Regulation of Stroke Volume and Heart Rate

Question 1

Explain the effects of the sympathetic system on heart rate

Answer 1

• Noradrenaline (sympathetic nerves) and circulating adrenaline (adrenal medulla)
• Act on beta-1 receptors on SA node
• Increases slope of pacemaker potential
• Increases HR = tachycardia

Question 2

Explain the effects of the parasympathetic system on heart rate

Answer 2

• ACh (vagus nerve)
• Acts on muscarinic receptors on SA node
• Hyperpolarises cell and decreases slope of pacemaker potential
• Decreases HR = bradycardia

Question 3

Explain the effects of the sympathetic system on stroke volume

Answer 3

• Noradrenaline (sympathetic nerves) and circulating adrenaline (adrenal medulla)
• Act on beta-1 receptors on myocytes
• Increase contractility (positive inotropic effect)
• Stronger but shorter contractions

Question 4

Explain the effects of the parasympathetic system on stroke volume

Answer 4

• Little effect

- Vagus nerve does not innervate ventricular muscle

Question 5

Explain the effects of the preload on stroke volume

Answer 5

Preload = length of muscle fibres

A large preload causes a strong contraction, which decreases ESV and increases SV.

Question 6

Explain the effects of the afterload on stroke volume

Answer 6

Afterload = the load against which muscle tries to contract
Aortic pressure is controlled by TPR. If TPR increases, aortic pressure i.e. afterload increases, more work to push open aortic valve, less work available to eject blood, so SV decreases.

Question 7

Describe the importance of SNS, PNS, preload and afterload in the control of cardiac output

Answer 7

CO = HR x SV

HR increases, contractility increases, venous return increases, TPR decreases, CO increases 4-6 times.

Question 8

What pathologies affect stroke volume?

Answer 8

• Hypercalcaemia: shifts curve up and left
  Decrease in EDV, increase in SV
• Hypocalcaemia: shifts curve down and right
• Ischaemia: shifts curve down and right
  Increases in EDV, decrease in SV

Question 9

What pathologies affect stroke volume?

Answer 9

• Hypercalcaemia
  Decrease in EDV, increase in SV
• Hypocalcaemia
• Ischaemia
• Barbiturates (CNS depressant)
  Increases in EDV, decrease in SV

Question 10

Increase in HR causes increases in CO but slight decrease in SV, why?

Answer 10

Shortened cardiac interval cuts into rapid filling phase, reducing EDV and preload, thus reducing SV.