Regulation of Stroke Volume and Heart Rate Flashcards
How does the sympathetic system affect heart rate?
Sympathetic nerves release noradrenaline
This noradrenaline plus circulating adrenaline from the adrenal medulla act on the beta-1 receptors on the sinoatrial node
This increases the slope of the pacemaker potential and increases the heart rate
How does the parasympathetic system affect heart rate?
Vagus nerve releases acetylcholine
This acts on muscarinic receptors on the sinoatrial node
This hyperpolarises the cells and decreases the slope of the pacemaker potential, decreasing heart rate
What receptors on the sinoatrial node do adrenaline and noradrenaline act on?
Beta 1
What receptors on the sinoatrial node does acetylcholine act on?
Muscarinic
How does the sympathetic system affect stroke volume?
Nerves release noradrenaline
This plus circulating adrenaline from adrenal medulla act on beta-1 receptors on myocytes
This increases contractility and causes stronger, shorter contractions - stroke volume is increased
How does the parasympathetic system affect stroke volume?
Little effect, probably because vagus nerve does not innervate the ventricular muscle
What does Starling’s law state?
Energy of contraction is proportional to the initial length of the cardiac muscle fibre
What is peak tension?
The point of maximum overlap between myosin and actin on the cross bridges
How does venous return affect EDV and SV?
Increased venous return increases EDV and therefore increases stroke volume
(and vice versa)
Preload is affected by EDV, what does this ensure?
Self-regulation - it matches the stroke volume of the left and right ventricles
What is the afterload?
Load against which the muscle tries to contract
What is aortic pressure affected by?
How much blood is pushed into the aorta (cardiac output)
How easy it is for that blood to get out of the aorta (total peripheral resistance)
What effect does increasing total peripheral resistance have on aortic pressure?
If total peripheral resistance increases, aortic pressure will increase
What does increased aortic pressure mean?
Ventricle will have to work harder to push open the aortic valve and it will have less energy left due to the act of ejecting blood, so stroke volume will decrease
What is afterload set by?
Arterial pressure against which blood is expelled, which in turn depends on TPR
What vessels affect preload and afterload?
Venules and veins affect preload
Arterioles affect afterload
What effect does hypercalcaemia have on stroke volume?
Increases it
What effect does hypocalcaemia have on stroke volume?
Decreases it
What effect does ischaemia have on stroke volume?
Decreases it
What effect do barbiturates have on stroke volume?
Decrease it
What does Starling’s Law let the heart compensate for?
A reduced pumping ability - the heart will compensate by working around a bigger EDV
What effect does a reduced pumping ability have of the heart?
Results in a lower ejection fraction and reduced exercise capacity
What equation can be used to calculate ejection fraction?
Ejection fraction = SV/EDV
What equation can be used to calculate cardiac output?
CO = HR x SV
What effect does increasing heart rate caused by an electronic pacemaker have? Why is this?
Causes a small increase in CO but then SV decreases
This is because of the shortened cardiac interval
What do heart rates above 150bpm cut into?
Rapid filling phase
What effect does reduced EDV have?
Reduces preload and so reduces stroke volume
What causes an increase in heart rate?
Decreased vagal tone
Increased sympathetic tone
What causes an increase in contractility?
Increased sympathetic tone
What causes an increase in venous return?
Venoconstriction
Skeletal muscle and respiratory pumps
What causes a decrease in total peripheral resistance?
Arteriolar dilation in muscle, skin and heart
