Regulation of Stroke Volume and Heart Rate

Question 1

Explain fully the effect of the sympathetic nervous system on heart rate.

Answer 1

• Sympathetic nerves release noradrenaline
• Plus circulating adrenaline form the adrenal medulla
• Both act on beta 1 receptors on the SA node
• Increases the slope of the pacemaker potential
• Increases heart rate = tachycardia

Question 2

Explain fully the effect of the parasympathetic nervous system on heart rate.

Answer 2

• Parasympathetic nerves (vagus) release acetylcholine which act on muscarinic receptors on the SA node
• This hyper polarises the cell and decreases the slope of the pacemaker potential
• Causes a decrease in heart rate = bradycardia

Question 3

State Starling’s Law

Answer 3

The energy of contraction is proportional to the initial length of the cardiac muscle fibre

Question 4

In vivo, preload is affected by what?

Answer 4

End diastolic volume

Question 5

What does increased venous return result in?

Answer 5

An increase in end diastolic volume and therefore an increase in stroke volume
(vice versa for decrease in venous return)

Question 6

How is self- regulation of stroke volume maintained?

Answer 6

Stroke volumes of the left and right ventricles are matched

Question 7

What is afterload?

Answer 7

The load against which the muscle tries to contract

Question 8

Why does an increase in total peripheral resistance decrease stroke volume?

Answer 8

As TPR 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, ie stroke volume will decrease

Question 9

In vivo, afterload is set by what?

Answer 9

The arterial pressure against which the blood is expelled, this in turn depends on the total peripheral resistance

Question 10

What happens if total peripheral resistance increases?

Answer 10

Stroke volume will decrease

Question 11

Capacitance vessels affect preload/afterload

Answer 11

Preload

Question 12

Resistance vessels affect preload/afterload

Answer 12

Afterload

Question 13

Explain fully the effect of the sympathetic nervous system on stroke volume.

Answer 13

• Sympathetic nerve fibres release noradrenaline
• Plus circulating adrenaline form the adrenal medulla
• Both act on beta 1 receptors on the myocytes
• Increases contractility (an inotropic effect)
• Gives stronger, but shorter contraction and increases stroke volume

Question 14

Explain fully the effect of the parasympathetic nervous system on stroke volume.

Answer 14

• No effect

- Probably because the vagus nerve does NOT innervate ventricular muscle