---
title: Atria and ventricles
description: How the four chambers of the heart are built, where blood enters and leaves each one, how their walls and septa differ, and how the chambers take turns to contract and relax.
type: anatomy
aliases:
  - heart chambers
  - chambers of the heart
  - atrium
  - ventricle
  - left ventricle
  - right ventricle
sources:
  - tier: reference
    org: statpearls
    title: Anatomy, Thorax, Heart
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK470256/
  - tier: reference
    org: statpearls
    title: Anatomy, Thorax, Heart and Pericardial Cavity
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK482452/
  - tier: textbook
    title: Anatomy and Physiology 2e (OpenStax), 19.1 Heart Anatomy
    year: 2022
    isbn: 9781711494067
    url: https://openstax.org/books/anatomy-and-physiology-2e/pages/19-1-heart-anatomy
  - tier: textbook
    title: Anatomy and Physiology 2e (OpenStax), 19.3 Cardiac Cycle
    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 heart has four chambers: two upper chambers, the right and left atria, and two lower chambers, the right and left ventricles.<Cite n={1} /> The atria receive blood, and the ventricles are the heart's main pumps.<Cite n={3} /> The right atrium and right ventricle together are called the right heart, and the left atrium and left ventricle the left heart.<Cite n={1} /> The overall structure of the heart is described in the article [Structure of the heart](../heart-structure/); this article looks more closely at the chambers themselves.

<Diagram name="heart-chambers">
  <p slot="diastole">Ventricular diastole. The ventricles are relaxed and fill with blood from the atria; most of the blood flows in passively, and at the end of the phase the atria contract and push in the rest (about 20–30%).<Cite n={[3, 4]} /></p>
  <p slot="systole">Ventricular systole. The ventricles contract and eject blood: the right into the pulmonary trunk, the left into the aorta. Meanwhile the atria are relaxed, and blood from the veins keeps flowing into them.<Cite n={[3, 4]} /></p>
</Diagram>

## How it is built

<Patient>

The **<Structure id="right-atrium">right atrium</Structure>** receives oxygen-poor blood from the whole body except the lungs: it arrives through the superior and inferior venae cavae.<Cite n={[1, 2]} /> Blood from the heart muscle itself also drains here, through the coronary sinus.<Cite n={1} /> From the right atrium, blood passes through the tricuspid valve into the **<Structure id="right-ventricle">right ventricle</Structure>**.<Cite n={[2, 3]} /> The right ventricle pumps it through the pulmonary valve into the pulmonary trunk, and from there it goes to the lungs.<Cite n={[1, 3]} />

The **<Structure id="left-atrium">left atrium</Structure>** collects oxygen-rich blood returning from the lungs through the four pulmonary veins.<Cite n={[1, 3]} /> Through the mitral valve it passes into the **<Structure id="left-ventricle">left ventricle</Structure>**.<Cite n={2} /> The left ventricle is the main pumping chamber of the left heart: it sends blood through the aortic valve into the aorta and on to the whole body.<Cite n={[1, 3]} />

Each atrium has an auricle, a small thin-walled pouch that can fill with blood; the name means "ear-like", because its shape resembles the outer ear.<Cite n={3} />

The wall of the left ventricle is much thicker than that of the right.<Cite n={3} /> The muscle layer of the heart wall, the myocardium, is thickest in the left ventricle.<Cite n={2} />

The chambers are separated by partitions called septa. The interatrial septum lies between the atria and the interventricular septum between the ventricles; the interventricular septum is substantially thicker than the interatrial one.<Cite n={3} /> The septum between the atria and the ventricles is called the atrioventricular septum.<Cite n={3} />

</Patient>

<Student>

The interatrial septum bears an oval depression, the fossa ovalis, a remnant of the foramen ovale, which in the fetal heart allowed blood to pass directly from the right atrium to the left atrium.<Cite n={3} /> This opening closes soon after birth.<Cite n={[1, 3]} /> If the septum primum and septum secundum fail to fuse, the foramen ovale remains patent.<Cite n={2} /> The opening of the coronary sinus lies on the posterior surface of the right atrium, immediately superior and slightly medial to the opening of the inferior vena cava.<Cite n={3} />

The anterior surface of the right atrium and the right auricle have prominent ridges of muscle, the pectinate muscles (mm. pectinati); in the left atrium, pectinate muscles are found only in the auricle.<Cite n={3} /> The walls of both ventricles are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium.<Cite n={3} />

The flaps of the atrioventricular valves are attached by chordae tendineae to papillary muscles. There are three papillary muscles in the right ventricle and two in the left.<Cite n={3} /> The right ventricle has a moderator band, a band of cardiac muscle covered by endocardium; the left ventricle has none.<Cite n={3} /> The openings between the atria and ventricles are surrounded by rings of dense connective tissue, part of the fibrous skeleton of the heart.<Cite n={3} />

</Student>

<HeartModel highlight={['right-atrium', 'left-atrium', 'right-ventricle', 'left-ventricle']} />

## How it works

<Patient>

Contraction is called systole, and relaxation, during which the chambers fill with blood, is called diastole.<Cite n={4} /> The atria and ventricles do not contract at the same time: atrial systole ends before ventricular systole begins.<Cite n={4} />

Most blood flows passively into the ventricles while both the atria and the ventricles are relaxed.<Cite n={3} /> Towards the end of ventricular relaxation, the atria contract and push more blood into them: this accounts for about 20–30% of ventricular filling, and the remaining 70–80% flows in passively.<Cite n={[3, 4]} /> Then the ventricles contract and eject blood into the pulmonary trunk and the aorta, while the atria are already relaxed.<Cite n={[3, 4]} />

The right and left ventricles pump the same amount of blood per contraction.<Cite n={3} /> The left one needs a thicker wall because it pumps blood around the long systemic circulation and must generate high pressure; the pulmonary circuit is shorter and offers less resistance to blood flow.<Cite n={3} />

</Patient>

<Student>

Ventricular systole follows depolarisation of the ventricles and is represented by the QRS complex on the ECG; ventricular diastole follows repolarisation and is represented by the T wave.<Cite n={4} /> Atrial systole lasts approximately 100 ms.<Cite n={4} /> The phases are covered in more detail in the article [Cardiac cycle](../cardiac-cycle/), and the ECG in the article [ECG](../../diagnostic/ecg/).

When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises.<Cite n={3} /> At the same time the papillary muscles contract, generating tension on the chordae tendineae, which prevents the flaps of the atrioventricular valves from being forced into the atria.<Cite n={3} /> The tricuspid valve lets blood into the right ventricle during diastole and closes during systole.<Cite n={2} /> Coordinated contraction of the ventricles begins at the apex and moves towards the base.<Cite n={2} /> How the valves are built and how they work is covered in the article [Heart valves](../heart-valves/).

</Student>

## In disease

For diseases that affect the septa and chambers of the heart, see the separate articles: [Atrial septal defect](../../disease/atrial-septal-defect/), [Atrial fibrillation](../../disease/atrial-fibrillation/) and [Heart failure](../../disease/heart-failure/).

<Quiz />