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103 Cards in this Set
- Front
- Back
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What property of cardiac muscle lets us think of it as a syncytium?
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intercalated discs have gap junctions that allow for communication and impulse conduction
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Why is there a delay in conduction between atrial and ventricular muscle?
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the fibrous tissue between the atria and ventricles does not have many gap junctions and impulse is restricted to the nerve fibers
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Why does there need to be a plateau in the action potential of ventricular muscle?
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It is about .2 seconds, allows the contraction to last 15 times longer then skeletal muscle.
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What causes the plateau in the ventricular muscle action potential?
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slow calcium/sodium channels and increased impermeability to potassium until repolarization
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What occurs with regard to the potassium channels while the slow calcium channels are open in ventricular muscle fibers?
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potassium permeability is decreased about 5 fold (may result from excess calcium influx).
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Slow calcium channels are open for how long in ventricular muscle fibers?
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.2-.3 seconds
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Refractory period of the ventricles? Atria?
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.25-.30 seconds. Atria are .15 sec.
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What is the relative refractory period of ventricular muscle?
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0.05 seconds at the end of action potential where action potential can occur prematurely with a strong signal
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Why does cardiac muscle rely on extracellular calcium concentrations?
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Cardiac muscle relies on T-tubules to move extracellular calcium into the cell to activate calcium release channels (ryanodine receptors) in the sarcoplasmic reticulum membrane (which releases more ca). Without it, strength of contraction would be reduced considerably because the SR is less developed than that in Skeletal muscle and does not store enough calcium
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How does calcium recycle itself in the cardiac muscle after contraction?
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Some is pumped into the sarcoplasmic reticulum (by calcium-ATPase pump). Some are also removed via a sodium-calcium exchanger, and then the Na moves out (Na/K pump maintains Na coming in)
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Why is cardiac muscle more affected by extracellular calcium levels than skeletal muscle?
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Sarcoplasmic reticulum in cardiac muscle is less developed than that in skeletal muscle. T-tubules (which bring extra cellular calcium into cell) are responsible for more calcium influx in cardiac cells as opposed to skeletal cells
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The plateau in atrial muscle action potentials is how long?
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.2 seconds
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The plateau in ventricular muscle action potentials is how long?
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.3 seconds
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What is the "duration of cardiac cycle" as a function of "heart rate"
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duration = 1/(heart rate) in "minutes per beat or seconds per beat"
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The first heart sound (S1) is caused by ...
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closing of AV valves
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The second heart sound (S2) is caused by ...
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closing of aortic/pulmonary valves (splitting = pulmonary closing a little slower due to lower pressure)
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Increased rate of cardiac cycle decreases systole and diastole. Which is decreased more?
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diastole shortens more than systole
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When rate of cardiac cycle is increased, why does cardiac output decrease?
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Since diastole is shortened, ventricles do not fill as much blood. This leads to decreased stroke volume
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The "a" wave in atrial pressure is caused by ...
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atrial contraction
Right = 4-6mm HG increase Left = 7-8 mm Hg increase |
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The "c" wave in atrial pressure is caused by ...
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ventricular contraction (back flow into atria)
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The "v" wave in atrial pressure is caused by ...
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filling of the atria when AV valves are closed, disappears when they open.
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The "c" wave in atrial pressure is caused by the closing of what valves...
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A/V valves - bulge backwards toward atria because of increasing pressure in ventricles
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During isovolumetric contraction of ventricles, what pressure does the ventricle need to reach before the semilunar valves open?
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left about 80 mmHg/ right ventricular pressure slightly above 8 mm Hg
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Isovolumetric contraction corresponds with what EKG wave?
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QRS = ventricles contracting
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The "a" wave in atrial pressure corresponds with what part of the EKG?
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P wave is atrial contraction
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Why is it not a big problem if a person does not have atrial contraction?
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80% of the blood gets into the ventricles without atrial pumping.
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QRS waves on an EKG correspond with the closing of what valve?
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A/V valve closes when ventricles begin to contract
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Ventricular volume is lowest at what part of the EKG?
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T wave, after ventricles have contracted
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Ventricular filling (diastole) is broken into thirds. What occurs during the first third?
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rapid filling, 70% of blood empties (30% in the other 2/3rds)
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Ventricular filling is broken into thirds. What occurs during the last third?
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atrial contraction of the remaining 20% of blood
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Isometric ventricular contraction occurs for how long?
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.02-.03 seconds are required for the ventricle to build up enough pressure to push the semilunar valves open. Contraction of ventricles but NO blood leaves (no shortening of fibers)
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What is the ejection fraction?
