Regulation of Stroke Volume and Heart Rate Flashcards
What is the control mechanism of heart rate?
Neural
What is the regulation mechanism for stroke volume?
Preload
Afterload
Neural
Pathological
How does the sympathetic nervous system regulate heart rate?
Releases noradrenaline from nerves and adrenal medulla.
Act on Beta 1 receptors on sinoatrial node
This increases the slope of the pace maker potential between heart beats
Increases heart rate
What is the effect of the parasympathetic nervous system on heart rate?
Vagus nerve releases acetycholine
Acts on muscarinic receptors on the sinoatrial node
Hyperpolarises cells and decreases the slope of the pacemaker potential
Decreases heart rate = bradycardia
What does starlings law state?
Starling’s Law states - the energy of contraction is proportional to the initial length of the cardiac muscle fibre
Why is the slope decreasing to the left side of the peak tension?
Actin filaments start to interfere with each other
What is the length of the cardiac muscle fibres proportional to?
End diastolic volume
What is the stroke volume related to?
Tension
What is preload affected by?
End diastolic volume
What ensures self regulation of stroke volume of left and right ventricles?
The proportional relationship between the volume of venous return and the stroke volume
What is the afterload?
The load against which the muscle tries to contract
What is the aortic pressure affected by?
How much blood is pushed into the aorta (cardia output)
How easy it is for the blood to get out of the aorta (total peripheral resistance)
Why does an increase in TPR result in a reduction in stroke volume?
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 is responsible for the variable portion of the total peripheral resistance?
The arterioles
What is the total peripheral resistance set by?
The arterial pressure - what sets the afterload
What affects preload?
Venules/veins and capacitance vessels
These all affect the venous return
What affects afterload?
Arterioles and resistance vessels
What does a bigger end diastolic volume mean?
Bigger pre-load and therefore more tension and more strength of contraction
How does the sympathetic nervous system affect stroke volume?
Noradrenaline from nerves and the adrenal medulla act on beta 1 receptors of the myocytes - which increases contractility - stronger but shorter contraction
What does ionotropic refer to?
The strength of contraction
What is the effect of the parasympathetic nervous system on the contractility of the heart?
Little effect - vagus nerve does not innervate the ventricular muscle
What is the effect of hypercalcaemia, hypocalcaemia and ischaemia on stroke volume?
•Hypercalcemia
–shifts curve up and left – more strength of contraction, more X bridges, more extracellular and therefore more intracellular calcium when the action potential reaches the muscle
•Hypocalcemia
–shifts curve down and right
•Ischaemia
–shifts curve down and right
How does the heart compensate for a reduced pumping ability?
Works around a bigger end diastolic volume - lower ejection fraction and reduced excersize capacity.
What is the effect of barbituates on stroke volume?
–shifts curve down and right
