Regulation of stroke volume and heart rate

Question 1

What is the effect of the sympathetic nervous system on regulation of heart rate?

Answer 1

sympathetic nerves release noradrenaline

plus circulating adrenaline from adrenal medulla

both act on ß1-receptors on sinoatrial node

increases slope of the pacemaker potential

increases heart rate = tachycardia

Question 2

What is the effect of the parasympathetic nervous system on the regulation of the heart?

Answer 2

vagus releases ACh

acts on muscarinic receptors on sinoatrial node

hyperpolarises cells and decreases slope of pacemaker potential

decreases heart rate = bradycardia

Question 3

What happens to the EDV and stroke volume when you get increased venous return?

Answer 3

Increased EDV and stroke volume

Question 4

What happens to the EDV and stroke volume when you get decreased venous return?

Answer 4

Decreased EDV and stroke volume

Question 5

What does preload ensure?

Answer 5

Ensures self-regulation – matches SV of left & right ventricles

Question 6

What is the aortic pressure affected by?

Answer 6

how easy it is for that blood to get out through the arterioles - ie the total peripheral resistance

Question 7

What will happen to the aortic pressure, work of ventricle, energy and stroke volume if the total peripheral resistance is high?

Answer 7

Increase pressure aorta and the ventricle will have to work harder to push open the aortic valve, and it will have less energy left to do the useful bit of ejecting blood, ie stroke volume will decrease

Question 8

What do veins and venules affect and why?

Answer 8

Affect preload because they are capacitance

Question 9

What do arterioles affect and why?

Answer 9

Affect afterload because they are resistance vessels

Question 10

What is the effect of the sympathetic nervous system on stroke volume?

Answer 10

sympathetic nerves releasing noradrenaline

plus circulating adrenaline from adrenal medulla

both act on ß1-receptors on the myocytes

increases contractility (an inotropic effect)

gives stronger, but shorter contraction

Question 11

What is the effect of the parasympathetic nervous system on stroke volume?

Answer 11

little effect

probably because the vagus does not innervate the ventricular muscle

Question 12

What are the main three factors that affect stroke volume?

Answer 12

Preload
Contractility
Afterload

Question 13

Define preload

Answer 13

How full the ventricle is before it starts contracting
(ie the EDV)
Affected by the state of contraction of venules/veins

Question 14

Define contractility

Answer 14

How strong a contraction is produced for any given preload or afterload
Affected by the sympathetic system

Question 15

Define afterload

Answer 15

How difficult it is for the heart to pump out the blood
(ie the TPR)
Affected by the state of contraction of arterioles

Question 16

Where does the starling curve shift during hypercalcemai?

Answer 16

shifts curve up and left

Question 17

Where does the starling curve shift during hypocalcemia?

Answer 17

shifts curve down and right

Question 18

Where does the starling curve shift during ischaemia?

Answer 18

shifts curve down and right

Question 19

How does the starling law let the heart compensate for a reduced pumping ability?

Answer 19

ie the heart will compensate for a reduced pumping ability by working around a bigger EDV. Results in lower ejection fraction, and reduced exercise capacity.

Question 20

Where does the starling curve shift during barbiturates injection?

Answer 20

shifts curve down and right

Question 21

What is the formula for cardiac output?

Answer 21

Cardiac output = Heart rate x Stroke volume

Question 22

What is the effect of increasing heart rate with an electronic pacemaker cause on CO and SV?

Answer 22

Small increase in Co
Stroke volume decreases
Due to starlings curve

Explaination:Start contraction point earlier and earlier when using electronic pacemaker (decrease time for diastole ) eg: 140 beats for min

Eventually can cut into he rapid filling phase and cosenquences = less effective cross bridge formation and so stroke volume decreases

Question 23

What can cause venous return increase?

Answer 23

via venoconstriction
& skeletal/respiratory pumps
maintains preload

Question 24

What factors increase cardiac output?

Answer 24

HR increases

• via decreased vagal tone
• & increased sympathetic tone

Contractility increases

• via increased sympathetic tone
• alters inotropic state & shortens systole

Venous return increases

• via venoconstriction
• & skeletal/respiratory pumps
• maintains preload

Total peripheral resistance falls

• due to arteriolar dilation in muscle, skin & heart
• reduces afterload

CO increase 4-6 times