Regulation of Stroke Volume and Heart Rate Flashcards
What is the control mechanism of heart rate?
Neural
What is the regulation mechanism for stroke volume?
Preload
Afterload
Neural
Pathological
How does the sympathetic nervous system regulate heart rate?
Releases noradrenaline from nerves and adrenal medulla.
Act on Beta 1 receptors on sinoatrial node
Increases slope of pace maker potential between heart beats
Increases heart rate
What is the effect of the parasympathetic nervous system on heart rate?
Vagus nerve releases acetycholine
Acts on muscarinic receptors on sinoatrial node
Hyperpolarises cells and decreases the slope of pacemaker potential
Decreases heart rate = bradycardia
What does starlings law state?
Starling’s Law states -
Energy of contraction is proportional to initial length of cardiac muscle fibre
Why is the slope decreasing to the left side of the peak tension?
Actin filaments start to interfere with each other
What is the length of the cardiac muscle fibres proportional to?
End diastolic volume
What is the stroke volume related to?
Tension
What is preload affected by?
End diastolic volume
What ensures self regulation of stroke volume of left and right ventricles?
Proportional relationship between volume of venous return & stroke volume
What is the afterload?
The load against which muscle tries to contract
What is the aortic pressure affected by?
How much blood is pushed into aorta (cardia output)
How easy it is for blood to get out of aorta (total peripheral resistance)
Why does an increase in TPR result in a reduction in stroke volume?
TPR- Total Peripheral Resistance
Aortic pressure will increase,
Ventricle will have to work harder to push open the aortic valve
It will have less energy left to do useful bit of ejecting blood.
What is responsible for the variable portion of the total peripheral resistance?
The arterioles
What is the total peripheral resistance set by?
The arterial pressure - what sets the afterload
What affects preload?
Venules/veins and capacitance vessels
These all affect the venous return
What affects afterload?
Arterioles and resistance vessels
What does a bigger end diastolic volume mean?
Bigger pre-load and therefore more tension and more strength of contraction
neural
How does the sympathetic nervous system affect stroke volume?
Noradrenaline from nerves & adrenal medulla act on beta 1 receptors of myocytes
Increases contractility - stronger but shorter contraction
What does ionotropic refer to?
strength of contraction
What is the effect of the parasympathetic nervous system on the contractility of the heart?
Little effect - vagus nerve does not innervate the ventricular muscle
How does the heart compensate for a reduced pumping ability?
Works around a bigger end diastolic volume - lower ejection fraction and reduced excersize capacity.
What is the effect of barbituates on stroke volume?
–shifts curve down and right
Why does stroke volume decrease with an increased heart rate?
Shortened cardiac cycle cuts into rapid filling stage - smaller EDV (small length of muscle), less tension and therefore less stroke volume
What are the combined mechanisms that increase heart rate?
Via decreased vagal tone and increased sympathetic tone
What shortens systole?
Effect of increased sympathetic tone on the contractility - alters ionotropic state
How does venous return increase?
Via venoconstriction
And skeletal / respiratory pumps
Maintains preload when we expect it to fall with a reduced EDV brought on with a fast heart rate
Why does total peripheral resistance fall with a fast heart rate?
Arteriolar dilation in muscle, skin & heart
Reduced afterload - makes it easier for heart to pump out blood
What are the 4 ways CO increases when workload increases?
HR increases
Contractility increases
Venous return increases
Total peripheral resistance falls
Summary of regulation of stroke volume
