Regulation of stroke volume and heart rate Flashcards
What is the effect of the sympathetic nervous system on regulation of heart rate?
sympathetic nerves release noradrenaline
plus circulating adrenaline from adrenal medulla
both act on ß1-receptors on sinoatrial node
increases slope of the pacemaker potential
increases heart rate = tachycardia
What is the effect of the parasympathetic nervous system on the regulation of the heart?
vagus releases ACh
acts on muscarinic receptors on sinoatrial node
hyperpolarises cells and decreases slope of pacemaker potential
decreases heart rate = bradycardia
What happens to the EDV and stroke volume when you get increased venous return?
Increased EDV and stroke volume
What happens to the EDV and stroke volume when you get decreased venous return?
Decreased EDV and stroke volume
What does preload ensure?
Ensures self-regulation – matches SV of left & right ventricles
What is the aortic pressure affected by?
how easy it is for that blood to get out through the arterioles - ie the total peripheral resistance
What will happen to the aortic pressure, work of ventricle, energy and stroke volume if the total peripheral resistance is high?
Increase pressure aorta and the ventricle will have to work harder to push open the aortic valve, and it will have less energy left to do the useful bit of ejecting blood, ie stroke volume will decrease
What do veins and venules affect and why?
Affect preload because they are capacitance
What do arterioles affect and why?
Affect afterload because they are resistance vessels
What is the effect of the sympathetic nervous system on stroke volume?
sympathetic nerves releasing noradrenaline
plus circulating adrenaline from adrenal medulla
both act on ß1-receptors on the myocytes
increases contractility (an inotropic effect)
gives stronger, but shorter contraction
What is the effect of the parasympathetic nervous system on stroke volume?
little effect
probably because the vagus does not innervate the ventricular muscle
What are the main three factors that affect stroke volume?
Preload
Contractility
Afterload
Define preload
How full the ventricle is before it starts contracting
(ie the EDV)
Affected by the state of contraction of venules/veins
Define contractility
How strong a contraction is produced for any given preload or afterload
Affected by the sympathetic system
Define afterload
How difficult it is for the heart to pump out the blood
(ie the TPR)
Affected by the state of contraction of arterioles
Where does the starling curve shift during hypercalcemai?
shifts curve up and left
Where does the starling curve shift during hypocalcemia?
shifts curve down and right
Where does the starling curve shift during ischaemia?
shifts curve down and right
How does the starling law let the heart compensate for a reduced pumping ability?
ie the heart will compensate for a reduced pumping ability by working around a bigger EDV. Results in lower ejection fraction, and reduced exercise capacity.
Where does the starling curve shift during barbiturates injection?
shifts curve down and right
What is the formula for cardiac output?
Cardiac output = Heart rate x Stroke volume
What is the effect of increasing heart rate with an electronic pacemaker cause on CO and SV?
Small increase in Co
Stroke volume decreases
Due to starlings curve
Explaination:Start contraction point earlier and earlier when using electronic pacemaker (decrease time for diastole ) eg: 140 beats for min
Eventually can cut into he rapid filling phase and cosenquences = less effective cross bridge formation and so stroke volume decreases
What can cause venous return increase?
via venoconstriction
& skeletal/respiratory pumps
maintains preload
What factors increase cardiac output?
HR increases
- via decreased vagal tone
- & increased sympathetic tone
Contractility increases
- via increased sympathetic tone
- alters inotropic state & shortens systole
Venous return increases
- via venoconstriction
- & skeletal/respiratory pumps
- maintains preload
Total peripheral resistance falls
- due to arteriolar dilation in muscle, skin & heart
- reduces afterload
CO increase 4-6 times
