Regulation of stroke volume and heart rate

Question 1

Sympathetic system effect on heart rate

Answer 1

• Noradrenaline (and circulating adrenaline)
• Beta 1 receptors on SA node
• Increase HR (positive chronotrophy)
• Increase conduction velocity (positive dromotrophy)

Question 2

Parasympathetic system effect on heart rate

Answer 2

• Acetylcholine from vagus nerve
• Muscarinic receptors on SA node
• Hyperpolarises cells (increasing potassium and decreasing Ca and Na movement)
• Decreases HR

Question 3

Sympathetic system effect on stroke volume

Answer 3

• Noradrenaline and adrenaline
• Beta 1 receptors on myocytes
• Increases contractility
  Stronger, shorter contraction (more filling phase time)

Question 4

Parasympathetic system effect on stroke volume

Answer 4

Little effect

- Vagus does not innervate the ventricular muscle

Question 5

Preload effect on stroke volume

Answer 5

1. Greater end diastolic volume (venous return)
2. Greater preload
3. Greater SV
   Starling’s Law: energy of contraction is proportional to the initial length of the cardiac muscle fibre

Question 6

Afterload effect on stroke volume

Answer 6

1. Increased total peripheral resistance (determined by state of arteriole contraction)
2. Increased afterload
3. Decreased SV

Question 7

Physiological increases in HR (decreased vagal tone and/or increased sympathetic tone) are accompanied by…

Answer 7

1. Increased contractility (increased sympathetic tone)
2. Increased venous return (venoconstriction, skeletal/respiratory pump)
3. TPR decreased (arteriolar dilation in muscle, skin and heart)

Question 8

Effect of calcium on EDV and stroke volume

Answer 8

• Hypercalcaemia: greater SV for a smaller EDV

- Hypocalcaemia: lower SV for a greater EDV

Question 9

Effect of ischaemia on stroke volume

Answer 9

Decreased

- Heart compensates by lowering EJ and exercise capacity

Question 10

Effect of barbiturates on stroke volume

Answer 10

Depress CNS

- lower SV for a greater EDV