regulation of stroke volume and heart rate Flashcards
what regulates the heart rate?
sympathetic and parasympathetic nervous system
In what way dos sympathetuc nervous system regulate the 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
In what way does the parasympathetic system regulate the heart rate?
vagus releases ACh
acts on muscarinic receptors on sinoatrial node
hyperpolarises cells and decreases slope of pacemaker potential
decreases heart rate = bradycardia
what is starlings law associated with? and what is it?
preload- stroke volume
the energy of contraction is proportional to the initial length of the cardiac muscle fibre
what is preload affected by?
the End Diastolic Volume
is end diastolic volume and stroke volume directly or indirectly proportional?
directly
what happens to EDV and stroke volume when there is an increase in venous return?
increase EDV therefore increase in stroke volume
what happens to EDV and stroke volume when there is a decrease in venous return?
decrease in EDV and therefore decrease in stroke volume
what is the after load?
the load against which the muscle tries to contract
what is aortic pressure affected by?
how much blood is pushed into the aorta (ie the cardiac output)
how easy it is for that blood to get out of the aorta (ie the total peripheral resistance)
why would stroke volume decrease if aortic pressure will increase?
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
what hapens to stroke volume if total peripheral resistance is increased?
stroke volume will decrease as it increases aortic pressure
what is the difference between preload and afterload?
preload-
venules/veins
capacitance vessels
afterload -
arterioles
resistance vessels
what regulates stroke volume?
preload
afterload
neural (symp)
pathological
how does sympathetic nervous system regulate 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
does parasympathetic system have an effect on stroke volume and why?
no-
probably because the vagus does not innervate the ventricular muscle
what are the pathological effects that regulate stroke volume?
Hypercalcemia
Hypocalcemia
Ischaemia
Barbiturates
whats the effect of Ischaemia
on the curve
shifts curve down and right
whats the effect of Hypocalcemia on the curve
shifts curve down and right
whats the effect of hypercalcemia on the curve?
shifts curve up and left
what is the effect of Barbiturates on the curve?
shifts curve down and right
whats the formula for cardiac output?
heart rate x stroke volume
what is the effect of increasing HR on CO and stroke volume?
small increase on CO but stroke volume decreases
why does Increasing HR with an electronic pacemaker, causes a small increase in CO, but then SV decreases
Shortened cardiac interval cuts into the rapid filling phase
Reduced end diastolic volume reduces preload
So by Starling’s law, reduces stroke volume
what increases HR
via decreased vagal tone
and increased sympathetic tone
how is contractility increased?
via increased sympathetic tone
alters inotropic state and shortens systole
how is venous return increased?
via vasoconstriction
and skeletal/ respiratory pumps
maintains preload
what causes total peripheral resistance falls?
due to arteriolar dilation in muscle, skin and heart
reduces afterload
