Session 2 - The Heart as a Pump Flashcards
What does the myocardium consist of?
Individual cardiac muscle cells joined by low electrical resistance connections
What are action potentials generated by in the heart?
The SAN and the AVN
What is contraction of each muscle cell directly produced by?
A rise in intracellular calcium
Why is conduction quick in the heart?
Due to gap junctions, branching and intercalated discs.
How long is a single cardiac contraction?
280ms
Define systole
Period when myocardium is contracting
Define diastole
Period when the whole heart relaxes between contractions - allows for ventricular filling
Describe the process of excitation in systole
1) The SA node fires an action potential which spreads over the atria causing atrial systole
2) Excitation reaches AV node, where it is delayed for about 120ms
3) From the the AV node, excitation spreads down the septum via purkinje fibres
4) Excitation spreads from endocardium to epicardial surface
5) Ventricles contract from apex upward, forcing blood towards the outlflow valves
What is specific to the structure of muscle fibres in the heart which aids ejection of blood?
Figure of 8 structure of muscle fibres squeezes causes most efficient contraction of ventricles
Describe six differences between right and left sides of the heart
- R - Thin myocardium L - Thick ventricular wall
- R - Pulmonary circulation/ L - Systemic circulation
- R - Deoxygenated blood/ L - Oxygenated blood
- R - Inflow: vena cavae, coronary sinus/ L - pulmonary veins
- R - Outflow: pulmonary arteries/L - Outflow: aorta
- R - Valves: tricuspid, pulmonary/L -Valves: mitral, aortic
Describe process of ventricular systole
- Wave of excitation spreads to the AVN where it is delayed for » 120 seconds > bundle of His > purkinje fibres > ventricles contract
- Ventricular pressure exceeds atrial pressure > brief backflow > mitral valve closes
- Isovolumetric contraction occurs (ventricles contract but volume remains the same) > pressure increases rapidly
- Ventricular pressure exceeds aortic > aortic valve opens > blood flows into aorta
Describe what occurs as ventricles begin to relax is diastole
o Ventricular pressure falls \ aortic pressure exceeds ventricular > aortic valve shuts
o Isovolumetric relaxation: pressure decreases, ventricle relaxes but no blood is entering as mitral valve still closed
Why does blood return to ventricles during diastole?
Intraventricular pressure follows below that of atra, opening mitral valve
How fast is the rapid filling phase of diastole?
o Lasts about 200 – 300 ms
o <50% filling occurs during first 100 ms
What happens after rapid filling phase of diastole?
- As diastole continues the ventricles fill more slowly
o Intraventricular pressure rises as walls stretch
o Filling stops until intraventricular pressure = atrial
