Regulation of Heart Rate and Stroke Volume

Question 1

Describe the effect of the sympathetic system on HR

Answer 1

NA released and circulating adrenaline from adrenal medulla.
Act on Beta 1 receptors in sinoatrial node
Increase slope of pacemaker potential
Increased heart rate = tachycardia

Question 2

What is the effect of the sympathetic system on HR in terms of the slope of the pacemaker potential

Answer 2

Increased slope

Question 3

Describe the effect of the parasympathetic system on HR

Answer 3

ACh released
ACh acts on muscarinic receptors
Decrease slope of pacemaker potential
Decreased HR = bradycardia

Question 4

What is the effect of the parasympathetic system on HR in terms of the slope of the pacemaker potential

Answer 4

Decreased slope

Question 5

State the three physiological mechanisms that regulate stroke volume

Answer 5

Preload
Afterload
Neural (sympathetic and parasympathetic)

Question 6

State Starling’s law

Answer 6

Energy of contraction proportional to initial length of cardiac muscle fibre

Question 7

Define preload

Answer 7

End diastolic volume

Question 8

How does starling’s law apply to preload and thus SV

Answer 8

Starling’s Law = Energy of contraction proportional to initial length of cardiac muscle fibre
Preload = EDV
Explanation: A big EDV means the cardiac muscle is very stretched initially so contraction strength is greater. A small EDV means cardiac muscle is not that stretched initially to contraction strength is smaller.

Question 9

What vessels affect preload?

Answer 9

Venules and veins

Question 10

Describe the effect of venous return and EDV/preload on SV

Answer 10

Increased venous return = increased EDV = increased SV

Decreased venous return = reduced EDV = reduced SV

Question 11

Define afterload

Answer 11

Load against which heart must contract = total peripheral resistance (TPR)

Question 12

What vessels affect afterload?

Answer 12

Arterioles

Question 13

Describe the effect of TPR/afterload on SV

Answer 13

As TPR increases ->
Aortic pressure increases ->
More energy needed for ventricle to push aortic valve open -> 
less energy for ejection -> 
reduced SV

Question 14

Describe the effect of the sympathetic system (neural) on SV

Answer 14

NA and adrenaline released
Act on Beta 1 receptors of myocytes
Inotropic effect - increase contractility
Stronger but shorter contraction

Question 15

Does the parasympathetic system affect SV?

Answer 15

No

Question 16

What does the parasympathetic system have little effect on SV?

Answer 16

Vagus nerve doesn’t innervate ventricular muscle

Question 17

Name 4 factors that can pathologically affect SV

Answer 17

Hypercalcaemia
Hypocalcaemia
Ischaemia
Barbiturates

Question 18

Describe the effect of hypercalcaemia on SV and the starling curve

Answer 18

More Ca2+ to form cross bridges.
Increased contractility
Curve shifts UP and TO LEFT

Question 19

Describe the effect of hypocalcaemia on SV and the starling curve

Answer 19

Less Ca2+ to form cross bridges.
Decreased contractility
Curve shifts DOWN and TO RIGHT

Question 20

Describe the effect of ischaemia on SV and the starling curve

Answer 20

Less blood supply to cardiac muscle.
Less energy for cardiac muscle to contract
Curve shifts DOWN and TO RIGHT

Question 21

Describe the effect of barbiturates on the starling curve

Answer 21

Curve shifts DOWN and TO RIGHT

Question 22

Describe how the heart compensates for reduced pumping ability

Answer 22

The heart works around its reduced EDV by reducing its ejection fraction which in turn reduces exercise capacity

Question 23

Describe how cardiac output (CO) is controlled in an electronic pacemaker

Answer 23

Increased HR -> increased CO w/ small reduction in SV because the cardiac interval is shorter. Therefore part of the rapid filling phase is cut meaning reduced EDV and preload

Question 24

List the physiological components that can increase CO

Answer 24

HR
Contractility
Venous Return
TPR

Question 25

What factors increase HR to increase CO?

Answer 25

Increased sympathetic tone

Decreased vagal tone

Question 26

How is contractility increased to increase CO?

Describe the mechanism that enables this to occur

Answer 26

Increased sympathetic tone alters inotropic state, shortening systole and leaving more time for diastole

Question 27

How is venous return increased to increase CO?

Answer 27

Venoconstriction
Skeletal and respiratory pumps
These maintain the preload

Question 28

How is TPR reduced to increase CO?

Answer 28

Arteriolar dilatation in muscle, skin and heart

Reduced afterload

Question 29

What receptors are acted upon by what NT to bring about arteriolar dilation which reduces TPR

Answer 29

Noradrenaline (and adrenaline) acting on alpha 1 receptors

Question 30

By how many times can CO be increased via physiological factors?

Answer 30

4-6 times