The heart is a muscular organ located in the middle mediastinum that pumps blood through the circulatory system. The heart is surrounded by the pericardium and is divided into four chambers: two atria and two ventricles. The right atrium and ventricle are often referred to as the right heart while the left atrium and ventricle are often referred to as the left heart. The atria and ventricles are separated by the atrioventricular valves, while the ventricles and the arterial outflow tracts of the heart (namely the pulmonary trunk and the aorta) are separated by the semilunar valves. The heart wall consists of the endocardium (innermost), the myocardium, and the epicardium. The conduction system of the heart is composed of specialized nodes and pacemaker cells that initiate and coordinate the contraction of the heart.
The right heart receives deoxygenated blood from the systemic circulation and pumps it through the pulmonary circulation, where it becomes oxygenated. The left heart then receives the oxygenated blood from the pulmonary circulation and pumps it through the blood vessels of the systemic circulation. The coronary arteries, namely the right coronary artery and the left coronary artery, arise in the root of the aorta and supply the myocardium and endocardium. The heart develops embryologically from the heart tube, which undergoes looping and septation to separate it into the four chambers.

Gross anatomy


  • Characteristics
    • Two ventricles and two atria, which connect the pulmonary circulation with the systemic circulation
    • Four valves, which ensure that blood flow occurs in only one direction
    • Roughly the size of a fist
    • Weighs approx. 300–500 g
    • Surrounded by pericardium (a fibroserous, fluid-filled sac)
  • Location: in the middle mediastinum between the lungs
  • Function
    • Pumps blood through the body via the circulatory system
    • ANP synthesis

The left atrium is the posteriormost part of the heart, located directly in front of the esophagus. It can be visualized using TEE. The right ventricle is the anteriormost part of the heart and is at greatest risk of injury following chest trauma.

The cardiac apex beat can typically be palpated to the left of the sternum, medial to the midclavicular line at the 4th–5th intercostal space. In patients with dextrocardia, the orientation of the heart is inverse so the apex is located to the right of the mediastinum rather than the left.

Heart chambers

Two atria

Separated by the interatrial septum (The fossa ovalis is visible on the septum as a small oval-shaped depression in the interatrial septum.)

Two ventricles of the heart

Cardiac borders

The cardiac borders form the cardiac silhouette on chest x-ray!

Heart valves

This mnemonic provides the order in which blood flows through the heart valves: Try PULling My AORTA (Tricuspidal, Pulmonary, Mitral, Aortic)!

Atrioventricular valves

This mnemonic provides the rule of twos and threes for the atrioventricular valves: The tricuspid valve has three leaflets and is located on the right side, as is the three-lobed right lung. The bicuspid (mitral) valve has two leaflets and is located on the left side, as is the two-lobed left lung!

Semilunar valves

Coronary arteries

Coronary arteries
Source Important branches Territory
Left coronary artery (LCA)
  • Left anterior descending artery (LAD): descends between right and left ventricles on anterior surface of heart (in the anterior interventricular sulcus) towards cardiac apex → gives off several diagonal branches on its course
  • Left circumflex artery (LCX): courses left around the heart in the coronary sulcus towards the posterior aspect, ending before the posterior interventricular sulcus → gives off left marginal artery
Right coronary artery (RCA)
  • Various branches
  • Posterior ⅓ of the interventricular septum
  • Posteroinferior aspect of heart
  • Posteromedial papillary muscle
  • Atrioventricular nodal artery

The LAD is the most commonly occluded coronary artery and is often referred to as the “widow maker” due to the high mortality rate associated with LAD infarction.

The RCA usually supplies the heart's conduction system (sinus and AV node) so that stenosis or occlusion of this vessel often leads to cardiac arrhythmias!

Venous drainage


The lymphatics of the heart drain into the anterior mediastinal nodes and the tracheobronchial nodes.


Nervous system Nerves Function
Somatic nervous system

Phrenic nerve

Sympathetic nervous system

Cardiac plexus

Parasympathetic nervous system

Vagal nerve

Structures of cardiac conduction system

Name Anatomic localization Characteristics Frequency

Sinoatrial node

ca. 60–80/min

Atrioventricular node

ca. 40–50/min
Bundle of His
  • Directly below the cardiac skeleton within the membranous part of the interventricular septum
ca. 30–40/min

Purkinje fibers
  • Terminal conducting fibers in the subendocardium

ca. 30–40/min

See conducting system of the heart in cardiovascular physiology for more details.

During states of increased heart rate (e.g. during exercise), the duration of diastole decreases so that there is less time for the coronary arteries to fill with blood and supply the heart with oxygen. Patients with narrow coronary arteries, e.g., due to atherosclerosis, will therefore experience chest pain (angina pectoris) during exertion!


Microscopic anatomy

Layers of the heart



Damaged myocardial tissue is replaced by noncontractile scar tissue (fibrosis) that does not conduct electrical impulses well and, thus, predisposes to cardiac arrhythmias.


