Regulation of Stroke Volume and Heart Rate

Question 1

What effect does the sympathetic system have on heart rate?

Answer 1

• Sympathetic nerves release noradrenaline
• Circulating adrenaline from adrenal medulla in bloodstream
• Both act on B1 adrenergic receptors on the SA node, increases permeability to Ca2+
• Causes the cells to depolarise quicker
• Increases slope of pacemaker potential, smaller intervals between each potential
• Increases heart rate = tachycardia

Question 2

What effect does the parasympathetic system have on heart rate?

Answer 2

• Vagus nerve releases acetylcholine
• Acts on muscarinic receptors on the SA node
• Influence the increased permeability of K+ and decreased permeability of Ca2+
• Hyperpolarises cell so it takes longer to reach threshold, so there are longer intervals between potentials
• Decreases slope of pacemaker potential
• Decreases heart rate = bradycardia

Question 3

What does stroke volume mean?

Answer 3

The volume of blood pumped out by the LV per contraction.

Question 4

Describe how the sympathetic nervous system affects the stroke volume

Answer 4

• Sympathetic nerves release noradrenaline
• Circulating adrenaline from adrenal medulla (in brain)
• Both act on B1 adrenergic receptors on myocytes
• Increases contractility - inotropic effect
• Gives stronger but shorter contractions
• Increases stroke volume

Question 5

Describe how the parasympathetic nervous system affects the stroke volume

Answer 5

Little affect probably because the Vegas nerve does not innervate the ventricular muscle

Question 6

Describe how hypercalcemia can affect stroke volume

Answer 6

More calcium, so more cross bridges formed - shifts curve up and left

Increase stroke volume

Question 7

Describe how hypocalcemia can affect stroke volume

Answer 7

Less calcium, less cross bridges formed - shifts curve down and to the right

Decrease stroke volume

Question 8

Describe how ischaemia can affect stroke volume

Answer 8

Reduced blood supply to tissues, resulting in damage. Shifts curve down and to the right.

Question 9

Describe how barbiturates can affect stroke volume

Answer 9

Used as an anaesthetic, reduces heart rate, shift curve down and to the right

Question 10

Define the term preload

Answer 10

Degree of myocardial stretch before contraction begins ie. the load that is placed on the heart before contraction.

Question 11

What does starlings law state?

Answer 11

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

Question 12

In what ways can preload be affected by end diastolic volume?

Answer 12

• Increased venous return = increased EDV = increased preload = increased stroke volume
• Decreased venous return = decreased EDV = decreased preload = decreased stroke volume
• Ensures self-regulation - matches stroke volume of left and right ventricles

Question 13

Describe how starlings law lets the heart compensate for a reduced pumping ability eg. after heart attack

Answer 13

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 14

Define the term afterload

Answer 14

The load at which the muscle tries to contract

Question 15

What is the after load determined by?

Answer 15

The arterial pressure against which the blood is expelled. The mean arterial pressure (in the aorta) affects the blood being pushed from the heart

Question 16

What two things affect the arterial pressure?

Answer 16

• How much blood is pushed into the aorta (cardiac output)

- How easy it is for that blood to get out of the aorta (total peripheral resistance)

Question 17

What would happen to stroke volume if total peripheral resistance increases?

Answer 17

TPR increases, aortic pressure will increase, 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 18

What is the cardiac output determined by?

Answer 18

Cardiac output = Heart rate x Stroke volume

Question 19

What affect will an increased heart rate have not he cardiac output?

Answer 19

Increasing heart rate with an electronic pacemaker causes a small increase in cardiac output but then stroke volume decreases, this is because…

• Shortened cardiac interval cuts into the rapid filling phase - Reduced end diastolic volume (EDV) reduces preload
• Results in a reduced stroke volume

Question 20

Describe what happens to HR during exercise

Answer 20

Increases via decreased vagal tone and increased sympathetic tone