Regulation of stroke volume & heart rate

Question 1

How does the sympathetic nervous system regulate heart rate?

Answer 1

• Sympathetic nerves release noradrenaline
• plus circulating adrenaline from adrenal medulla
• Both act on B1-receptors on the surface of sinoatrial node
• Increases heart rate = tachycardia

Question 2

What is Tachycardia?

Answer 2

• Heart rate that exceeds the normal resting rate.

- In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults.

Question 3

How does the parasympathetic nervous system regulate heart rate?

Answer 3

• Vagus releases ACh
• acts on muscarinic receptors in sinoatrial node
• Hyperpolarises cells
• Decreases heart rate = bradycardia

Question 4

What is Bradycardia?

Answer 4

It is a slower than normal heart rate

Question 5

What is normal stroke volume?

Answer 5

60-120 ml/beat

Question 6

What 3 things affect stroke volume?

Answer 6

• Contractility
• Preload
• Afterload

Question 7

What does Starling’s law state?

Answer 7

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

Question 8

What is Preload?

Answer 8

Preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand.
- It is how much volume is in the ventricles at the end of diastole : EDV.

Question 9

What volume is preload affected by, in vivo?

Answer 9

End diastolic volume

Question 10

What are factors determining cardiac Preload? (to increase it)

Answer 10

• Increased EDV
• Increased atrial contractility
• Increased aortic pressure
• Increased atrial contractility
• Decreased heart rate
• Increased central venous pressure

Question 11

Which valve closes after diastole?

Answer 11

The mitral valve

Question 12

What is Afterload?

Answer 12

It is the pressure required to open the aortic valve.

- It is the load against which the muscle tries to contract

Question 13

What is total (systemic) peripheral resistance?

Answer 13

The total resistance to flow of blood in the systemic circuit

Question 14

What occurs to Aortic pressure, if total peripheral resistance increases?

Answer 14

If TPR increases, aortic pressure will increase, the ventricle will have to work harder to push open the aortic valve..
- Stroke volume will decrease.

Question 15

What is required for the aortic valve to open?

Answer 15

The pressure in the ventricle needs to be greater than the pressure distal to the aortic valve.

Question 16

In vivo, how is afterload set?

Answer 16

• Afterload is set by the arterial pressure against which the blood is expelled - this in turn depends on the Total Peripheral Resistance.

Question 17

Which vessels affect preload?

Answer 17

Capacitance vessels

e.g. venules/veins

Question 18

Which vessels affect afterload?

Answer 18

Resistance vessels

e.g. Arterioles

Question 19

What is contractility?

Answer 19

The inherent vigor of contraction of the heart muscles during systole.

Question 20

How does the sympathetic system affect the regulation of stroke volume?

Answer 20

• 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 21

Does the parasympathetic nervous system affect regulation of stroke volume?

Answer 21

• Little effect

- Probably because the vagus does not innervate the ventricular muscle.

Question 22

How does Hypercalcemia affect the stroke volume-EDV curve?

Answer 22

Shifts curve up and left

Question 23

How does Hypocalcemia affect the stroke volume-EDV curve?

Answer 23

Shifts curve down and right

Question 24

How does Ischaemia affect the stroke volume-EDV curve?

Answer 24

Shifts curve down and right

Ischaemia - loss of blood supply to the cardiac muscle.

Question 25

How will the heart compensate for a reduced pumping ability?

Answer 25

By working around a bigger EDV.

- Results in lower ejection fraction, and reduced exercise capacity.

Question 26

How do Barbiturates affect the stroke volume-EDV curve?

Answer 26

Shifts curve down and right

• A barbiturate is a drug that acts as a central nervous system depressant.

Question 27

Equation to calculate cardiac output

Answer 27

Cardiac output = Heart rate x Stroke volume

Question 28

Increasing heart rate with an electronic pacemaker, causes a small increase in CO, but then SV(stroke volume) decreases, Why?

Answer 28

• Shortened cardiac interval cuts into the rapid filling phase.
• Reduced end diastolic volume reduces preload
• So by Sterling’s law, reduces stroke volume.

Question 29

Mechanisms that cause heart rate to decrease for control of cardiac output

Answer 29

• via decreased vagal tone

- & increased sympathetic tone

Question 30

How does contractility increase for control of cardiac output?

Answer 30

• via increased sympathetic tone

- alters inotropic state & shortens systole

Question 31

How does venous return increase for control of cardiac output?

Answer 31

• Via venoconstriction (via a1 receptors)
• & skeletal/respiratory pumps
• maintains preload

Question 32

How does total peripheral resistance fall (TPR) to control cardiac output?

Answer 32

• Due to arteriolar dilation in muscle, skin & heart (via b2 receptors)
• reduces afterload

Question 33

How much does Cardiac output increase during exercise?

Answer 33

4-6 times