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% of blood ejected out of ventricles. stroke volume/ end diastolic volume. typically 60%
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What is stroke volume?
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amount of blood ejected. end diastolic - end systolic volumes
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Function of valves
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prevent backflow of blood
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Function of the papillary muscles/chordae tendineae
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prevent bulging of the AV valves into the atrium
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Why do the semilunar valves need to be more tough than the AV valves?
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High pressure in arteries after systole (strong back flow), smaller openings (faster flow), no chordae tendineae
Subjected to much greater mechanical abrasion. |
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What causes the incisura in the aortic pressure curve?
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closing of the aortic valve: backward flow of blood immediately before closure of the valve then backflow is shut off quickly.
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minute work output =
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stroke work output * heart rate
Total amount of energy converted to work in 1 min |
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Right ventricle work output is what fraction of left ventricle work output?
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1/6
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What valve closes once the left ventricle has reached its end diastolic volume?
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mitral valve closes
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What valve opens once the left ventricle end systolic volume is reached?
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mitral valve opens
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The preload of the ventricle is equal to the ...
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end diastolic pressure when ventricle is filled
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The "after load" of the ventricle is equal to the ...
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end systolic pressure in aorta (phase III of VP diagram)
Resistence of circulation rather than pressure |
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In heart failure, when the ventricle may be dilated (have a high pressure due to back-up of blood), is more or less work required to pump this ventricle?
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more work and energy is required
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Efficiency of the heart
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20-25% (decrease to 5-10% in heart failure)
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What does the Frank-Starling Mechanism say?
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Increased venous return will lead to increased cardiac output = all blood returned to the heart will be pumped out
Greater heart muscle is stretched while filling, greater contraction force and larger volume of blood. |
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Why does the Frank-Starling Mechanism work?
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Increased venous load distends the cardiac muscles and positions the muscle fibers in a more optimal arrangement to accommodate the excess blood
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Complete inhibition of sympathetics has what affect on heart rate?
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decreases heart rate, and strength of ventricular muscle contraction = cardiac pumping 30% below normal
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Parasympathetics use what neurotransmitter to decrease heart rate?
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Ach
20-40 beats/min is tone |
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Vagus parasympathetics work to ...
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Mainly decrease heart rate (also weaken force of contraction by 20-30%)
Distributed to atria. Decrease ventricular pumping 50% or more |
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High blood potassium levels have what affect on the heart?
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slows down heart (action potential repolarizes more slowly)
Heart becomes dilated and flaccid, conduction can be blocked (AV bundle) |
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High blood calcium levels have what affect on the heart?
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increase heart rate by helping depolarization, cause spastic contraction.
(deficiency results in flaccidity of heart) |
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How does increased temperature affect heart rate?
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increases heart rate by increasing permeability to self-excitatory ions, self-excitation is accelerated.
Contractile forces are also increased, but is exhausted after long term high temps |
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What does the division of the heart muscle into two functional syncytiums allow for?
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The atria to contract a short time head of the ventricles to allow for effective heart pumping.
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What is the average action potential in a ventricular muscle fiber?
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105mV
Rises from -85mV to +20mV during each heart beat |
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What is the action potential of skeletal muscle caused by?
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Sudden opening of large numbers of fast sodium channels. Remain open for only a few thousanths of a second.
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What happens when the fast sodium channels close in skeletal muscle?
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Repolarization occurs and the action potential is completed within another thousandth of a second.
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In cardiac muscle, what is the action potential caused by?
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Opening of the same type of fast sodium channels in skeletal muscle, and slow calcium channels (calcium-sodium channels).
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How long do slow sodium channels stay open?
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Several tenths of a second
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What activates the cardiac muscle contraction?
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Ca2+ from influx during action potential (in contrast to skeletal which gets it from the SR)
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What is the velocity of excitatory action potentials in cardiac muscle?
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.3-.5 m/sec
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What is the velocity of conduction in the purkinje fibers?
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As great as 4m/sec
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Can premature contractions of cardiac muscle cause wave summation?
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NO! Only in skeletal muscle.
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What is the mechanism by which the action potential causes the myofibrils of muscle to contract?
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Excitation-contraction coupling
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What happens when an action potential spreads to the cardiac muscle fibers?
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1) Spreads to the T tubules, and the AP's there act on the longitudinal sarcoplasmic tubules to cause release of the CA ions into the muscle sarcoplasm from the cisternae of SR
2) CA ions diffuse into the sarcoplasm from T tubules opening voltage-dep CA channels in the membrane of the T-tubule. CA activates ryanodine receptor channels in the SR and CA is released into sarcoplasm. |
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What is the diameter of the T tubules in cardiac muscle?
