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
What is the regulation mechanism for stroke volume ?
Preload
Afterload
Neural
Pathological
How does the sympathetic nervous system regulate heart rate ?
Sympathetic nerves release noradrenaline
Plus circulating adrenaline from adrenal medulla
Both act on B1 receptors on SAN
Increases slope of the pacemaker potential
Increases heart rate = tachycardia
What is the effect of the parasympathetic nervous system on heart rate ?
Vagus nerve releases Ach
Acts on muscarinic receptors on SAN
Hyperpolarises cells and decreases slope on pacemaker potential
Decrease 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
In vivo, what is preload affected by ?
Preload is affected by the end diastolic volume
What does increased venous return do ?
Increases EDV and therefore increases stroke volume
What does decreased venous return do ?
Decreases EDV and therefore decreases stroke 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 afterload ?
Load against which the muscles 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
How is cardiac output calculated ?
HR X SV = cardiac output
Why does stroke volume decrease with an increased heart rate ?
Shortened cardiac cycle cuts into rapid filling stage - smaller EDV (small length of muscle), less tension and therefore less stroke volume
What are the combined mechanisms that increase heart rate ?
Via decreased vagal tone and increased sympathetic tone
What shortens systole ?
Effect of increased sympathetic tone on the contractility - alters ionotropic state
How does venous return increase ?
Via venoconstriction and skeletal / respiratory pumps
Maintains preload when we expect it to fall with a reduced EDV brought on with a fast heart rate
Why does total peripheral resistance fall with a fast heart rate ?
Arteriolar dilation in muscle, skin & heart
Reduced afterload - makes it easier for heart to pump out blood
