Regulation Of Stroke Volume And Heart Rate

Question 1

Effect of sympathetic nervous system on heart rate

Answer 1

Release noradrenaline and adrenaline from adrenal medulla

They act on B1 receptors on sinoatrial node

Increasing the slope of the pacemaker potential and therefore heart rate goes up

Question 2

Effect of parasympathetic nervous system on heart rate

Answer 2

The vagus releases ACh which acts on muscarine receptors on the sinoatrial node

This hyperpolarises cells and decreases the slope of the pacemaker potential

Decreasing heart rate

Question 3

Starling’s law

Answer 3

The energy of contraction is proportional to the initial length of the cardiac muscle fibre (preload)

Question 4

What is preload

Answer 4

Initial length of cardiac muscle fibre

Question 5

What is preload

Answer 5

Initial length of cardiac muscle fibres

Question 6

What affects preload

Answer 6

End diastolic volume (end volume of blood in ventricles)

Question 6

What affects preload

Answer 6

End diastolic volume (end volume of blood in ventricles)

Question 7

How does EDV affect preload

Answer 7

If there is lots of blood in the ventricles the fibres are stretched and stroke volume increases to match return and vice versa

Question 8

What is afterload

Answer 8

The load against which the muscle tries to contract

Question 9

What affects afterload

Answer 9

The total peripheral resistance (size of arterioles)

Increasing aortic pressure so the ventricle will have to work harder to push open the aortic valve and then less energy will be left to eject blood so stroke volume will decrease

Question 10

Preload Vs afterload

Answer 10

Preload affected by capacitance vessels

Afterload affected by resistance vessels

Question 11

Effect of sympathetic nervous system on stroke volume

Answer 11

Noradrenaline and adrenaline act on B1 receptors in myocytes to increase contractibility (ionotropic) to give a stronger but shorter contraction

Question 12

Parasympathetic nervous system effect on stroke volume

Answer 12

Little effect probably because vagus doesn’t innervate the ventricular muscles

Question 13

Hypercalcaemia on stroke volume

Answer 13

Curve goes up and left (pumps more with less in it)

Question 14

Hypocalcemia and ischaemia and barbiturates on stroke volume

Answer 14

Shift curve down and right (doesn’t work as well)

Question 15

Cardiac output equation

Answer 15

Heart rate X stroke volume

Question 16

Why will increasing HR with a pacemaker cause SV to decrease if too high

Answer 16

A shortened cardiac interval cuts into the filling phase so the end diastolic volume is decreased

Preload is decreased and stroke decreases

Question 17

How is venous return increased

Answer 17

Via venoconstriction
Skeletal/respiratory pumps
Maintaining preload

Question 18

How does total peripheral resistance fall

Answer 18

Arteriolar dilation in muscle, skin and heart

Reduces afterload

Question 19

How is contractibility increased

Answer 19

Increased sympathetic tone which alters inotropic state and shortens systole

Question 20

How is heart rate increased

Answer 20

Decreased vagal tone and increased sympathetic tone

Question 21

Factors increasing cardiac output

Answer 21

HR increase
Contractibility increase
Venous return increase
Total peripheral resistance decrease
Will increase by 4-6 times