mitral valve opens and a new round of filling begins.<Cite n={[1, 2]} />

Textbooks describe the first heart sound as "lub" and the second as "dub".<Cite n={2} /> The interval between the first and second sounds is systole, and from the second sound to the next first sound is diastole.<Cite n={1} /> The heart's electrical events are covered in "[The conduction system of the heart](../conduction-system/)" and "[ECG](../../diagnostic/ecg/)".

</Patient>

<Student>

Relation to the ECG: the P wave represents atrial depolarisation, which is followed by atrial contraction; ventricular contraction follows ventricular depolarisation (the QRS complex), and ventricular relaxation follows repolarisation (the T wave).<Cite n={2} /> The QRS complex directly precedes ventricular systole: although depolarisation has gone through the myocardium, there is little or no contraction yet.<Cite n={1} />

In textbooks the Wiggers diagram shows the ECG, ventricular pressure, ventricular volume and the heart sounds on a shared time scale.<Cite n={1} /> The heart sounds are not drawn on the "heart clock". When ventricular pressure drops below the pressure in the pulmonary trunk and aorta, blood flows back towards the heart, producing the dicrotic notch, a small dip seen in blood pressure tracings.<Cite n={2} />

Left ventricular volumes in a resting adult: at the end of diastole it holds about 130 mL of blood (end-diastolic volume), a contraction normally ejects 70–80 mL (stroke volume), and 50–60 mL remains (end-systolic volume).<Cite n={2} />

Extra heart sounds are sometimes heard. A third heart sound (S3) is rarely heard in healthy individuals; it may be heard in young people, some athletes and pregnant people.<Cite n={2} /> A fourth heart sound (S4) occurs before S1: it results from the atria pushing blood into a stiff or hypertrophic ventricle.<Cite n={2} />

</Student>

<HeartModel cycle="page" />

## In disease

<Patient>

If a valve is narrowed, it does not let blood through properly to the next chamber; if it does not close fully, it lets blood leak back.<Cite n={1} /> A heart attack, heart rhythm problems and blood vessel disease can all disrupt the normal cycle.<Cite n={1} />

You can read more in the articles on [aortic stenosis](../../disease/aortic-stenosis/), [heart failure](../../disease/heart-failure/), [coronary heart disease](../../disease/coronary-heart-disease/) and [atrial fibrillation](../../disease/atrial-fibrillation/).

</Patient>

<Quiz />