Heart

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.

Overview

• 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
  • Anterior to the heart: sternum and rib cartilage
  • Posterior to the heart
    • Vertebral column (T5–T8)
    • Esophagus
    • Carina and primary bronchi
  • The upper part of the heart is at the level of the third costal cartilage.
    • The site of attachment for the venae cavae, aorta, and pulmonary trunk
  • The lower part of the heart (cardiac apex) lies left of the sternum at the level between the fourth and fifth ribs.
• 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 4^th–5^th 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.)

• Right atrium
  • Receives deoxygenated blood via the superior vena cava (SVC), inferior vena cava (IVC), and the coronary veins
  • Pumps deoxygenated blood into the right ventricle
  • Right auricle (right atrial appendage)
    • Muscular pouch that acts to increase the capacity of the atrium
    • Located close to the ascending aorta
    • Separated from the atrium by the terminal sulcus (sulcus terminalis) and terminal crest (crista terminalis).
    • Common site for the development of thrombi in patients with atrial fibrillation
• Left atrium
  • Receives oxygenated blood from the four pulmonary veins
  • Pumps oxygenated blood into the left ventricle to enter the systemic circulation
  • Left auricle (left atrial appendage)
    • Muscular pouch that acts to increase the capacity of the atrium
    • Extends from superior aspect of the chamber in close proximity to the root of the pulmonary trunk
    • Common site for the development of thrombi in patients with atrial fibrillation

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

Cardiac borders

• Frontal view
  • 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
  • Superior border is formed by the atria and great vessels
• Lateral view
  • Anterior border is formed by the right ventricle
  • Posterior border is formed by the left atrium and left ventricle

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

Heart valves

• Two types of cardiac valves that differ in location and morphology
• Cardiac skeleton
  • Consists of four fibrous rings (annuli fibrosi cordis) that surround the atrioventricular and arterial orifices
  • Function
    • Separates atria and ventricles
    • Provides anchor and structural support for the valves
    • Provides electrical insulation layer between atria and ventricles
• Closure of heart valves produces heart sounds (See auscultation of the heart in cardiovascular examination for details.)

“Try PULling My AORTA“: Tricuspidal, Pulmonary, Mitral, Aortic is the order in which blood flows through the heart valves.

Atrioventricular valves

• Structure: leaflets supported by subvalvular apparatus
• Valves
  • Tricuspid valve
    • Consists of three leaflets
    • Located between right atrium and right ventricle
  • Mitral valve (bicuspid valve)
    • Consists of two leaflets
    • Located between left atrium and left ventricle
• Subvalvular apparatus
  • Chordae tendineae: fibrous cords that support the AV valves and connect them to the papillary muscles
  • Papillary muscles (two in the left ventricle; three in the right ventricle)
    • Derive from the myocardium
    • Extend from the anterior and posterior ventricular walls and the septum
    • Have apices that are attached to the 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

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

• Structure: three crescent-shaped cusps without subvalvular apparatus
• Valves
  • Pulmonary valve: located between right ventricle and pulmonary trunk
  • Aortic valve: located between left ventricle and aorta (consists of three leaflets and the aortic sinuses)

Coronary arteries

• The left and right coronary arteries arise from the root of the aorta and supply the heart muscle with arterial blood.
• Coronary arterial dominance
  • Right-dominant (∼ 85% of the population): posterior descending artery (PDA) supplied by the RCA
  • Left-dominant (∼ 8% of the population): PDA supplied by the left circumflex artery (LCX)
  • Codominant (balanced; ∼ 7% of people): PDA supplied by both RCA and LCX
• Coronary blood flow peaks during early diastole at a point when the pressure differential between the aorta and the ventricle is the greatest (see left ventricular pressure-volume diagram.)

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

• Coronary sinus
  • Largest vein of the heart, into which all other coronary veins drain
  • Lies in the left posterior atrioventricular groove
  • Drains into the right atrium between the IVC orifice and the right atrioventricular orifice
  • Veins draining into coronary sinus (coronary veins):
    • Left side of the heart: great cardiac vein and posterior cardiac vein(s)
    • Right side of the heart: small cardiac vein and middle cardiac vein

Lymphatics

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

Innervation

Structures of cardiac conduction system

• Definition: collection of nodes and specialized conduction cells that initiate and coordinate contraction of the heart muscle.
• Components
  • Sinoatrial node (SA node): in the upper wall of the right atrium (at the junction where the SVC enters)
  • Atrioventricular node (AV node): within the AV septum (superior and medial to the opening of the coronary sinus in the right atrium)
  • Atrioventricular bundle (bundle of His): directly below the cardiac skeleton within the membranous part of the interventricular septum
  • Purkinje fibers (modified myocytes): terminal conducting fibers in the subendocardium

See “Conducting system of the heart“ in “Cardiac physiology“ for more details.

