Lecture 3 Physiology of the Heart II Flashcards Preview

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Flashcards in Lecture 3 Physiology of the Heart II Deck (48)
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What percentage of their volume do the ventricles expel per beat



What are the two methods of increasing cardiac output

Increasing the heart rate or increasing the stroke volume by increasing the volume of blood expelled by the ventricles in each beat


What is the equation for cardiac output

Cardiac output = stroke volume x heart rate


What are the units of cardiac output

Litres per minute


What is the standard resting stroke volume



What is the resting heart rate



What is the standard resting cardiac output

4.9L min-1


What is the standard blood volume



How many times per minute does the entire blood volume circulate

Once per minute


To what extent can cardiac output increase during exercise

Cardiac output can increase 7-fold to around 35L min-1


What is meant by ejection fraction

The percentage of the ventricles volume ejected with each cardiac contraction


What is the rough ejection fraction of patients in heart failure

Around 35%


Cardiac output is controlled by heart rate and stroke volume. How can heart rate be regulated

Heart rate is mainly regulated by the autonomic nervous system. The sympathetic nervous system increases heart rate and contractility of the heart whole the parasympathetic nervous system decreases heart rate only. In addition circulating catecholamines such as noradrenaline also act although much slower to increase heart rate


How does the action of the two divisions of the autonomic nervous system differ in terms of their effects on stroke volume

The sympathetic nervous system acts on both the SAN and the cardiac myocytes themselves hence increasing the rate of impulse generation and the contractility of the muscle itself. In contrast the parasympathetic nervous system has very little effect on the ventricles themselves and mainly just decreases heart rate


How is stroke volume intrinsically regulated

Intrinsic regulation of stroke volume occurs due to changes in contractility of the muscle itself. This is determined by the availability of intracellular Ca2+ oxygen free fatty acids and ATP.


How is stroke volume regulated by extrinsic factors

Extrinsic regulation of stroke volume occurs due to changes in the preload or filling pressure and afterload as well as a contribution by the sympathetic nervous system


What is meant by preload

The preload is referred to as the filling pressure or the pressure created during diastole when the venous return brings blood back to the heart setting the stretch or tension in the muscle. It is also referred to as LV end diastolic pressure


What is meant by afterload

The afterload is the pressure against which the heart needs to contract. This is essentially the resistance of the circulation


How can the sympathetic nervous system extrinsically regulate the stroke volume

Due to its positive inotropic effects increasing the force of contraction


Describe the role of Ca2+ in the contraction of cardiac muscle

Depolarisation of the membrane of the cardiac myocyte spreads down T-tubules allowing it to get closer to the sarcoplasmic reticulum. During depolarisation there is an activation of voltage-gated L-type Ca2+ channels in the membrane of the cardiac myocyte which causes Ca2+ influx. This results in a rise in intracellular concentrations of Ca2+ specially during the plateau phase (100nM to 1-10μm). This Ca2+ entry from outside the cell leads to activation of the ryanodine receptor in the membrane of the sarcoplasmic reticulum. This in turn causes store calcium release of Ca2+ (CICR) by SR which acts to enhance the increase in intracellular Ca2+ as part of a positive feedback system. The massive increase in intracellular Ca2+ leads to it binding to and displacing the troponin/tropomyosin complex from actin. This allows myosin to contact actin and filament sliding to occur


What is the significance of the intercalated discs

The intercalated discs are crosslinkages in the cardiac myocytes that help electrical conduction between cells


Define the Frank-Starling law

Changing the preload changes the resting tension and the tension that the heart can generate when it actively contracts


What is the relationship between changing the filling pressure/preload and the force of contraction in the heart. How does this relate to stroke volume and cardiac output

Increases in filling pressure (LVEDP/myocardial fibre stretching) increase the force of contraction. This in turn increases stroke volume and cardiac output


Draw a diagram of the typical healthy Frank-Starling curve

See completed diagram below


What can be said about the gradient of the Frank-Starling curve in healthy patients

It is relatively steep


In healthy patients what would be the effect of changes in the preload on the cardiac output

Changes in venous return would have a massive effect on stroke volume


Normally venous return is continually fluctuating to maintain cardiac output T or F

F – its remains relatively unchanged as small changes in preload will have a massive effect on cardiac output


The main regulation of cardiac output in healthy patients in the Frank-Starling mechanism T or F

F – the sympathetic nervous system takes over


In what situation(s) is the heart reliant on the Frank-Starling relationship

Heart transplant patients do not have a nerve supply to the heart so their cardiac output is controlled entirely by circulating catecholamines and the Frank-Starling mechanism


What is the effect of denervating the heart what does this tell us about its autonomic nervous system regulation

Denervation of the heart causes an increase in heart rate this implies that the parasympathetic nervous system is dominant and that vagal stimulation is occurring under rest conditions