Physiology 3120 Lecture Notes - Lecture 33: Aortic Valve, Heart Valve, Atrioventricular Node
2 May 2018
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Human Physiology Lecture 33
Cardiac Cycle, Mechanical Performance of the Heart
The Cardiac Cycle
- The mechanical and electrical events during a single contraction at rest including:
o Pressures changes in aorta, left atrium and left ventricle
▪ Very dramatically during one cardiac cycle
o Volume changes in left ventricle
▪ At rest: 80-160mL
▪ Ventricles fill and empty as the heart is contracting
o ECG
o Phonocardiogram
▪ Heart sounds
▪ 2 sounds = lub + dub, BUT occasionally there is a third heart sound (in young
children)
o Opening and closing of valves
▪ Left AV valve, aortic valve
- NOTE: these values change during exercise, stress
- Cardiac cycle is initiated by the spontaneous generation of an AP in the SA node – impulse is rapidly
spread throughout atria and conducted into the AV node, then ventricles
- Consists of a period of ventricular:
o Diastole lasting 0.53 s
▪ Ventricles are relaxing
o Systole lasting 0.27 s
▪ Ventricles are contracting
o Initiated by AP generation in SA node (spontaneously)
▪ AP then propagates out through the atria
muscle
- P wave atria contract
o Depolarization of atria
- QRS vent contract
o Depolarization of ventricles
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- There are 7 phases:
1. Atrial systole
2. Isovolumetric (ventricular) contraction BRIEF
3. (Rapid) Ejection period
4. Protodiastole
5. Isovolumetric (ventricular) relaxation
6. Rapid Inflow
7. Diastasis
1. Atrial systole
1. P wave atria contract
o Represents atrial depolarization leading to atrial contraction
2. ↑ Atrial pressure > Ventricular pressure (AV valves are already open – OPENED BEFORE THIS
EVENT)
o Slight increase in atrial pressure b/c the atria are beginning to contract
o Not a large increase b/c the atria are very muscular
o THEREFORE, THERE IS A PRESSURE GRADIENT (high in left atrium, low in left ventricle)
3. Last 30% of Vent filling end diastolic volume (EDV)
o Majority of ventricle filling has occurred in the previous cycle BEFORE this event
o Ventricles are filling up as the atria contract
▪ Goes from 140-160
o Volume you are left with in the ventricle = EDV (volume of blood in the ventricle at the end
of the ventricular diastole)
Note: everything begins with the firing of the SA node
- AP spread through the atria muscle and depolarizing the atria muscle = P wave
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2. Isovolumetric (ventricular) contraction
1. QRS complex Ventricles begin contracting pressure builds in ventricles
o Squeeze on the ventricles, the pressure in the ventricles will increase
o QRS represents the depolarization of the ventricles + ventricles begin to contract, building
up pressure in ventricles
2. Ventricular P > Atrial P
o AV valves close
o Do not want the blood to flow back to the atria
o 1st heart sound
o PRESSURE GRADIENT!
3. No change in vent volume
o (Vent. pressure < aortic P)
o This is because the pressure in the ventricles is STILL lower than the pressure in the aorta
o ALL the valves are closed – blood can not go anywhere
- Note pressure in left atrium
o Slight increase in atria pressure
o Some of the pressure in the ventricle is feeding into the atria
o Valves are just leaflets. As the ventricles push on the blood, the leaflets of the AV valve push
into the atria a little, increasing the pressure
▪ Note: leaflet can not completely blow back into the atria due to the chordae
tendonae & papillary muscles
- Volume of blood in the ventricle HAS NOT CHANGED, even when the ventricles are contracting
- Very short period – does not last long
- Begins with QRS complex
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find more resources at oneclass.com