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5 times that of skeletal, volume is 25 times as great
Also have mucopolysaccarides that bind CA ions. |
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Where do the T tubules open into?
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Pass directly through the cardiac muscle cell membrane into the extracellular spaces surrounding the cells. Depend on extracellular fluid.
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What is the delay of cardiac impulse from atria to ventricles?
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More than .1 sec
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When the heart rate increases, what happens to the duration of the cardiac cycle?
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Decreases. Relaxation phase decreases more than systole. Hear beating at a fast rate does not remain relaxed enough to allow for complete filling of heart chambers.
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When does the QRS wave appear?
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About .16 sec after onset of P wave
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What is the P wave caused by?
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Spread of depolarization of the ventricles, initiates contraction of atria and a rise in atrial pressure
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What is the QRS complex?
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Electrical depolarization of the ventricles, initiates ventricular contraction and pressure rise.
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When does the QRS complex begin?
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Slightly before the onset of ventricular systole
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What is the T wave?
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Repolarization of ventricles, occurs slightly before the end of ventricular contraction.
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By what percent does atrial contraction increase the ventricular pumping effectiveness?
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By 20%, 80% comes from just passive filling of ventricle
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How long is isovolumic relaxation?
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.03-.06 seconds when the ventricular muscle relaxes even though the volume does not change.
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What is the end-diastolic volume?
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110 to 120 mL normal filling of ventricle
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What is the stroke volume output?
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70 mL
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What is the end-systolic volume?
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40-50mL
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What is the end systolic volume reduced to after strong contraction?
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10-20 mL
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What does the end diastolic volume become when the end systolic volume is decreased?
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150-180 mL
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What is stroke work output?
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Amount of energy that the heart converts to work during each heartbeat while pumping blood to arteries.
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What are the two forms of work output of the heart?
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1) Volume-pressure work (veins to arteries)
2) Kinetic energy of blood flow (velocity of blood through valves) |
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What is required to create the kinetic energy of blood flow?
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mass of blood ejected x (velocity of ejection)'2
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What happens to the total work output of the ventricle in aortic stenosis?
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More than 50% of total work output may be required to create kinetic energy of blood flow.
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At what volume does the ventricular diastolic pressure increase rapidly?
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Above 150mL
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Why would the systolic pressure fall after reaching 150-170 mL?
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Actin and myosin filaments are pulled so far apart that the contraction is less than optimal.
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Max pressure of left ventricle? Right ventricle?
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L = 250-300 mm Hg
R = 60-80 mm Hg |
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What are the phases of the volume pressure diagram?
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1) Period of filling
2) Period of isovolumic contraction 3) Period of ejection 4) Period of isovolumic relaxation |
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When does the period of filling begin?
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Vetnricle volume of 50mL and diastolic pressure of 2-3 mm Hg. Volume and pressure change here.
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What is special about isovolumetric contraction and relaxation?
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The volume of the ventricle does not change because of closed valves, pressure either increases (to 80 mm Hg) or decreases.
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What does the area inside the volume-pressure diagram equal?
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Net external work output of ventricle during cycle.
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What occurs during period of ejection?
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Systolic pressure rises, voluem of ventricle decreases.
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What happens to the volume-pressure diagram when the heart pumps large quantities of blood?
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The area of the work diagram becomes much larger, extends far to the right because the ventricles are filling with more blood. It is much higher because ventricle contracts with great pressure, and to the left because ventricle returns to smaller volume.
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Where does 70-90 % of energy for heart muscle normally derived from?
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Oxidation of fatty acids, other is from lactate and glucose.
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What is oxygen consumption of the heart proportional to?
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Tension that occurs in the heart muscle during contraction * duration of time (tension-time index)
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Heart muscle tension =
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pressure * diameter of ventricle
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Volume pumped by the heart is regulated by what?
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Intrinsic cardiac regulation of heart pumping (FS mech)
Control of heart by autonomic NS |
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What does stretch of the right atrial wall directly do to the heart rate?
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Increases it by 10-20 %
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What occurs when atrial pressure for each side of the heart increases?
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The stroke work output for that side increases until it reaches the limit of the ventricle's pumping activity.
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Sympathetic stimulation increases the heart rate to what?
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180-200, sometimes 250 beats per min
Increases the force of contraction to double normal Cardiac output to as much as 2-3 fold |
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What is a deadly concentration of potassium?
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8-12 mEq/L (2-3 times normal )
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When does the arterial pressure in the aorta decrease cardiac output?
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When pressure rises above 160 mm Hg
Most of the time the function of the heart is determined by resistance of blood flow through tissues/venous return. |
