Regulation of Stroke Volume and Heart Rate

Question 1

Explain the effects of the sympathetic system on heart rate

Answer 1

• Sympathetic nerves release noradrenaline
• Plus circulating adrenaline from adrenal medulla
• Both act on β1 receptors on sinoatrial node
• Increases slope of the pacemaker potential
• Increases heart rate = tachycardia

Question 2

Explain the effects of the parasympathetic system on heart rate

Answer 2

• Vagus nerve releases acetylcholine
• Acts on muscarinic receptors on sinoatrial node
• Hyperpolarises cells and decreases slope of pacemaker potential
• Decreases heart rate = bradycardia

Question 3

Explain the effects of preload on stroke volume

Answer 3

Increased venous return, ­increased EDV and therefore ­increased stroke volume

Decreased venous return, decreased EDV and therefore decrease stroke volume

Ensures self-regulation – matches SV of left and right ventricles

Question 4

What is afterload and explain the effects it has on stroke volume

Answer 4

Afterload is the load against which the muscle tries to contract.
if TOTAL PERIPHERAL RESISTANCE increases, 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, i.e. stroke volume will decrease

Question 5

Sympathetic nervous system regulation of stroke volume

Answer 5

• Sympathetic nerves releasing noradrenaline
• Plus circulating adrenaline from adrenal medulla
• Both act on β1 receptors on the myocytes
• Increases contractility (an inotropic effect)
• Gives stronger, but shorter contraction

Question 6

Parasympathetic regulation of stroke volume

Answer 6

• Little effect
• Probably because the vagus nerve does not innervate the ventricular muscle

Question 7

CARDIAC OUTPUT =

Answer 7

Heart rate x stroke volume

Question 8

Describe the importance of these factors in the control of cardiac output.

Answer 8

Increasing heart rate (e.g. with electronic pacemaker) causes a small increase in cardiac output but stroke volume starts decreasing

Question 9

What effect does physiological increases in heart rate have on the body

Answer 9

• HR increases
• Contractility increases
• Venous return increases
• Total peripheral resistance falls
• CO increases 4-6 times