Anatomy and Physiology of Heart

Anatomy and Physiology of the Heart

As I often tell my first‑year students, the heart is a fist‑sized, tirelessly working pump that prefers precision over drama. Master the map (anatomy) and the mechanics (physiology), and most exam questions fall into place.

Location and Orientation

  • Position: In the mediastinum, resting on the diaphragm, between the lungs. About two‑thirds lies left of the midline.
  • Size and mass: ≈12 cm long × 9 cm wide × 6 cm thick; ≈250 g in adult females, ≈300 g in adult males.
  • Apex: Tip of the left ventricle; points anterior, inferior, and left; contacts the diaphragm.
  • Base: Posterior surface, formed mainly by the left atrium.
  • Surfaces and borders:
    • Anterior surface: deep to the sternum and ribs.
    • Inferior surface: rests on the diaphragm, between the apex and the right border.
    • Right border: faces the right lung.
    • Left (pulmonary) border: faces the left lung, from base to apex.

Tip: In viva, use “apex left, base posterior” to orient models quickly.

Pericardium: The Protective Sac

  • Fibrous pericardium (outer layer): Tough, inelastic dense irregular connective tissue that anchors the heart to diaphragm and great vessels; prevents overstretching.
  • Serous pericardium (double inner membrane):
    • Parietal layer: lines inside of fibrous pericardium.
    • Visceral layer (epicardium): adheres to heart surface; also the outer layer of the heart wall.
  • Pericardial cavity: Thin, fluid‑filled potential space between parietal and visceral layers. Pericardial fluid minimizes friction during heartbeats.

Clinical pearl: Excess fluid here (pericardial effusion) can compress the heart—cardiac tamponade—limiting filling.

Layers of the Heart Wall

  • Epicardium: Thin outer layer (mesothelium + delicate connective tissue); smooth, slippery surface.
  • Myocardium: Middle, muscular layer (≈95% of wall); cardiac muscle is striated but involuntary; arranged in bundles that generate the pumping action.
  • Endocardium: Inner endothelial layer over connective tissue; smooth lining of chambers and valves; continuous with vascular endothelium—reduces friction and thrombosis.

Mnemonic: “Epi on top, Myo moves, Endo inside.”

Surface Landmarks and Coronary Grooves

  • Auricles: Wrinkled pouches on each atrium; slightly increase atrial capacity.
  • Sulci (grooves containing coronary vessels and fat):
    • Coronary sulcus: Encircles heart; boundary between atria and ventricles.
    • Anterior interventricular sulcus: Separates ventricles on anterior surface.
    • Posterior interventricular sulcus: Continuation on posterior surface.

Chambers and Key Internal Features


Right Atrium (RA)

  • Inflow: Superior vena cava, inferior vena cava, coronary sinus.
  • Walls: Posterior smooth; anterior with pectinate muscles (comb‑like ridges); auricle also has pectinate.
  • Septum: Interatrial septum bears fossa ovalis—remnant of fetal foramen ovale.
  • Outflow valve: Tricuspid (right atrioventricular) valve—three cusps of dense connective tissue covered by endocardium.

Exam cue: “SVC, IVC, and coronary sinus all end in RA.”

Right Ventricle (RV)

  • Interior: Trabeculae carneae (irregular muscular ridges).
  • Valve support: Chordae tendineae anchor tricuspid cusps to papillary muscles; prevent cusp prolapse during systole.
  • Function note: Forms most of anterior surface; pumps to the lungs at low pressure.

Left Atrium (LA)

  • Forms most of the base; receives oxygenated blood via four pulmonary veins.
  • Mostly smooth interior; pectinate muscles limited to the auricle.
  • LA → Left ventricle (LV) through the bicuspid (mitral, left AV) valve.

Left Ventricle (LV)

  • Thickest chamber (10–15 mm); forms the apex.
  • Similar internal features to RV: trabeculae carneae, papillary muscles, chordae tendineae.
  • LV → Aorta via aortic (semilunar) valve.
    • First branches: coronary arteries to the heart wall.
    • Then arch and descending aorta supply the body.

Fetal note: Ductus arteriosus shunts blood from pulmonary trunk to aorta; closes after birth to become ligamentum arteriosum.

Left Ventricle (LV) versus Right Ventricle

  • LV wall is much thicker than RV because it must generate higher pressures for systemic circulation.
  • LV cavity is roughly circular in cross‑section; RV cavity is crescent‑shaped.

Clinical correlation: Chronic hypertension → LV hypertrophy; echo reports will highlight wall thickness.

The Fibrous Skeleton of the Heart

  • Four dense connective tissue rings encircle the valves and merge with the interventricular septum.
  • Functions:
    • Structural support for valves; prevents overstretching.
    • Firm insertion for cardiac muscle.
    • Electrical insulator between atria and ventricles—forces impulses to travel through the specialized conduction system rather than directly through the myocardium.

Why this matters: This insulation ensures the atria contract before the ventricles, optimizing filling.

Linking Anatomy to Physiology: How the Heart Works as a Pump

  • One‑way flow: AV valves (tricuspid on right, mitral on left) allow flow from atria to ventricles; chordae + papillary muscles keep cusps from inverting during ventricular systole. Semilunar valves (pulmonary and aortic) open with ventricular ejection and snap shut to prevent backflow.
  • Pressure generation: Thick LV myocardium creates the high systemic pressure; thinner RV suits the low‑pressure pulmonary circuit.
  • Low‑friction design: Endocardium and pericardial fluid reduce resistance and wear.
  • Coordinated contraction: The fibrous skeleton’s insulation supports a top‑down activation pattern—atria first, ventricles second—for efficient stroke volume.

Simple flow summary:

  • Systemic veins → RA → tricuspid → RV → pulmonary valve → pulmonary trunk/arteries → lungs
  • Pulmonary veins → LA → mitral → LV → aortic valve → aorta → systemic circulation

Quick Clinical Checkpoints

  • Fossa ovalis persists as a patent foramen ovale in some adults; can allow right‑to‑left shunt under certain conditions.
  • Papillary muscle/chordae rupture (e.g., after MI) → acute valve regurgitation and pulmonary edema.
  • Pericarditis can produce pericardial friction rub and chest pain that eases on leaning forward.

Rapid Revision (exam bites)

  • Location: Mediastinum; apex left, base posterior.
  • Pericardium: Fibrous (protects/anchors) + serous (parietal/visceral) with lubricating fluid.
  • Wall layers: Epicardium, myocardium, endocardium.
  • Right atrium: SVC, IVC, coronary sinus; pectinate muscles; fossa ovalis.
  • Right ventricle: Trabeculae carneae; chordae tendineae; papillary muscles.
  • Left ventricle: Thickest wall; circular lumen; high‑pressure pump.
  • Fibrous skeleton: Valve support + electrical insulation.

Self‑test

  • What is the functional reason the LV wall is thicker than the RV?
  • Name the structures that prevent AV valve prolapse during systole.
  • Which groove separates atria from ventricles externally?
  • What is the adult remnant of the fetal foramen ovale?

If you can answer these crisply, you’re heart‑ready for the exam.

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