5/ regulation of stroke volume and HR

Question 1

where does HR regulation come from?

Answer 1

neural regulation

Question 2

what governs regulation of stroke volume?

Answer 2

preload, afterload, neural, pathological regulation

Question 3

what is the outcome of regulation of HR and stroke volume?

Answer 3

control of cardiac output

Question 4

what do sympathetic nerves release?

Answer 4

noradrenaline

Question 5

what other hormone is released by sympathetic system?

Answer 5

circulating adrenaline from adrenal medulla

Question 6

where do noradrenaline and adrenaline act in HR regulation?

Answer 6

B1 receptor on sinoatrial node

Question 7

what is the effect of the sympathetic system on the HR?

Answer 7

increases slope of the pacemaker potential (repolarisation of pacemaker cells becomes faster) -> increases HR, tachycardia if HR > 100

Question 8

which nerve is by the parasympathetic system to influence the HR?

Answer 8

vagus nerve

Question 9

what does this system release?

Answer 9

acetylcholine

Question 10

what is the effect of the parasympathetic system on the HR?

Answer 10

hyperpolarises cells AND decreases slope of pacemaker potential, decreases heart rate (brachycardia, HR < 60)

Question 11

what is the energy of contraction of a muscle fibre proportional to according to Starling’s law?

Answer 11

initial length of muscle fibre

Question 12

what is preload affected by?

Answer 12

end diastolic volume

Question 13

when increased venous return increases, how does EDV and stroke volume vary?

Answer 13

EDV increases, and so does stroke volume

Question 14

what do these relations ensure?

Answer 14

ensures self-regulation: matching stroke volumes in left and right ventricle

Question 15

what is afterload?

Answer 15

the load against which the muscle tries to contract (i.e: total peripheral resistance)

Question 16

what is preload?

Answer 16

how much blood is pushed into the aorta (i.e: the cardiac output)

Question 17

What happens to aortic pressure if total periphery resistance (= afterload) increases?

Answer 17

it increases

Question 18

what happens to stroke volume if aortic pressure increases?

Answer 18

stroke volume will decrease as the ventricle will have to work harder to push open the aortic valve and will therefore have less energy left to do the useful bit of ejecting blood (= stroke volume) + (more energy is “wasted” building up sufficient pressure to open the aortic valve)

Question 19

what happens to stroke volume if TPR increase?

Answer 19

stroke volume decreases

Question 20

which vessels affect preload?

Answer 20

venules and veins (capacitance vessels)

Question 21

which vessels affect afterload?

Answer 21

arterioles (resistance vessels)

Question 22

what does the sympathetic system release to influence stroke volume?

Answer 22

noradrenaline + adrenaline from adrenal medulla

Question 23

what do noradrenaline and adrenaline act on for regulation of stroke volume?

Answer 23

B1 receptors on the myocytes

Question 24

what is the effect of the sympathetic system on the stroke volume?

Answer 24

increases contractility (an inotropic effect), giving stronger but shorter contraction

Question 25

what is the effect of the parasympathetic system on the stroke volume?

Answer 25

little effect

Question 26

what explains the parasympathetic system’s effect on stroke volume?

Answer 26

probably because the vagus does not innervate the ventricular muscle

Question 27

what are the effects of hypercalcemia on the stroke volume?

Answer 27

shifts curve up and left; higher stroke volume for same EDV

Question 28

what are the effects of hypocalcemia on the stroke volume?

Answer 28

shifts curve down and right; lower stroke volume for same EDV

Question 29

what are the effects of ischaemia on the stroke volume?

Answer 29

shifts curve down and right; lower stroke volume for same EDV

Question 30

what are the effects of barbiturates on the stroke volume?

Answer 30

shifts curve down and right; lower stroke volume for same EDV

Question 31

how do you calculate cardiac output from HR and stroke volume?

Answer 31

CO = HR x SV

Question 32

what will be the impact on CO and SV if HR is increased with an electronic pacemaker?

Answer 32

small increase in CO then SV decreases because the shortened cardiac interval cuts into the rapid filling phase and this reduces end diastolic volume (= reduces preload); reduces stroke volume by Starling’s law

Question 33

what happens neurally when HR increases physiologically?

Answer 33

HR increases:

• via decreased vagal tone
• & increased sympathetic tone

Question 34

how does the body offset these effects to cope with physiological increases in HR? (think integration and co-ordinated increase in CO)

Answer 34

contractility increases: 
- via increased sympathetic tone
- alters inotropic state & shortens systole
venous return increases: 
- via venoconstriction 
- & skeletal/ respiratory pumps 
- maintains preload
total periphery resistance falls
- due to arteriolar dilation in muscle, skin and heart
- reduces afterload
CO increase 4-6 times

Question 35

SUMMARY: 
HR
- sympathetic supply (?) HR? 
- parasympathetic supply (?) HR
SV
- preload (= EDV) (?) SV
- afterload (= TPR) (?) SV
- neural (= symp supply) (?) SV

Answer 35

HR
- sympathetic supply increases HR
- parasympathetic supply decreases HR
SV
- preload (= EDV) increases SV
- afterload (= TPR) decreases SV
- neural (= symp supply) increase SV