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Lecture 10

BIOL 2P97 Lecture Notes - Lecture 10: Threshold Potential, Stroke Volume, Membrane Potential


Department
Biological Sciences
Course Code
BIOL 2P97
Professor
dr.
Lecture
10

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CARDIAC PHYSIOLOGY
Cardiac Cycle
-the heart contracts and relaxes during a cardiac cycle
-each cycle has 2 phases: Diastole – cardiac muscle relaxation (in the
textbook, its usually
referring to the ventricular phase)
Systole – cardiac muscle contraction
Fig 14.17 a)
1. Late diastole – both sets of chambers are relaxed and ventricles $ll passively (no
contraction, just &owing)
2. Atrial systole – atrial contraction forces a small amount of additional blood into
ventricles (contributes to about 20% of blood – aka atrial kick)
3. Isovolumic/isovolumetric ventricular contraction – $rst phase of ventricular
contraction pushes AV valves closed but doesn’t create enough pressure to open
semilunar valves (no change in volume – although there is tension in muscles, there
is no shortening)
4. Ventricular ejection: as ventricular pressure rises and exceeds pressure in the
arteries, the cusps of semilunar valves open and blood is ejected
5. Isovolumic ventricular relaxation – as ventricles relax; pressure in ventricles,
blood &ows back into cusps of semilunar valves and snaps them closed
-the pressure in pulmonary trunk and aorta is greater than pressure in ventricle; as
soon as pressure in arteries and increase above aorta, the cusps close and you get
relaxation of ventricle without any valves open (no blood coming in or going out)
Pressure-Volume Curve (Loop)
-another way to represent one cardiac cycle (one heart beat)
-represents the changes in volume and pressure that happen over one cardiac cycle
-Pressue=y axis -Volume=x axis
Pressure-volume curve Fig 14.17 b)
A. Ventricle is relaxed (low pressure) and is $lling with blood
AB. Atrial contraction adds $nal amount of blood to ventricle
B. The volume of blood in the ventricle at the end of $lling is called end-diastolic
volume
-end of diastolic phase and is the volume that is there at end of diastolic phase
-ventricular contraction begins
-both the AV valves and semilunar valves are closed so blood has no where to go
-pressure rises rapidly without a change in volume

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BC. This change in pressure without change in volume is called isovolumic
contraction
C. Pressure in ventricle exceeds that of the aorta and the aortic valve opens (he said
semilunar valve in lecture?)
CD. Pressure rises as ventricle continues to contract, but volume decreases as
blood leaves through the aorta
-the volume of blood that leaves the ventricle is known as stroke volume
(volume at C minus
volume at D)
D. The volume remaining in the ventricle is called the end-systolic volume (ESV)
DA. The ventricles relax/losing pressure, and the semilunar valves close
-the ventricle is once again a sealed chamber
-this region of the loop is called isovolumic relaxation
-cardiac work: how much work the heart has to do to pump a certain amount of
blood out
Fig. 14.18 The Wiggers diagram (Look this up in the textbook bc he was talking too
fast)
-puts together the atrial pressure (blue
(red) pressue in the ventricles
-puts together pressure in aorta
-puts together ECG
-depolarization begins in the sinoatrial (SA node) which are autorhythmic cells in the
right atrium that serve as the main pacemaker of the heart
-depolarization wave spreads though non contractile autorhythmic $bres
-a branched intermodal pathway connects the SA node to the atrioventricular node
(AV node) which is a group of autorhythmic cells near the &oor of the right atrium
-depolarization moves from the AV node into the ventricles
-depolarization of the atria occurs before atrial systole (contraction) actually
happens
-(the electrical impulse is ahead of the contraction impulse)
-bc you need time for calcium to bind to tryponin
Left ventricular volume curve
-left ventricle is $lli passively and atrial kick happens and volume increases eve n
more
-ventricuar systole, the volume decreases bc its getting ejected out the left
ventricle
-then another plateau phase
….
Atrial pressure
-little change in atrial pressure bc its &owing into atria and ventricles, during atriole
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