Regulation of Stroke Volume and Heart Rate

Question 1

How does the sympathetic system affect heart rate?

Answer 1

Sympathetic nerves release noradrenaline
This noradrenaline plus circulating adrenaline from the adrenal medulla act on the beta-1 receptors on the sinoatrial node
This increases the slope of the pacemaker potential and increases the heart rate

Question 2

How does the parasympathetic system affect heart rate?

Answer 2

Vagus nerve releases acetylcholine
This acts on muscarinic receptors on the sinoatrial node
This hyperpolarises the cells and decreases the slope of the pacemaker potential, decreasing heart rate

Question 3

What receptors on the sinoatrial node do adrenaline and noradrenaline act on?

Answer 3

Beta 1

Question 4

What receptors on the sinoatrial node does acetylcholine act on?

Answer 4

Muscarinic

Question 5

How does the sympathetic system affect stroke volume?

Answer 5

Nerves release noradrenaline
This plus circulating adrenaline from adrenal medulla act on beta-1 receptors on myocytes
This increases contractility and causes stronger, shorter contractions - stroke volume is increased

Question 6

How does the parasympathetic system affect stroke volume?

Answer 6

Little effect, probably because vagus nerve does not innervate the ventricular muscle

Question 7

What does Starling’s law state?

Answer 7

Energy of contraction is proportional to the initial length of the cardiac muscle fibre

Question 8

What is peak tension?

Answer 8

The point of maximum overlap between myosin and actin on the cross bridges

Question 9

How does venous return affect EDV and SV?

Answer 9

Increased venous return increases EDV and therefore increases stroke volume
(and vice versa)

Question 10

Preload is affected by EDV, what does this ensure?

Answer 10

Self-regulation - it matches the stroke volume of the left and right ventricles

Question 11

What is the afterload?

Answer 11

Load against which the muscle tries to contract

Question 12

What is aortic pressure affected by?

Answer 12

How much blood is pushed into the aorta (cardiac output)

How easy it is for that blood to get out of the aorta (total peripheral resistance)

Question 13

What effect does increasing total peripheral resistance have on aortic pressure?

Answer 13

If total peripheral resistance increases, aortic pressure will increase

Question 14

What does increased aortic pressure mean?

Answer 14

Ventricle will have to work harder to push open the aortic valve and it will have less energy left due to the act of ejecting blood, so stroke volume will decrease

Question 15

What is afterload set by?

Answer 15

Arterial pressure against which blood is expelled, which in turn depends on TPR

Question 16

What vessels affect preload and afterload?

Answer 16

Venules and veins affect preload

Arterioles affect afterload

Question 17

What effect does hypercalcaemia have on stroke volume?

Answer 17

Increases it

Question 18

What effect does hypocalcaemia have on stroke volume?

Answer 18

Decreases it

Question 19

What effect does ischaemia have on stroke volume?

Answer 19

Decreases it

Question 20

What effect do barbiturates have on stroke volume?

Answer 20

Decrease it

Question 21

What does Starling’s Law let the heart compensate for?

Answer 21

A reduced pumping ability - the heart will compensate by working around a bigger EDV

Question 22

What effect does a reduced pumping ability have of the heart?

Answer 22

Results in a lower ejection fraction and reduced exercise capacity

Question 23

What equation can be used to calculate ejection fraction?

Answer 23

Ejection fraction = SV/EDV

Question 24

What equation can be used to calculate cardiac output?

Answer 24

CO = HR x SV

Question 25

What effect does increasing heart rate caused by an electronic pacemaker have? Why is this?

Answer 25

Causes a small increase in CO but then SV decreases

This is because of the shortened cardiac interval

Question 26

What do heart rates above 150bpm cut into?

Answer 26

Rapid filling phase

Question 27

What effect does reduced EDV have?

Answer 27

Reduces preload and so reduces stroke volume

Question 28

What causes an increase in heart rate?

Answer 28

Decreased vagal tone

Increased sympathetic tone

Question 29

What causes an increase in contractility?

Answer 29

Increased sympathetic tone

Question 30

What causes an increase in venous return?

Answer 30

Venoconstriction

Skeletal muscle and respiratory pumps

Question 31

What causes a decrease in total peripheral resistance?

Answer 31

Arteriolar dilation in muscle, skin and heart