Week 7 - heart anatomy & physiology

• Mediastinum


  The cavity between the two pleural cavities where the heart sits and rests on the superior surface of the diaphragm
• Pericardium


  The protective sac surrounding the heart
• Components of the heart wall

  • Epicardium (visceral pericardium)
  • Myocardium
  • Endocardium
• Atrioventricular (AV) valves


  • Open when atrial pressure > ventricular pressure
  • Close when atrial pressure < ventricular pressure
• Semilunar (SL) valves


  • Open when ventricular pressure > arterial pressure
  • Close when arterial pressure > ventricular pressure
• Coronary circulation - arteries

  • Right and left coronary arteries arise from base of the aorta and encircle the heart in the coronary sulcus
  • Blood moves into the coronary arteries when the ventricles relax, in between heart beats
  • Left coronary artery gives rise to the anterior interventricular artery and supplies oxygenated blood to the anterior ventricles
  • Right coronary artery supplies the right atrium and gives rise to the posterior interventricular artery which supplies oxygenated blood to the posterior ventricles
• Coronary circulation - veins

  • Great cardiac vein drains deoxygenated blood from the anterior ventricles
  • Middle cardiac vein drains the posterior ventricles
  • All veins drain into the coronary sinus (thin-walled, expanded vein) which empties into the right atrium
• Coronary artery disease (CAD)

  • Coronary arteries become narrowed and hardened (less elastic), most commonly due to atherosclerosis (fatty plaques occluding the arteries)
  • Over time, reduced blood flow weakens the myocardium and contributes to heart failure
• Cardiac cycle


  1. Relaxation (atria and ventricles relaxed)

  2. Atria contract (ventricles relaxed)

  3. Ventricles contract (atria relaxed)

  4. Relaxation (atria and ventricles relaxed)
• Cardiac cycle


  • One complete heartbeat
  • Includes atrial diastole and systole, ventricular diastole and systole
• Phases of the cardiac cycle

  1. Ventricular filling

  2. Atrial contraction

  3. Ventricular systole - isovolumetric contraction

  4. Ventricular systole - ventricular ejection

  5. Ventricular diastole - isovolumetric relaxation

  6. Ventricular diastole - late filling
• Ventricular filling (phase 1): All 4 chambers are relaxed, blood moves passively from atria to ventricles, atrial systole completely fills ventricles
• Ventricular systole - isovolumetric contraction (phase 2a): Ventricles contract, AV valves close, ventricular pressure increases but not enough to open semilunar valves
• Ventricular systole - ventricular ejection (phase 2b): Ventricular pressure exceeds arterial pressure, semilunar valves open, blood ejected from ventricles
• Ventricular diastole - isovolumetric relaxation (phase 3): Ventricles relax, ventricular pressure drops below arterial pressure, semilunar valves close, AV valves remain closed
• Ventricular diastole - late filling: Ventricular pressure drops below atrial pressure, AV valves open, ventricles passively fill with blood
• Heart sounds


  S1 = closure of AV valves (lubb)

  S2 = closure of semilunar valves (dubb)

  Heart murmur = swishing sound from incompetent valve
• Heart sound locations


  • Tricuspid valve sounds in right sternal margin of 5th intercostal space

  Aortic valve sounds in 2nd intercostal space at right sternal margin

  Pulmonary valve sounds in 2nd intercostal space at left sternal margin

  Mitral valve sounds at heart apex in 5th intercostal space in line with middle of clavicle
• As heart rate increases, all phases of the cardiac cycle are shortened, especially ventricular diastole leading to reduced passive ventricular filling
• Intrinsic conduction system


  Allows the myocardium to stimulate its own contractions
• Components of the intrinsic conduction system

  • Sinoatrial (SA) node (pacemaker)

  Atrioventricular (AV) node

  Atrioventricular (AV) bundle (Bundle of His)

  Bundle branches

  Purkinje fibres
• The SA node acts as the pacemaker, depolarizing 80-100 times per minute to set the heart rate
• The internodal pathway conducts the depolarization from the SA node to the atrial myocardium, leading to atrial contraction
• The AV node delays the depolarization by 0.1 seconds, allowing the atria to fully contract before the ventricles
• The AV bundle, bundle branches and Purkinje fibres conduct the depolarization through the ventricles, leading to ventricular contraction
• Extrinsic innervation


  The autonomic nervous system modifies the myocardial activity
• Myocardium


  Includes some auto-rhythmic cells called pacemaker cells
• Internodal pathway


  Depolarisation of atrial myocardium leading to contraction
• Sinoatrial (SA) node


  Depolarizes 100 times per minute
• Atrioventricular (AV) node


  Depolarisation pauses for 0.1 seconds
• Atrioventricular (AV) bundle (Bundle of His)


  Connects atria to ventricles
• Bundle branches


  Conducts depolarization through the interventricular septum
• Purkinje fibers


  Subendocardial conducting network that depolarises the ventricular myocardium (from apex upwards) leading to contraction
• Sinoatrial node


  • Right atrial wall, inferior to entry point of superior vena cava
  • Depolarises 80-100x per minute (fastest component)
  • Acts as pacemaker and determines heart rate (sinus rhythm)
  • Parasympathetic NS reduces this to 75x per minute at rest
• Internodal pathway


  • SA node myocardial cells depolarize the surrounding myocardial cells until all atrial myocardium is depolarized
  • Depolarisation triggers atrial contraction
• Atrioventricular node


  • At the junction between the atria and ventricles
  • Depolarises 40-60x per minute (max. 230x per min = upper limit of heart rate)
  • Delays depolarisation for 0.1 s while atria complete contraction
  • Becomes the pacemaker if SA node damaged
• Atrioventricular bundle (bundle of His)


  • In the upper interventricular septum
  • Only electrical connection between the atria and ventricles
  • Damage à heart block à neither SA or AV node can control heart rate
• Bundle branches (right and left)


  Travels in the interventricular septum to the apex of the heart
• Purkinje fibres (subendothelial conducting network)


  • Penetrate ventricle walls, depolarise ventricular myocardium
  • Depolarises 30x per minute (this heart rate is too slow for adequate CO)
• Because the heart contains auto-rhythmic cells, it can continue to beat even when separated from the body as long as it has a supply of oxygen