Regulation of SV and HR Flashcards
What is the role of the SNS in HR?
Sympathetic nerves release noradrenaline.
The adrenal medulla releases adrenaline in circulation.
Both act on B1 receptors on the SAN.
Increases the slope of the pacemaker potential.
Increases HR (tachycardia).
What is the role of the PNS in HR?
Vagus nerve releases acetylcholine.
Acts on muscarinic receptors on the SAN.
Hyperpolarises cells and decreases the slope of the pacemaker potential.
Decreases HR (bradycardia).
What is Starling’s Law?
The energy of contraction is proportional to the initial length of the cardiac muscle fibre.
What is preload?
Affected by the EDV (in vivo).
Increased venous return - increases EDV and SV.
Decreased venous return - decreases EDV and SV.
Ensures self-regulation - matches SV of both ventricles.
What is afterload?
The load against which the muscle tries to contract.
Determined by the arterial pressure against which the blood is ejected (in vivo) - this depends on the TPR.
What happens in afterload if TPR increases?
Aortic pressure increases.
The ventricle will have to work harder to push open the aortic valve.
It will have less energy to eject blood.
Decreased SV.
What is the role of the SNS in SV?
Adrenaline and noradrenaline both act on B1 receptors in myocytes.
Increases contractility (an inotropic effect).
Stronger but shorter contractions.
What is the role of the PNS in SV?
Parasympathetic - little effect, since the vagus nerve does not innervate the ventricular muscle.
Increasing HR causes a small increase in CO. How is CO reduced?
The shortened cardiac interval cuts into the rapid filling phase.
The reduced EDV reduces preload and SV.
CO decreases.
How does an increase in HR control CO?
Vagal tone decreases.
Sympathetic tone increases.
How does an increase in contractility control CO?
Sympathetic tone increases.
Alters the inotropic state and shortens systole.
How does an increase in venous return control CO?
Venoconstriction and skeletal/respiratory pumps maintain preload.
How does a decrease in TPR control CO?
Arteriolar dilation in muscle, skin and heart.
Afterload decreases.
What physiological events occur in a person that has a heart attack?
Decreased SV.
EDV increases (if venous return is the same).
Preload and SV increase.
The heart reaches a steady state and restores SV, compensating a reduced pumping ability by working at a higher EDV.
Ejection fraction is reduced (sign of an unhealthy heart).
What is a summary of the different factors controlling SV?
Preload - how full the ventricle is before contraction, affected by venules and veins.
Contractility - how strong a contraction is, affected by the SNS.
Afterload - how difficult it is for the heart to pump blood, affected by arterioles.
