Heart

• Located in the middle mediastinum
• Has two ventricles and two atria that connect the two major circulatory systems of the body: the pulmonary circulation with the systemic circulation
• Has valves to ensure blood flow only in one direction
• Surrounded by pericardium (fibroserous, fluid-filled sac)
• About the size of a fist and weighs ∼ 300–500 g

References:^[1]

Heart chambers

The heart has:

• Two atria
  • Right atrium
    • Receives deoxygenated blood from the upper and lower body via the superior vena cava (SVC), inferior vena cava (IVC), and the coronary veins
    • Pumps deoxygenated blood into the right ventricle
  • Left atrium
    • Receives oxygenated blood from the four pulmonary veins
    • Pumps oxygenated blood into the left ventricle to enter the systemic circulation
• Two ventricles of the heart
  • Right ventricle
    • Receives deoxygenated blood from the right atrium
    • Pumps deoxygenated blood into the pulmonary trunk as part of the pulmonary circulation
  • Left ventricle
    • Receives oxygenated blood from the left atrium
    • Pumps oxygenated blood into the aorta as part of the systemic circulation

Heart valves

Atrioventricular valves

Consist of leaflets that are supported by the attachment of fibrous cords (chordae tendineae) and papillary muscles.

• Tricuspid valve: : consists of three leaflets located between the right atrium and right ventricle
• Bicuspid valve (mitral valve) : consists of two leaflets located between the left atrium and the left ventricle

The tricuspid valve has three leaflets and is located on the right side, as is the three-lobed right lung. The mitral valve has two leaflets and is located on the left side, as is the two-lobed left lung!

Semilunar valves

Consist of three crescent-shaped cusps (semilunar valves) without chordae tendineae or papillary muscles involvement.

• Pulmonary valve: between right ventricle and pulmonary trunk
• Aortic valve: between left ventricle and aorta

Circulatory system

• Pulmonary circulation: Deoxygenated blood in the right heart flows into the lungs, where it is oxygenated, and delivered back to the left atrium.
  • Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk → pulmonary arteries → lungs → four pulmonary veins → left atrium
• Systemic circulation: Oxygenated blood flows from the left ventricle into the systemic circulation and, after passing through the capillary bed, flows back in a deoxygenated state to the right atrium of the heart to restart the process.
  • Left atrium → mitral valve → left ventricle→ aortic valve → aorta→ systemic circulation (capillary beds) → SVC and IVC → right atrium

Cardiac skeleton

• Definition: : A layer of connective tissue that separates the atria from the ventricles and anchors the valves with fibrous rings (annuli fibrosi cordis) surrounding the atrioventricular and arterial orifices.
• Functions
  • Electrical insulation layer between atria and ventricles
  • Does not conduct electricity (stimuli can only be conducted by the specialized conduction system of the heart as described below)
  • Anchors the valves

Internal structures of the heart

	Left	Right
Atrium	Left auricle (left atrial appendage): extends from superior aspect of the chamber in close proximity to the root of the pulmonary trunk Valve of foramen ovale: remnant of septum primum	Right auricle (right atrial appendage): close to the ascending aorta Small crests (sulcus terminalis and crista terminalis) separate the right auricle from the right atrium Coronary sinus opens into the right atrium between the IVC orifice and the right atrioventricular orifice Fossa ovalis Small oval-shaped depression in the interatrial septum Remnant of the foramen ovale, which usually closes after birth
Ventricle	Contains two papillary muscles Trabeculae carneae cordis: meshwork of muscular ridges	Trabeculae carneae cordis Papillary muscles Extend from the anterior and posterior ventricular walls and the septum Their apices are attached to chordae tendineae Contract during systole and thereby tighten the chordae tendineae: prevent prolapse of valve leaflets and regurgitation into the atria when pressure rises during ventricular contraction

Structures of cardiac conduction system

Collection of nodes and specialized conduction cells that initiate and coordinate contraction of the heart muscle.

• Consists of:
  • Sinoatrial node (SA node)
  • Atrioventricular node (AV node)
  • Atrioventricular bundle (bundle of His)
  • Purkinje fibers (modified myocytes)
• Normal course of electrical conduction: SA node (pacemaker) creates an action potential → signal spreads across atria and causes their contraction . → signal reaches AV node and is slowed down → AV node conducts the signal to bundle of His down the interventricular septum to Purkinje fibers in myocardium → they spread the signal across the ventricles → the ventricles contract

