Regulation of Stroke Volume and Heart Rate Flashcards
Regulation of the heart rate is _______.
neural
The sympathetic nervous system has what effect on regulation of heart?
Tachycardia
What does the sympathetic nervous system release?
Noradrenaline released from sympathetic nerves and;
adrenaline released from the adrenal medulla.
What does noradrenaline and circulating adrenaline act on?B1 receptors on sinoatrial node
Beta-1 receptors on sinoatrial node
What impact does the sympathetic nervous system have on the SA node?
Increase slope of the pacemaker potential, pacemaker cells reach threshold quicker.
Increases heart rate (tachycardia)
What does the parasympathetic system release?
Acetylcholine (ACh)
What nerve from the parasympathetic system acts on the heart?
Vagus nerve (CN X)
What does the parasympathetic system act on?
Muscurinic receptors on sinoatrial node
What effect does the parasympathetic system have on the sinoatrial node?
Hyperpolarises cells and decreases slope of pacemaker potential.
Decreases heart rate (bradycardia)
What are the 4 things that can alter the strength of contraction (stroke volume) of the heart?
Preload
Afterload
Neural
Pathological
What does Starling’s Law state?
The energy contraction is proportional to the initial length of the cardiac muscle fibre.
How does tension of cardiac muscle fibres change with length (preload)?
Tension increases until it reaches a maximum then it decreases as the length increases.
What is the relationship between tension and length (preload) due to?
Because the over lap of the actin and myosin filaments changes, therefore the number of potential cross bridges changes also.
What is the curve of tension against length (preload) essentially the same as?
End diastolic volume against stroke volume
EDV v SV
In vivo, what affects preload?
EDV
What impact does increases venous return have on EDV and stroke volume?
EDV: increases
SV: increases
What effect does decreasing venous return have on EDV and stroke volume?
EDV: decreases
SV: decreases
What does the relationship between EDV and stroke volume ensure?
Self regulation so that the stroke volume of the left and right ventricles are the same.
Resting EDV is neither at the max or min SV.
What is afterload?
The load against which a muscle tries to contract.
What is the aortic pressure impacted by?
Total peripheral resistance (TPR)
What happens to the aortic pressure if total peripheral resistance increases?
- aortic pressure increases
- LV will have to use more work (energy) in opening the aortic valve
- SV will decrease as there is less energy for the ejection phase of contraction.
What happens to stroke volume if total peripheral resistance increases?
It decreases
What affects the afterload of the heart?
The aortic pressure which in turn is determined by TPR.
What kind of vessels affect preload?
capacitance (veins/venules)
What kind of vessels affect afterload?
resistance (arterioles)
How is the stroke volume regulated neurally?
By the sympathetic nervous system
What does the sympathetic system act on to impact stroke volume?
Beta-1 receptors on the myocytes
What does the sympathetic system acting on B1 receptors of myocytes cause?
Increases contractibility (inotropic effect)
Gives stronger, but shorter contractions.
What impact does the parasympathetic system have on stroke volume?
Why?
Little effect
Due to vagus nerve not innervating the ventricular muscle.
What is preload?
How full the ventricle is before it starts contracting (EDV).
What is afterload?
What is it affected by?
How difficult it is for the heart to pump out the blood (i.e. the TPR).
Affected by the state of contraction of arterioles.
What happens to stroke volume when veins/venules contract?
Why?
It increases
- increased venous return to the heart
- increases EDV
- increases preload therefore increasing stroke volume.
Does vasoconstriction affect stroke volume?
No, just arteriole constriction.
Describe how arteriole constriction affects stroke volume?
SV decreases because:
- TPR increases
- aortic pressure increases
- afterload increases
- LV needs to work harder
What is an inotropic effect?
A change in the force or speed of contraction of muscle.
What are examples of pathological impacts on stroke volume?
4 points
Hypercalcemia
Hypocalcemia
Ischaemia
Barbiturates
How does hypercalcemia impact the stroke volume/EDV curve?
Why?
Shifts it up and left
- more Ca outside myocytes
- more actin-myosin cross bridge formation
- increased SV
How does hypocalcemia impact the stroke volume/EDV curve?
Shifts it down and right
How does ischaemia impact the stroke volume/EDV curve?
Shifts it down and right
How does barbiturates impact the stroke volume/EDV curve?
Shifts it down and right, producing an anaesthetic effect.
What is ischaemia?
Restriction in blood supply to tissues.
Why does ischaemia cause the stroke volume vs EDV curve to shift down and to the right?
Down: stroke volume decreases as there are less living myocytes to contract.
Right: As the other side of the heart is still healthy and has a higher SV. This means the EDV in the ischaemic side of the heart increases and SV increases again by preload.
What are barbiturates?
Drug that acts on the CNS as a depressant.
Cardiac output = _____ + ______
SV + HR
Increasing HR with an electronic pacemaker, causes a small increase in ____, but then _____ decreases.
Why?
CO
SV
- Shortened cardiac interval cuts into the rapid filling phase.
- Reduced EDV, reduces preload
- SV decreases (Starlings)
How does the heart compensate for a reduced pumping ability?
By working around a bigger EDV, resulting in a lower ejection fraction and reduced exercise capacity.
Outline the things that accompany physiological increases in heart rate to offset the effect of decreased stroke volume? (4)
- HR increases
- Contractility increases
- Venous return increases
- TPR falls
What causes the HR to increase? (2)
- increased sympathetic tone
- decreased vagal tone
What causes contractility to increase? (2)
- increased sympathetic tone
- alters inotropic state & shortens systole (shorter but stronger contractions)
What causes the venous return to increase? (3)
- vasoconstriction
- skeletal/resp. pumps
- maintains preload
What causes the TPR to fall? (2)
- arteriole dilation
- reduces afterload
How much can a pacemaker increase the CO by?
2x
How much can the physiological mechanisms increase the CO by?
4-6x
