Physiology 3120 Lecture Notes - Lecture 31: Aortic Valve, Heart Valve, Atrioventricular Node
Lecture 30 – Cardiac Cycle
- The mechanical and electrical events during a single contraction at rest including:
o Pressures changes in aorta, left atrium and left ventricle (0 – 120 mmHg at rest)
o Volume changes in left ventricle (80 – 160 mL at rest)
o ECG
o Phonocardiogram: heart sounds (lub dub)
o Opening and closing of valves
o Looking mainly at the left side of the heart (aortic valve and left AV valve)
General information
- Consists of a period of ventricular:
o Diastole lasting 0.53 s (ventricles relaxing)
o Systole lasting 0.27 s (ventricles contracting)
- Event is initiated by spontaneous AP generation in SA node
- Action potential propagates through the atrial muscle
- P wave → depolarization of atrial muscle leads to atria contracting
- AP propagates and spreads to ventricles
- QRS → depolarization of ventricles, ventricles contract = ventricular systole
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Phases of the cardiac cycle
There are 7 phases
1. Atrial systole
- i. Everything starts with the firing of the SA node and AP spreads through the atrial muscle
- ii. P wave → atrial muscle depolarize → atria contract
o Slight increase in atrial pressure because the atria are starting to contract
o Not a large increase in pressure because the atria are not that muscular
- iii. ↑ Atrial pressure > Ventricular pressure (AV valves are already open)
o There is a pressure gradient
o AV valves opened before this event
- iv. Last 30% of vent filling
o Blood flows into the ventricles
o Majority of ventricular filling as occurred before this in the previous cycle (70%)
o Atria contracting just top off the last 30%
o Ventricular volume increases (140 → 160 mL) and left with EDV
o End diastolic volume (EDV): volume of blood in the ventricules at the end of
ventricular diastole
▪ Volume remaining in the ventricles
▪ After this the ventricles start contracting
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2. Isovolumetric (ventricular) contraction
- VOLUME OF BLOOD IN THE VENTRICLES DOES NOT CHANGE EVEN THOUGH THE
VENTRICLES ARE CONTRACTING
- Very brief period, does not last very long
- AP spread down to the AV node then to the Bundle of His
- i. QRS complex → Ventricles begin contracting → pressure builds in ventricles
- ii. Ventricular P > Atrial P
- iii. AV valves close →1st heart sound (lub)
o Do not want blood flowing back into the atria
- iv. No change in vent volume
o This is because the vent. pressure is less than aortic P
o All the valves are closed = blood is not going anywhere
- Pressure in the left atrium slightly increases
o This is because some of the pressure from the ventricles is feeding into the atria
o Valves are just leaflets – as the ventricles start to push on the blood, the leaftlets
push back into the atria a little bit and as they do that, they increase the pressure in
there
o AV valve pushes back into the atria and increases the pressure in the atrium
o Leaflet cannot blow back into the atria because of the chordinae tendonae and
papillary muscles helping to keep the leaflets from blowing open, allowing blood to
flow into the atria
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Document Summary
Consists of a period of ventricular: diastole lasting 0. 53 s (ventricles relaxing, systole lasting 0. 27 s (ventricles contracting) Event is initiated by spontaneous ap generation in sa node. Action potential propagates through the atrial muscle. P wave depolarization of atrial muscle leads to atria contracting. Qrs depolarization of ventricles, ventricles contract = ventricular systole. Volume of blood in the ventricles does not change even though the. Very brief period, does not last very long. Brief, short period: ventricles just begin to relax. Left with end systolic volume at the end of the phase. Ventricular volume is not changing even though the ventricles are. P > atrial p: all valves are closed and there is no change in vent volume. Note pressure in atria: pressure in the atria is beginning to increase as the ventricles contract and relax, this is because the atria are filling with blood, increasing the pressure, rapid inflow, ventricles still relaxing, vent.