Name	Anatomic localization	Characteristics	Frequency	Action potential
Sinoatrial node	Upper wall of the right atrium (at the junction where the SVC enters)	Natural pacemaker center of the heart with specialized pacemaker cells Spontaneously generates electrical impulses that initiate a heartbeat Influenced by autonomic nervous system Supplied by sinus node artery (branch of the right coronary artery)	ca. 60–80/min
Atrioventricular node	Lies within the AV septum (superior and medial to the opening of the coronary sinus in the right atrium)	Receives impulses from the SA node and passes it to the bundle of His Has the slowest conduction velocity Delays conduction for 60–120 ms (without this delay, the atria and ventricles would contract at the same time) Supplied by the AV nodal artery (posterior descending artery of right coronary artery)	ca. 40–50/min
Bundle of His	Runs in membranous part of the interventricular septum Splits into left and right bundle branches (Tawara branches), which terminate into terminal conducting fibers (Purkinje fibers) of the left and right ventricle	Purkinje fibers have a long refractory period Prevent retrograde conduction Filter high-frequency action potentials so that high atrial rates (e.g. in atrial fibrillation) are not conducted to the myocardium Form functional syncytium: forward incoming stimuli very quickly via gap junctions to allow coordinated contraction	ca. 30–40/min
Purkinje fibers	Terminal conducting fibers in the myocardium

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

Vasculature

Coronary arteries

• The left and right coronary artery arise from the ascending aorta and supply arterial blood to the heart muscle.
• Coronary blood flow peaks during early diastole at a point when the pressure differential between the aorta and the ventricle is the greatest

Right coronary artery (RCA)

• Course: arises from right aortic sinus of the ascending aorta
• Supplies:
  • Right atrium and right ventricle
  • Inferior margin of the right ventricle (via right marginal artery)
  • Parts of the left ventricle and ⅓ of the interventricular septum (via posterior descending artery)
  • Sinus node and AV node (via sinoatrial nodal artery and AV node artery)

Left coronary artery (LCA)

• Course: arises from left aortic sinus of the ascending aorta and anastomoses with the RCA
• Supplies:
  • Anterior aspects of the right and left ventricles
  • Cardiac apex and ⅔ of interventricular septum
  • Left atrium and left ventricle (via left circumflex artery)

Types of circulation

• Right-dominant (∼ 85% of people): posterior descending artery (PDA) arises from RCA
• Left-dominant (∼ 8% of people): PDA arises from left circumflex artery (LCX)
• Codominant (∼ 7% of people): PDA arises from both RCA and LCX

The RCA usually supplies the heart's specialized pacemaker centers (sinus and AV node), meaning that narrowing of this vessel often leads to cardiac arrhythmias!

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!

Venous drainage

The venous blood from the heart is drained into the right atrium by the coronary sinus.

• Left side drains into great cardiac vein → coronary sinus → right atrium
• Right side drains into middle and small cardiac veins → coronary sinus → right atrium

Innervation

• Sympathetic fibers
  • Present throughout the heart
  • Increase heart rate (positive chronotropy), contractility (positive inotropy), conduction velocity (positive dromotropy), dilate coronary arteries
• Parasympathetic fibers (from the vagal nerve)
  • Primarily innervates the atria (SA node and AV node)
  • Decrease heart rate (negative chronotropy), contractility (negative inotropy), and conduction velocity (negative dromotropy)
• Phrenic nerve: sensory innervation of the pericardium

Lymphatics

Lymph of the heart drains into anterior mediastinal nodes and tracheobronchial nodes.

Appearance on imaging

• Cardiac silhouette on chest x-ray
  • Frontal projection
    • Right border is formed by the right atrium
    • Left border is formed by the left ventricle and left atrial appendage
    • Inferior border is formed by the right ventricle
  • Lateral projection
    • Anterior border is formed by the right ventricle
    • Posterior border is formed by the left atrium and left ventricle

References:^{[1][2][3][4][5]}

• Heart tissue consists of four layers.
  • Endocardium: inner layer with three sublayers
    1. Endothelium (innermost): simple squamous epithelium
    2. Smooth muscle and connective tissue
    3. Subendocardium (outermost) - loose connective tissue
       • Site of Purkinje fibers: consist of modified cardiac muscle cells with more glycogen, extensive gap junctions, and fewer myofibrils, specialized for conduction
  • Myocardium: thick myocardial layer
    • Cross-striated muscle cells (cardiomyocytes) with single centrally located nucleus
      • Each cell contains myofibrils organized into sarcomeres (smallest functional contractile unit of cardiac muscle).
      • Contain many mitochondria to produce ATP for contraction
      • Atrial cardiomyocytes release atrial natriuretic peptide (ANP) when stretched to lower BP→ ↑ water and sodium excretion by the kidneys.
      • Cardiomyocytes are joined together by intercalated discs to form long fibers.
        • Intercalated discs connect the single cardiomyocytes to a functional syncytium and force transmission during muscle contraction.
        • Contain adherent junctions (transmit mechanical stimuli) and gap junctions (transmit electrical stimuli)
        • Appear as dark-staining lines between cardiac muscle cells under light microscopy
      • Cardiac Purkinje cells: Specialized/modified cardiomyocytes of the heart conduction system with fewer contractile myofibrils and more mitochondria and glycogen
    • Fibroblasts (turn into myofibroblasts after injury)
    • Extracellular matrix: collagen, elastin, and glycosaminoglycans
  • Epicardium: a connective tissue layer where the visceral layer of the serous pericardium comes into contact with the heart
    • Contains blood vessels, lymphatics, and adipose tissue
  • Pericardium: a fibroserous sac enclosing the heart that consists of a 2-layered inner serous pericardium and an outer fibrous pericardium
    1. Visceral layer of serous pericardium (innermost)
       • Pericardial cavity: space between the visceral and parietal layers of the serous pericardium that contains serous, pericardial fluid
    2. Parietal layer of serous pericardium
    3. Fibrous pericardium (outermost)
• Heart valves: : composed of connective tissue and endocardium
  • Mostly vessel-free and derive nutrition from the surrounding blood.

