Regulation of Stroke Volume and Heart Rate

Question 1

What is heart rate set by?

Answer 1

Pacemakers of the heart

Question 2

Which type of nervous system regulates heart rate?

Answer 2

Autonomic NS

Question 3

Which branch of the autonomic NS increases heart rate?

Answer 3

Sympathetic branch

Question 4

Which branch of the autonomic NS reduces heart rate?

Answer 4

Parasympathetic branch

Question 5

Describe how the sympathetic branch of the autonomic NS increases HR.

Answer 5

-Sympathetic nerves release noradrenaline
-Plus circulating adrenaline from adrenal medulla
-Both act on β1 receptors on sinoatrial node
-Increases slope of the pacemaker potential
-Increases heart rate

Question 6

Describe how the parasympathetic branch of the autonomic NS decreases HR.

Answer 6

Vagus nerve releases acetylcholine
Acts on muscarinic receptors on sinoatrial node
Hyperpolarises cells and decreases slope of pacemaker potential
Decreases heart rate

Question 7

What is the medical term for a decrease in HR?

Answer 7

Bradycardia

Question 8

What is the medical term for an increase in HR?

Answer 8

Tachycardia

Question 9

RECAP- what releases noradrenaline in the sympathetic ANS?

Answer 9

Sympathetic nerves

Question 10

RECAP- what releases acetylcholine in the parasympathetic ANS?

Answer 10

Vagus nerve

Question 11

RECAP- what does noradrenaline act on?

Answer 11

β1 receptors on sinoatrial node

Question 12

RECAP- what does acetylcholine act on?

Answer 12

Muscarinic receptors on sinoatrial node

Question 13

What is preload?

Answer 13

The force that stretches the cardiac muscle prior to contraction

Question 14

What does Starling’s Law state regarding preload?

Answer 14

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

Question 15

What happens at peak tension in the heart?

Answer 15

Optimal number of cross bridges forming between actin and myosin.
This means there is a strong muscle contraction.

Question 16

In physiology, what stretches cardiac muscle?

Answer 16

End diastolic volume (volume of blood is filled into the ventricles before the contract)

Question 17

If the ventricles are stretched, describe the size of the end diastolic volume, contraction and stroke volume.

Answer 17

Larger end diastolic volume
Larger contraction
Larger stroke volume

Question 18

Describe what may happen to end diastolic volume and stroke volume upon exercise.

Answer 18

Exercise would increase venous return to the heart.
Ventricles would stretch more and EDV volume increases.
This means there is a larger contraction and therefore larger stroke volume.

Question 19

Describe what would happen to end diastolic volume and stroke volume upon rest.

Answer 19

At rest, ventricles would not be stretched so lower EDV.
Smaller contraction and smaller stroke volume.

Question 20

Why is the regulation of stoke volume important?

Answer 20

Ensures self-regulation
Ensures blood volume is fairly equal on the left and right sides of the heart.

Question 21

Define stroke volume.

Answer 21

The volume of blood pumped out of the left ventricle of the heart during each systolic cardiac contraction.

Question 22

Define afterload.

Answer 22

Afterload is the load against which the muscle tries to contract

Question 23

What stops the muscle shortening during systole?

Answer 23

The mitral valve being closed due to pressure in the ventricle.

Question 24

What is aortic pressure affected by?

Answer 24

How easily blood gets out through the arterioles – i.e. the total peripheral resistance (TPR)

Question 25

Recap- Which vessels are the resistance vessels?

Answer 25

Arterioles.

Question 26

What is the main thing which influences total peripheral resistance?

Answer 26

How dilated the arterioles are.

Question 27

What happens to stroke volume if TPR increases?

Answer 27

Aortic pressure increases Ventricle will have to work harder to push open the aortic valve and will have less energy left to do the useful bit of ejecting blood Stroke volume decreases

Question 28

What is afterload determined by?

Answer 28

The arterial pressure against which the blood is ejected - this in turn depends on the total peripheral resistance (TPR)

Question 29

Describe how the sympathetic NS can regulate stroke volume.

Answer 29

Sympathetic nerves releasing noradrenaline Plus circulating adrenaline from adrenal medulla Both act on β1 receptors on the myocytes Increases contractility (an inotropic effect) Gives stronger, but shorter contraction

Question 30

Name the different places you can find beta 1 receptors.

Answer 30

Sinoatrial node of the heart Myocytes

Question 31

What effect does the parasympathetic NS have on stroke volume?

Answer 31

Little effect as vagus nerve does not innervate ventricular muscle

Question 32

Define contractibility

Answer 32

How strong a contraction is produced for any given preload or afterload.

Question 33

What is contractibility affected by?

Answer 33

Sympathetic NS

Question 34

What is preload affected by?

Answer 34

The state of contraction of venules/veins i.e. contracted, relaxed, constricted, dilated etc.

Question 35

What is afterload affected by?

Answer 35

Affected by the state of contraction of arterioles i.e. contracted, relaxed, dilated, constricted etc.

Question 36

Which are resistance vessels- veins or arteries?

Answer 36

Arteries/ arterioles

Question 37

Which are capacitance vessels?

Answer 37

Veins/ venules

Question 38

At rest, how much blood is stored in the capacitance vessels?

Answer 38

2/3

Question 39

Cardiac volume= ?

Answer 39

heart rate x stroke volume

Question 40

What does cardiac volume determine?

Answer 40

How much blood is getting to your tissues

Question 41

What would happen if you used an electrical pacemaker to increase HR?

Answer 41

You cut into the filling phase which decreases end diastolic volume. Cardiac muscle will be stretched less and there will be a smaller contraction. This reduces stroke volume.

Question 42

Why does HR increase during exercise?

Answer 42

Decreased vagal tone Increased sympathetic tone

Question 43

Why does contractibility increase during exercise?

Answer 43

Controlled via sympathetic NS via increased sympathetic tone Alters inotropic state as stronger force of contraction due to formation of more myosin-actin cross bridges. Short contraction and shortens systole

Question 44

Why does venous return increase during exercise?

Answer 44

Via venoconstriction and skeletal/respiratory pumps pushing more blood back to the heart. Increases EDV and maintains preload

Question 45

Why does TBR fall during exercise?

Answer 45

Due to arteriolar dilation in muscle, skin and heart Reduces afterload as easier to pump blood

Question 46

Name the three factors which affect stroke volume.

Answer 46

Preload Afterload Contractibility