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
This increases slope of pace maker potential between heart beats
Increases heart rate = tachycardia
What is the effect of the parasympathetic nervous system on heart rate?
Vagus nerve releases acetylcholine
Acts on muscarinic receptors on sinoatrial node
Hyperpolarises cells and decreases slope of pacemaker potential
Decreases heart rate = bradycardia
What does starlings law state?
Starling’s Law - 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 and stroke volume
What is the afterload?
Load against which the muscle tries to contract
What is the aortic pressure affected by?
How much blood is pushed into the aorta (cardia output)
How easy it is for blood to get out of the aorta (total peripheral resistance)
Why does an increase in TPR result in a reduction in stroke volume?
Aortic pressure will increase, ventricle will have to work harder to push open the aortic valve
Will have less energy left to eject blood
What is responsible for the variable portion of the total peripheral resistance?
arterioles
What is the total peripheral resistance set by?
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
How does the sympathetic nervous system affect stroke volume?
Noradrenaline from nerves and 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 ventricular muscle
What is the effect of hypercalcaemia, hypocalcaemia and ischaemia on stroke volume?
Hypercalcemia
- shifts curve up and left – more strength of contraction, more X bridges, more extracellular and intracellular calcium when action potential reaches the muscle
Hypocalcemia
- shifts curve down and right
Ischaemia
- shifts curve down and right
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 barbiturates 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 contractility - alters ionotropic state
How does venous return increase?
Via venoconstriction and skeletal / respiratory pumps
Maintains preload (expect it to fall with reduced EDV with 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