As the pericardium is rather stiff, the capacity of the pericardial space is limited. An acute pericardial effusion (e.g., following cardiac wall rupture, a complication of myocardial infarction), can lead to pericardial tamponade and a life-threatening reduction in cardiac output. In such cases, quick pericardial fluid drainage (pericardiocentesis is required).

References:^[6]

Overview

• The mesoderm is the embryonic progenitor of all muscle (cardiac muscle, smooth muscle, and skeletal muscle).
• The following steps are important during heart development:
  1. Development of a heart tube from two single endocardial tubes
  2. Transformation of the straight heart tube into an S-shaped heart loop
  3. Separation of the atria and ventricles through endocardial cushions that grow towards each other and fuse
  4. Formation of two atria, two ventricles, and division of one common outflow tract into an aortic trunk and a pulmonary trunk.

Heart structures and their embryonic origins

Structure	Embryonic origin
Ascending aorta Pulmonary trunk	Truncus arteriosus
Left and right ventricle outflow tract	Bulbus cordis
Atrial septum Interventricular septum Heart valves	Endocardial cushion
Trabeculated portion of atria	Primitive atrium
Trabeculated portion of ventricles	Primitive ventricle
Smooth portion of left atrium	Primitive pulmonary vein
Coronary sinus	Left horn of sinus venosus
Smooth portion of right atrium	Right horn of sinus venosus
Superior vena cava	Right common cardinal vein Right anterior cardinal vein

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

Chamber septation

• Atrial septation
  • Initially, the two atria communicate via the foramen primum.
  • The cranially located septum primum extends caudally towards the dorsal endocardial cushion, which narrows the foramen primum.
  • As the foramen primum disappears, the two atria communicate via a newly-formed foramen secundum in the septum primum.
  • A newly-formed cranially located septum secundum extends caudally towards the dorsal endocardial cushion. During this time, the septum primum expands, which narrows the foramen secundum and leaves the residual foramen ovale.
  • The septum primum then develops into the valve of the foramen ovale.
  • The septum secundum and remaining septum primum ultimately join to form the atrial septum.
  • Foramen ovale closes shortly after birth as the left atrial pressure increases (due to increased pulmonary circulation upon lung inflation).

Abnormal development of the septum primum or secundum can result in an atrial septal defect (e.g., patent foramen ovale).

• 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.
  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 and an initial left-to-right shunt.

Valves

Structure	Embryonic origin
Aortic valve Pulmonary valve	Endocardial cushions of outflow tract
Mitral valve Tricuspid valve	Fused endocardial cushions of AV canal

Defective development of the heart valves can result in:
- Stenosis
- Regurgitation
- Ebstein anomaly: a tricuspid valve leaflet is caudally displaced
- Tricuspid atresia: the tricuspid valve is hypoplastic or completely absent
- Bicuspid aortic valve: the most common congenital valvular heart defect.

Formation of the outflow tract

1. Neural crest and endocardial cells migrate to form truncal ridges and bulbar ridges from the truncus arteriosus and bulbus cordis, respectively.
2. Truncal and bulbar ridges spiral and fuse to form the aorticopulmonary septum (AP septum).
3. The AP septum develops into the ascending aorta and pulmonary trunk

Associated conotruncal abnormalities:
- Failure to spiral: transposition of the great vessels
- Malaligned AP septum: tetralogy of Fallot
- Partial AP septum development: persistent truncus arteriosus
References:^[1]

• Cardiovascular examination
• Acute coronary syndrome
• Overview of cardiac arrhythmias
• Artificial cardiac pacemaker
• Congestive heart failure
• Cardiac myxoma
• Cardiac tamponade
• Valvular heart diseases
• Myocarditis
• Pericarditis
• Pericardial effusion and cardiac tamponade
• Rheumatic fever
• Infective endocarditis
• Overview of major types of cardiomyopathy
• Cyanotic congenital heart defects
• Acyanotic congenital heart defects
• Aging cardiovascular system