---
title: Structure of the heart
description: Where the heart sits, the layers of its wall, what the pericardium is, how the four chambers are arranged and how blood passes through the heart along the two circuits of the circulation.
type: anatomy
aliases:
  - heart anatomy
  - cardiac anatomy
  - anatomy of the heart
  - heart wall
  - pericardium
sources:
  - tier: reference
    org: statpearls
    title: Anatomy, Thorax, Heart and Pericardial Cavity
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK482452/
  - tier: reference
    org: statpearls
    title: Anatomy, Thorax, Heart
    year: 2023
    url: https://www.ncbi.nlm.nih.gov/books/NBK470256/
  - tier: textbook
    title: Anatomy and Physiology 2e (OpenStax), chapter 19.1 Heart Anatomy
    year: 2022
    isbn: 9781711494067
    url: https://openstax.org/books/anatomy-and-physiology-2e/pages/19-1-heart-anatomy
sources_checked: 2026-10-11
review: null
---

The heart is a muscular organ in the centre of the chest, behind the breastbone (sternum).<Cite n={2} /> It has four chambers: two atria at the top and two ventricles below.<Cite n={2} /> The right and left halves of the heart work as two pumps: one sends blood through the lungs, the other around the rest of the body.<Cite n={[2, 3]} />

<Diagram name="blood-flow">
  <p slot="scheme">Oxygen-poor blood from the venae cavae enters the right atrium, then the right ventricle, and leaves through the pulmonary valve towards the lungs. In the lungs the blood gives up carbon dioxide and takes up oxygen, returns through the <Structure id="pulmonary-veins">pulmonary veins</Structure> to the left atrium, passes into the left ventricle and is pumped through the aortic valve into the <Structure id="aorta">aorta</Structure>.<Cite n={[2, 3]} /> Blue and red are a convention for oxygen-poor and oxygen-rich blood.</p>
</Diagram>

## How it is built

<Patient>

The heart lies in the mediastinum, the space in the middle of the chest between the lungs.<Cite n={3} /> Its front surface faces the breastbone and the rib cartilages, and its lower tip, the apex, points to the left.<Cite n={[1, 3]} /> The heart is roughly the size of its owner's fist.<Cite n={3} />

The heart sits inside a two-layered sac called the pericardium.<Cite n={1} /> Its outer layer is tough connective tissue.<Cite n={[1, 3]} /> A small amount of fluid between the layers of the pericardium reduces friction as the heart contracts.<Cite n={[1, 3]} />

The wall of the heart has three layers: the epicardium on the outside, the myocardium in the middle and the endocardium on the inside.<Cite n={[1, 3]} /> The myocardium is the thickest layer and is made mostly of heart muscle cells.<Cite n={3} /> The muscle of the left ventricle is especially thick.<Cite n={[1, 3]} /> The endocardium lines the chambers and covers the valves.<Cite n={3} />

Each half of the heart has one atrium and one ventricle.<Cite n={3} /> The atria are separated by the <Structure id="interatrial-septum">interatrial septum</Structure>, and the ventricles by the <Structure id="interventricular-septum">interventricular septum</Structure>.<Cite n={3} /> There is more about the chambers in [Atria and ventricles](../heart-chambers/) and about the valves between them in [Heart valves](../heart-valves/).

</Patient>

<Student>

The heart lies in the mediastinum between the third and sixth costal cartilages, fairly horizontally, with the apex directed to the left.<Cite n={1} /> The apex lies just to the left of the sternum; the base is the superior surface of the heart, where the great vessels attach.<Cite n={3} /> The heart weighs approximately 250–300 g in women and approximately 300–350 g in men.<Cite n={3} />

The pericardium consists of an outer fibrous pericardium and an inner serous pericardium.<Cite n={[1, 3]} /> The serous pericardium forms a parietal layer, fused to the fibrous pericardium, and a visceral layer (the epicardium), fused to the heart.<Cite n={[1, 3]} /> The two layers are continuous at the great vessels.<Cite n={1} /> The pericardial cavity between them usually holds 20–25 mL of fluid, secreted by the mesothelial cells of the serous pericardium, including the epicardium.<Cite n={1} /> The fibrous pericardium acts as a barrier against pathogens and hinders severe dilation of the heart.<Cite n={1} />

The endocardium lines the inside of the heart: it is endothelium, a single layer of flat (squamous) cells, over a thin layer of supporting connective tissue with smooth muscle fibres.<Cite n={[1, 3]} />

Besides the interatrial and interventricular septa, there is the atrioventricular septum between the atria and the ventricles.<Cite n={3} /> The interatrial septum bears an oval depression, the fossa ovalis, a remnant of an opening in the fetal heart (the foramen ovale).<Cite n={3} />

</Student>

<HeartModel
  highlight={[
    'interatrial-septum',
    'interventricular-septum',
    'aorta',
    'pulmonary-trunk',
    'superior-vena-cava',
    'inferior-vena-cava',
    'pulmonary-veins',
  ]}
/>

## How it works

<Patient>

Blood moves around two linked circuits.<Cite n={3} /> The pulmonary circuit carries blood to the lungs and back, and there the blood picks up oxygen.<Cite n={3} /> The systemic circuit carries oxygen-rich blood to almost all the tissues of the body.<Cite n={3} />

The atria receive blood and push it into the ventricles, and the ventricles are the heart's main pumps.<Cite n={3} /> The blood's path is:

1. Oxygen-poor blood from the superior and inferior venae cavae enters the right atrium.<Cite n={[1, 3]} />
2. It passes through the tricuspid valve into the right ventricle.<Cite n={2} />
3. The right ventricle pumps it through the pulmonary valve into the <Structure id="pulmonary-trunk">pulmonary artery</Structure>, towards the lungs.<Cite n={[2, 3]} />
4. In the lungs the blood gives up carbon dioxide and takes up oxygen.<Cite n={3} />
5. The blood returns to the left atrium through the pulmonary veins.<Cite n={[2, 3]} />
6. It passes through the mitral valve into the left ventricle.<Cite n={2} />
7. The left ventricle pumps it through the aortic valve into the aorta, and the blood travels out to the body.<Cite n={[2, 3]} />

</Patient>

<Student>

The right atrium receives deoxygenated blood from the entire body except the lungs.<Cite n={2} /> Blood from the lungs is collected by the four pulmonary veins, two from each lung, and all four open into the left atrium.<Cite n={2} /> The right ventricle pumps blood through the right ventricular outflow tract and across the pulmonary (pulmonic) valve.<Cite n={2} />

The left ventricular myocardium is much thicker than the right, because it must overcome the high resistance of the long systemic circuit.<Cite n={3} /> How the contractions of the chambers alternate in time is described in [The cardiac cycle](../cardiac-cycle/), and how the heart muscle itself is supplied with blood in [Coronary circulation](../coronary-circulation/).

</Student>

## In disease

Diseases that affect the parts of the heart described above have their own articles:

- the heart muscle and its blood supply: [Coronary heart disease and myocardial infarction](../../disease/coronary-heart-disease/), [Heart failure](../../disease/heart-failure/);
- the valves: [Aortic stenosis](../../disease/aortic-stenosis/);
- the rhythm: [Atrial fibrillation](../../disease/atrial-fibrillation/);
- the septa: [Atrial septal defect](../../disease/atrial-septal-defect/).

<Quiz />