Heart

Overview

Gross anatomy

Heart chambers

The heart has:

Heart valves

Atrioventricular valves

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

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.

Circulatory system

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
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.

Name Anatomic localization Characteristics Frequency Action potential

Sinoatrial node

  • 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

ca. 30–40/min

Purkinje fibers

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.
  • They fill with blood during diastole because they are compressed during ventricular systole.

Right coronary artery (RCA)

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 sinusright atrium
  • Right side drains into middle and small cardiac veins → coronary sinusright atrium

Innervation

Lymphatics

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

Appearance on imaging

Microscopic anatomy

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).

Embryology

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

Left and right ventricle outflow tract

  • Bulbus cordis

Atrial septum

Interventricular septum

Heart valves

Trabeculated portion of atria

Trabeculated portion of ventricles

  • Primitive ventricle

Smooth portion of left atrium

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

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

Abnormal development of the membranous interventricular septum results in a ventricular septal defect and an initial left-to-right shunt.

Valves

Structure Embryonic origin


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]

Clinical significance