Heart: structure, relations, and blood supply

1) Structure (gross anatomy)

• Location: Middle mediastinum, obliquely placed, about two-thirds left of midline.
• Coverings:
  • Fibrous pericardium (outer tough sac)
  • Serous pericardium: parietal and visceral layers (visceral layer = epicardium)
• Wall layers:
  1. Endocardium (inner lining)
  2. Myocardium (muscle, thickest in LV)
  3. Epicardium (outer serous layer)
• Chambers:
  • Right atrium (RA): receives SVC, IVC, coronary sinus
  • Right ventricle (RV): pumps to pulmonary trunk
  • Left atrium (LA): receives 4 pulmonary veins
  • Left ventricle (LV): pumps to aorta; thickest wall
• Valves:
  • Atrioventricular: tricuspid (RA-RV), mitral (LA-LV)
  • Semilunar: pulmonary and aortic
  • Aortic valve is tricuspid with sinuses above leaflets, two giving rise to coronary ostia (Bailey & Love, p. 975).
• External features:
  • Apex: mainly LV, in left 5th intercostal space (midclavicular line region)
  • Base: mainly LA (posterior surface)
  • Surfaces: sternocostal (anterior), diaphragmatic (inferior), right pulmonary, left pulmonary
  • Borders: right (RA), inferior (mostly RV), left (LV + left auricle), superior (atria + great vessels)

2) Relations

• Anterior: sternum, costal cartilages, thymic remnants, pleura/lungs
• Posterior: esophagus, descending thoracic aorta, thoracic vertebrae (via pericardium)
• Inferior: central tendon of diaphragm
• Lateral: mediastinal pleura and lungs; phrenic nerves/pericardiacophrenic vessels run along pericardium
• Superiorly continuous with great vessels: ascending aorta, pulmonary trunk, SVC, IVC, pulmonary veins

3) Blood supply of the heart

Arterial supply (coronary arteries)

1. Right coronary artery (RCA)
   • Runs in right AV groove
   • Branches: right marginal, usually posterior interventricular (PDA) in right-dominant hearts
   • Supplies: RA, most RV, inferior LV wall (variable), often SA node (~60%), AV node (~80-90%)
2. Left coronary artery (LCA)
   • Short main stem, divides into:
     • LAD (anterior interventricular): anterior septum, anterior LV wall, apex
     • Circumflex (LCx): lateral/posterior LV (extent varies), LA
   • May give left marginal branches

Coronary dominance

• Defined by artery giving PDA:
  • Right-dominant: ~70-85%
  • Left-dominant: ~8-15%
  • Co-dominant: remainder

Venous drainage

• Mostly via coronary sinus into RA:
  • Great cardiac vein
  • Middle cardiac vein
  • Small cardiac vein
• Also anterior cardiac veins (direct to RA) and venae cordis minimae (Thebesian veins).

Simple flow summary

• Heart muscle gets blood from coronary arteries (not directly from chamber blood).
• Perfusion is highest in diastole, especially for the LV subendocardium.
• Any critical coronary narrowing can reduce myocardial oxygen supply and cause ischemia/infarction.