References:^[1]

Layers of the heart

• The heart wall itself consists of three layers (from inside to outside):
  • Endocardium
  • Myocardium
  • Epicardium: connective tissue layer attached to the outside of the myocardium, i.e., visceral layer of serous pericardium
• Pericardium: membrane that directly surrounds the heart

Endocardium

• Description: innermost layer of heart tissue, consisting of the following three sublayers:
  • Endothelium (innermost): simple squamous epithelium
  • A layer of loose connective tissue
  • Subendocardium (outermost): loose connective tissue containing
    • Cardiac Purkinje cells
      • Specialized/modified cardiomyocytes that are part of the conducting system of the heart
      • Contain fewer contractile myofibrils and more mitochondria, glycogen, and gap junctions than normal cardiomyocytes.
    • Veins and nerves

Myocardium

• Description: thick myocardial layer composed of the following:
  • Cardiomyocytes: striated muscle cells containing a single, centrally located nucleus
    • Contain many mitochondria, which produce ATP for contraction.
    • Myofibrils within cardiomyocytes are organized into sarcomeres (smallest functional contractile unit of cardiac muscle).
    • Connected by intercalated discs to form long fibers.
      • Intercalated discs connect individual cardiomyocytes into a functional syncytium and force transmission during muscle contraction.
      • They contain adherent junctions (transmit mechanical stimuli) and gap junctions (transmit electrical stimuli)
      • They appear as slightly darker-staining lines between cardiac muscle cells under light microscopy and electron microscopy.
    • See also ”Cardiac muscle” in the article on muscle tissue.
    • Atrial cardiomyocytes release atrial natriuretic peptide (ANP) when stretched (i.e., at higher BPs) → ↑ water and sodium excretion by the kidneys → ↓ BP
  • Fibroblasts (these become myofibroblasts after injury)
  • Extracellular matrix: collagen, elastin, and glycosaminoglycans

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

Pericardium

• Description
  • Fibroserous sac enclosing the heart
  • Defines the pericardial cavity
• Pericardial layers
  • Serous pericardium (innermost)
    • Visceral layer of serous pericardium (epicardium)
      • Outermost layer of the heart wall
      • Separated from the parietal layer of the serous pericardium by the pericardial cavity.
      • Secretes lubricating serous fluid into the pericardial cavity
      • Contains blood vessels, lymphatics, and adipose tissue
    • Parietal layer of serous pericardium
  • Fibrous pericardium (outermost)
• Pericardial cavity: space between the visceral and parietal layers of the serous pericardium that contains serous, pericardial fluid
• Innervation: innervated by the phrenic nerve

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.)

References:^[2]

Overview

• Mesodermal origin
  • Mesoderm → myocardium
  • Mesoderm → mesothelium → pericardium
  • Mesoderm → endothelium → endocardium, blood vessels, lymphatics
• 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 article.

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

• Ectopia cordis
• 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
   • Initially, the two atria communicate via the foramen primum.
   • The septum primum, located cranially, grows caudally towards the dorsal endocardial cushion.
2. Narrowing of the foramen primum occurs as the septum primum grows towards the endocardial cushion.
3. Development of the foramen secundum
   • Before fusion of the septum primum with the endocardial cushion is complete, an opening appears in the cranial end of the septum primum, known as the foramen secundum, through which interatrial communication continues.
   • Ensures right-to-left shunting of the blood
4. Development of the septum secundum
   • A second muscular septum begins to grow on the right side of the septum primum in two segments:
     • The superior part grows caudally from the roof of the primitive right atrium.
     • The inferior part grows cranially from the endocardial cushion.
5. Development of the foramen ovale cordis
   • The space between the two parts of the septum secundum is known as the foramen ovale.
   • The superior portion of the septum primum regresses.
   • The inferior portion of the septum primum persists and acts as a one-way valve, allowing blood to flow from the right atrium into the left atrium.
6. Formation of the atrial septum (no further interatrial communication)
   • Closure of the foramen ovale occurs shortly after birth, when the left atrial pressure increases (due to loss of low resistance placental circulation) and the right atrial pressure decreases (due to increased pulmonary circulation upon lung inflation).
   • The septum secundum completely fuses with the remnant of the septum primum to form the atrial septum during infancy.

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, and 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

• Sequence of events
  1. Development of the aorticopulmonary septum (AP septum)
     • Neural crest and endocardial cells migrate to form truncal ridges and bulbar ridges from the truncus arteriosus and bulbus cordis, respectively.
     • Truncal and bulbar ridges spiral and fuse to form the AP septum.
  2. Division of the ventricular outflow tract
     • The aorticopulmonary septum fuses and rotates 180°, dividing the outflow tract into the aorta (from the left ventricle) and pulmonary trunk (from the right ventricle).
• 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

• 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)

References:^[3]

• 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

1. Ramanathan T, Skinner H. Coronary blood flow. Contin Educ Anaesth Crit Care Pain. 2005; 5 (2): p.61-64. doi: 10.1093/bjaceaccp/mki012 . | Open in Read by QxMD
2. Shad J, Budhwani K, Biswas R. Thoracic ectopia cordis. Case Reports. 2012; 2012 (sep26 1): p.bcr1120115241-bcr1120115241. doi: 10.1136/bcr.11.2011.5241 . | Open in Read by QxMD
3. Cardiovascular System - Heart Histology. https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Heart_Histology. Updated: December 15, 2018. Accessed: December 16, 2018.
4. Cardiovascular System - Heart Histology. https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Heart_Histology. Updated: April 16, 2016. Accessed: June 28, 2018.