regulation of stroke volume and heart rate Flashcards
what is the function of pacemaker cells?
responsible for the autorhythmicity of the heart
what controls the sympathetic action of the heart?
noradrenaline released from sympathetic nerves and adrenaline released from the adrenal medulla
what does noradrenaline act on in the heart?
Beta1 receptors
where are the beta1 adrenergic receptors in the heart?
the sinoatrial node
what is the action of NA and adrenaline on the sinoatrial node?
it increases the slope of the pacemaker potential so the pacemaker cells reach their threshold and depolarise more rapidly, this causes an increase in heart rate
which nerve supplies the heart with parasympathetic supply?
the vagus nerve
what neurotransmitter does the vagus nerve release and what receptors does it act on?
ACh, acting on muscarinic receptors in the sinoatrial node
what does ACh from the vagus nerve acting on muscarinic receptors do to the pacemaker cells?
hyperpolarises them and decreases the slope of the pacemaker cells so the cells reach their threshold slower and heart rate decreases
what are the 4 things that can alter the strength of contraction of the heart?
- preload
- afterload
- neural
- pathological
what does Starling’s law state?
the energy of contraction is proportional to the initial length of the cardiac muscle fibre
how does the tension that a muscle can exert change with starting length of that muscle fibre?
tension increases steadily. the increase then slows until the peak tension is reached. After this the tension decreases
why does the tension muscle can produce change when length is changed?
because the over lap of the actin and myosin filaments changes so the number of possibe cross bridges changes
in vivo, what affects preload?
end diastolic volume
how does an increased venous return affect stroke volume?
it causes an increase in end diastolic volume which causes an increased preload and so an increased stroke volume
how does a decrease in venous return affect stroke volume?
it causes an increase in end diastolic volume which causes a decrease in preload and so stroke volume increases
what does preload allow , in terms of cardiac output from left and right sides of the heart?
it ensures self-regulation as the resting EDV is not at the maximum for the max stroke volume, nor is at its minimum. this means stroke volume can vary to match the stroke volume on the other side of the heart
what is afterload?
the load against which the muscle tries to contract
what affects the afterload of the heart?
the aortic pressure which in turn is determined by the total peripheral resisitance
what happens to stroke volume as total peripheral resistance increases?
it would decrease, as aortic pressure would increase so the pressure required in left ventricle to exceed this pressure would also increase. Afterload has increases so this means more energy would be used to create this pressure, and less on ejecting blood
what happens to stroke volume when veins and venules contract? why?
it increases, as blood is forced from the capacitance vessels into the heart, increases EDV, so increasing preload and stroke volume
does vasoconstriction affect stroke volume?
no, just arteriole constriction
describe how arteriole constriction affects stroke volume?
it causes it to decrease, as total peripheral resistance increases and so aortic pressure increases and afterload increases
how does noradrenaline and adrenaline affect the myocytes of the heart?
they bind to beta1 receptors on the myocytes which causes an inotropic effect that gives a stonger but shorter contraction
what is an inotropic effect?
a change in the force or speed of contraction of muscle
what is the parasympathetic effect on the myocytes of the heart?
little effect
how does hypercalcaemia affect stroke volume? why?
it increases the stroke volume as there is more calcium outside the myocytes so more enters the cell, causing more myosin binding sites to be uncovered
how does hypercalcaemia affect the stroke volume vs EDV curve?
shifts it up and to the left
how does hypocalcaemia affect the stroke volume vs EDV curve?
shifts it down and to the right
how does hypocalcaemia affect stroke volume? why?
causes a decrease in stroke volume, as less calcium enters the myocytes when calcium channels in their membranes open
how does ischaemia shift the stroke volume vs EDV curve?
down and to the right
why does ischaemia cause the stroke volume vs EDV curve to shift down and to the right?
down:
for the same EDV as before, stroke volume decreases as there are less living myocytes to contract
to the right:
as the other side of the heart is still healthy and has a higher stroke volume. this means the EDV in the ischaemic side of the heart increases and stroke volume increases again by preload.
how do barbituates affect the stroke volume vs EDV curve?
causes it to shift down and to the right, producing an anaesthetic effect
how can cardiac output be increased by 4-6 times physiologically but only by 2 times by a pacemaker?
a pacemaker can only increases the HR which can only increase cardiac output by 2 times until it cuts into the rapid filling time. Physiologically heart rate can be increases by:
- increasing heart rate
- increasing contractility
- increasing venous return
- decreasing total peripheral resistance
