Week 5 & 6: Cardiovascular System (inc. practical)

Electrical conduction pathway

Sinoatrial (SA) node
  • Located in the right atria.
  • Generates cardiac action potentials spontaneously- allows the heart to beat spontaneously and independently of nerves. cardiac action potentials is at a greater frequency than other cardiac muscle cells.
  • Activation leads to excitation along internodal tracts towards the left atrium and the AV node. This results in atrial contraction.

   

Atrioventricular (AV) node and bundle of His
  • Located medial to the right atrioventricular valve.
  • Action potentials are propagated slowly through these areas compared to the rest of the heart's conduction system.
  • Ventricular depolarisation precedes ventricular contraction.
  • The propagation delay allows atrial contraction to be completed, and the ventricles to fill with blood before ventricular contraction begins.

   

Left and right bundle branches
  • Electrical conduction pathway divides at the interventricular septum to form the left and right bundle branches, which descend to the apex of each ventricle.
  • At the ventricle apex, the bundle branches ÷ repeatedly for distribution throughout the ventricular walls.
  • Ventricular depolarisation precedes ventricular contraction.

   

Purkinje fibers
  • Terminal branches of the bundle branches become Purkinje fibres→ cardiac muscle fibres that have special structural modifications. Which allow APs to travel more rapidly than they would in cardiac muscle tissue.
  • Found towards the endocardial surface. Thus, electrical activity at any level travels outwards towards the epicardial surface.

 

\ \ Three clear waves occur

  1. P wave = atrial depolarisation
  2. QRS complex = ventricular depolarisation
  3. T wave = ventricular repolarisation    Three intervals/segments determine function

   \    P-Q interval = conduction time from atrial to    ventricular excitation

   S-T segment = time ventricular contractile fibers are    depolarised

   Q-T interval = time of ventricular depolarisation to    repolarisation

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Cardiac cycle

Atrial Systole – Contraction (~0.1 seconds) During atrial systole, the ventricles are in diastole

  1. Depolarisation of the SA node causes atrial    depolarization (P wave in Fig A)
  2. Causes systole that exerts pressure, forcing the blood    through the AV valves (blue & green line in Fig B)
  3. Contributes a final ~25 mL of blood into each    ventricle. (Fig D)    • At the end of atrial systole, the volume of blood in the    ventricle is known as the end-diastolic volume (EDV).
  4. The QRS complex (Fig A) = the onset of ventricular depolarisation

\ Ventricular Systole – Relaxation (~0.3 seconds) During ventricular systole, the atria are in diastole 5. Ventricular depolarization (QRS complex, Fig A) causes systole. • Increases pressure in the ventricle (blue line Fig B). • Pressure causes AV and semilunar valves to close = isovolumetric contraction 6. Pressure rises sharply (Fig B). Left ventricular pressure above aortic pressure & right ventricular pressure above pulmonary trunk= both valves open = ventricular ejection 7. Ejects blood into the aorta and pulmonary artery (Fig D) • Volume at the end of systole = end-systolic volume (ESV). 8. The T wave (Fig A) = ventricular repolarisation

\ Isovolumetric Relaxation (~0.4 seconds)

 Atria and ventricles are relaxed 9. Ventricular repolarisation (T wave, Fig A) causes ventricular diastole.

• Pressure falls, closing all semilunar valves to prevent backflow.

• All four valves close, and no blood is leaving = isovolumetric relaxation (Fig D) 10. Ventricular pressure drops below atrial pressure (Fig B) = AV valves open = ventricular filling (Fig D).

\ Cardiac output= SV x HR

\ Blood Pressure

  1. Baroreceptor monitor blood
  2. The baroreceptor reflex helps to maintain blood pressure    * Rapid negative feedback loop    * ↑vascular resistance if BP drops    * Mediated by receptors in the carotid sinus

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