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 120ms3) From the the AV node, excitation spreads down the septum via purkinje fibres 4) Excitation spreads from endocardium to epicardial surface5) 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 wallR - 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 shutso 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 mso
What happens after rapid filling phase of diastole?
- As diastole continues the ventricles fill more slowlyo Intraventricular pressure rises as walls stretcho Filling stops until intraventricular pressure = atrial
When are the tricuspid/mitral valves open?
- Open: when atrial pressure exceeds ventricular (as ventricles relax and pressure drops)
What are the tricuspid/mitral valves closed?
- Close: ventriclar systole > intraventricular pressure exceeds atrial > back flow
What are the aortic/pulmonary valves open?
- Open: ventricular systole > intraventricular pressure exceeds outflow vessel
When are the aortic/pulmonary valves closed?
- Close: after ventricular systole > outflow vessel pressure exceeds ventricular > back flow
What is the origin of the first heart sound?
1st : AV valves closing ‘lup’ – first sound after longest silence (diastole)
What is the origin of the second heart sound?
2nd : outflow valves closing ‘dup’ – this sound is affected by inspiration.
What does inspiration do to heart beat?
Inspiration causes blood to be drawn into the thorax > increased right ventricular pressure > closure of pulmonary valve is delayed. Conversely, left atrial pressure is reducedcausing decreased ventricular filling so the aortic valve closes early.
When may sound of valves closing split?
Sounds may split if valves of right and left heart do not close at the same time
What generates murmurs?
Turbulent blood flow
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