The Heart

• Heart

  Muscular organ that contracts rhythmically, pumping blood in the circulatory system
• Heart wall and chambers

  • Consist of 3 main layers: Endocardium (internal), Myocardium (middle), Epicardium (external)
• Pericardium


  Fluid and capsule surrounding the heart
• Heart contains cardiac valves and an impulse generating and conducting system
• Size of normal adult heart reached at 17-20 years, weighing on average 275-325g
• Pericardium

  • Surrounds the heart and consists of a fibrous and a serous sac
  • Envelopes the heart and is reflected off the great vessels (ascending aorta, pulmonary trunk, terminal 2-4cm of the superior vena cava)
  • Composed of collagen, which is inelastic, may also contain variable amounts of adipose tissue towards the apex of the heart
  • Parietal pericardium is lined by a thin layer of mesothelium on its inner surface
• Epicardium


  • Covers heart surface
  • Single layer of mesothelial cells, continuous with that of the pericardium
  • Delicate membrane covers heart but also contains variable amounts of adipose tissue which is embedded with coronary arteries, lymphatic vessels, nerves, fibroblasts, macrophages
  • Contains only a few lymphocytes
  • Between the 2 mesothelial layers of the pericardium and epicardium, a potential space containing 1-30ml of clear serous fluid that allows the surfaces to glide over one another
• Myocardium

  • Myocytes separated by fibrous bands, form a syncytium with end-to-end junctions (intercalated discs) and occasional side-to-side junctions
  • Central ovoid nucleus, clear zone at poles, contains lipofuscin granules (lysosomes), filled with contractile myofibrils (actin and myosin)
  • Fibrous skeleton is the central supporting structure, muscle fibres are attached, heart valves connected, dense connective tissue with some elastic fibres
  • Dense connective tissue sometimes contains fat but consists predominantly of collagen bundles in layers
  • Fibrous tissue of the mitral and aortic valve rings is more substantial than that of the valves of the right side
• Endocardium


  • Single layer of endothelial cells, with a sub-endothelial portion containing a loose elastic framework and collagen bundles as well as nerves and blood vessels
  • Endocardium of the atria is thicker than that of the ventricles
• Internal anatomy of the heart

  • Internal septum separates the right and left atria, thickness varies
  • 4 chambers: right atrium, left atrium, right ventricle, left ventricle
  • Histology of interatrial septum shows atrial muscle, adipose tissue, fibrous tissue
• Right atrium


  • Venous return via inferior vena cava and superior vena cava
  • Wall ~0.2cm thick, so thin that connective tissue of endocardium blends with epicardium
  • Epicardial surface rich in nerves and ganglia
  • Superior vena cava has no valve, inferior vena cava has an inconsistent, rudimentary Eustachian valve
  • Covered by endothelium and sub-endocardium with elastic fibres, blood vessels, nerves, branches of conducting system
• Left atrium


  • Venous return via four pulmonary veins entering posterolaterally and left atrial appendage
  • Wall avg. 0.3cm (slightly thicker than right)
  • Endocardium thicker and contains more collagen than right, particularly near pulmonary valves
  • No true valves at junction of pulmonary veins and left atrium
  • Luminal surface smooth, derived from single pulmonary vein, endothelium thicker and more opaque than right due to higher pressures
• Right ventricle


  • Anterior to other chambers, has inflow and outflow portions separated by a muscular arch
  • Luminal surface coarsely trabeculated
  • Wall 4-5mm thick, thinner than left ventricle
  • Endocardium similar to other chambers, variable thickness slightly thicker on septal wall
  • Sub-endocardial space includes fenestrated elastic membrane, smooth muscle bundles, blood vessels, nerves, left bundle branch
  • Myocardium richly supplied with small vascular channels, extensive web of capillaries, anastomosing network of myocardial sinusoids
• Left ventricle


  • Receives blood from left atrium during diastole, ejects blood into systemic circulation through aortic valve during systole
  • Surrounded by thick muscular wall 8-15mm thick, ~2-3 times right ventricle
  • Endocardium slightly thicker due to higher pressures
  • Myocardium arranged in spiral inward from superficial to deeper layers
• Cardiac valves


  • Semilunar valves (pulmonary and aortic) have 3 semicircular cusps each, located between ventricles and great arteries

  Atrioventricular valves (mitral and tricuspid) have 3 layers: fibrosa (dense collagen), spongiosa (proteoglycans, loose collagen, cells), ventricularis (elastic fibres)
• Cardiac conduction system


  • Mammalian myocardium has contractile myocardial fibres and specialised fibres for impulse initiation and conduction

  Sinoatrial (SA) node is primary pacemaker, located at junction of superior vena cava and right atrium, histologically arranged around a central artery with sparse muscle fibres and abundant nerves

  Atrioventricular (AV) node on right side of interatrial septum anterior to coronary sinus
• Causes of cardiovascular dysfunction

  • Failure to pump
  • Obstruction to flow
  • Regurgitant flow
  • Disorders of cardiac conduction
  • Disruption of circulatory system continuity
• Genetics and environmental factors disrupt networks controlling morphogenesis, myocyte survival, biomechanical stress responses, contractility and electrical conduction
• Abnormalities often culminate in heart failure, an extremely common result of many forms of heart disease
• Categories of heart disease accounting for nearly all cardiac mortality

  • Congenital
  • Ischaemic
  • Hypertensive
  • Valvular
  • Non-ischaemic myocardial disease
• The heart is composed of three layers, including the epicardium (visceral pericardium), myocardium, and endocardium.
• The epicardium consists of mesothelial cells that form a smooth serous membrane covering the surface of the heart.