The capacity of the pericardial cavity is limited by the stiff, fibrous pericardium. If fluid abnormally accumulates in the pericardial space (pericardial effusion), intrapericardial pressure increases and may impair cardiac function → pericardial tamponade.

Because of the sensory innervation of the pericardium by the phrenic nerve, pericarditis can result in referred pain to the neck, arms, or shoulders (often the left side).

Microscopy of the heart valves

  • Composed of connective tissue and endocardium
  • Mostly vessel-free, with nutrition derived from the surrounding blood (This makes valvular involvement in endocarditis difficult to treat because both the cells of the immune system and antibiotics typically reach sites of infection via the circulatory system.)




  • Mesodermal origin
  • Steps of heart development
    1. Two single endocardial tubes merge to form the heart tube.
    2. The straight heart tube transforms into an S-shaped heart loop.
    3. Endocardial cushions grow towards each other and fuse to separate the atria and ventricles.
    4. Two atria and two ventricles form and one common outflow tract divides into an aortic trunk and a pulmonary trunk.
    5. Valves form from the endocardial cushion.
  • Fetal circulation is covered in the corresponding section of prenatal and postnatal physiology.
  • Postnatal derivatives of fetal vascular structures are covered in the section on postnatal adaptation of the circulatory and respiratory system in the prenatal and postnatal physiology learning card.
Embryonic structures Details Give rise to

Endocardial cushions

Two protuberances located on the dorsal and ventral inner surfaces of the primitive heart tube

Atrial septum, interventricular septum, and heart valves

Primitive atrium

A single cavity separated from the ventricular cavity by the endocardial cushion

Trabeculated portions of the atria

Primitive ventricle

A single cavity separated from the atrial cavity by the endocardial cushion

Trabeculated portions of ventricles

Primitive pulmonary vein

Forms from a pouch in the dorsal wall of the primitive left atrium

Smooth portion of the left atrium

Sinus venosus

Cavity at the caudal end of the embryonic heart tube in which the veins from the embryonic circulatory arcs unite

Right horn of the sinus venosus gives rise to the smooth part of the right atrium and the left horn to the coronary sinus

Bulbus cordis

Connects the primitive ventricle to the truncus arteriosus

Left and right ventricular outflow tracts
Truncus arteriosus

A single arterial trunk that originates from both ventricles of the embryonic heart

Ascending aorta and pulmonary trunk

Posterior, subcardinal, and supracardinal veins

Veins of primitive cardiovascular system in the embryo that empty into the sinus venosus

Inferior vena cava

Right common cardinal vein and right anterior cardinal vein

Superior vena cava

Persistent truncus arteriosus is a congenital heart disease in which the truncus arteriosus fails to divide into the aorta and pulmonary artery before birth.

Cardiac morphogenesis

See acyanotic congenital heart defects and cyanotic heart defects for associated heart defects.

Cardiac looping

Week 4 of gestation: looping of the primary heart tube establishes left-right polarity

The heart begins to beat spontaneously by week 4 of gestation and is detectable via transvaginal ultrasound by week 6 of gestation.

A defect in cardiac looping is caused by a defect in the dynein arm of microtubules and results in Kartagener syndrome with situs inversus and dextrocardia. See primary dyskinesia for details.

Chamber septation

Atrial septation

  1. Development of the septum primum and foramen primum
  2. Narrowing of the foramen primum occurs as the septum primum grows towards the endocardial cushion.
  3. Development of the foramen secundum
  4. Development of the septum secundum
    • A second muscular septum begins to grow in two segments:
  5. Development of the foramen ovale cordis
  6. Formation of the atrial septum (no further interatrial communication)

If the septum primum and septum secundum fail to fuse after birth, a patent foramen ovale (PFO) remains. Later in life, a PFO can result in a paradoxical embolus, in which a venous thrombus may travel via the PFO from the venous to the arterial circulation and cause end-organ infarction (e.g., stroke).

Ventricular septation

  1. A caudally located muscular interventricular septum forms with an interventricular foramen between the two ventricles.
  2. The cranially developing aorticopulmonary septum rotates and caudally fuses with the muscular interventricular septum → this fusion forms the membranous interventricular septum and closes the interventricular foramen (heart).
  3. Endocardial cushions grow to further separate the ventricles and the atria.

Abnormal development of the membranous interventricular septum results in a ventricular septal defect, the most common congenital cardiac anomaly.

Development of the outflow tract

  1. Development of the aorticopulmonary septum (AP septum)
  2. Division of the ventricular outflow tract

Associated conotruncal abnormalities:
- Failure to spiral: transposition of the great vessels
- Malaligned AP septum: tetralogy of Fallot
- Partial AP septum development: persistent truncus arteriosus

Valve formation

Stage Embryonic origin Structures
Development of semilunar valves
Development of atrioventricular valves

Defective development of the heart valves can result in
- Stenotic valves
- Regurgitant valves
- Displaced valves (e.g., Ebstein anomaly)
- Atretic valves (e.g., Tricuspid atresia)
- Minor abnormalities (e.g., Bicuspid aortic valve)


Clinical significance