Regulation of Stroke Volume and Heart Rate

Question 1

What is the control mechanism of heart rate?

Answer 1

Neural

Question 2

What is the regulation mechanism for stroke volume?

Answer 2

Preload

Afterload

Neural

Pathological

Question 3

How does the sympathetic nervous system regulate heart rate?

Answer 3

Releases noradrenaline from nerves and adrenal medulla.

Act on Beta 1 receptors on sinoatrial node

This increases the slope of the pace maker potential between heart beats

Increases heart rate

Question 4

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

Answer 4

Vagus nerve releases acetycholine

Acts on muscarinic receptors on the sinoatrial node

Hyperpolarises cells and decreases the slope of the pacemaker potential

Decreases heart rate = bradycardia

Question 5

What does starlings law state?

Answer 5

Starling’s Law states - the energy of contraction is proportional to the initial length of the cardiac muscle fibre

Question 6

Why is the slope decreasing to the left side of the peak tension?

Answer 6

Actin filaments start to interfere with each other

Question 7

What is the length of the cardiac muscle fibres proportional to?

Answer 7

End diastolic volume

Question 8

What is the stroke volume related to?

Answer 8

Tension

Question 9

What is preload affected by?

Answer 9

End diastolic volume

Question 10

What ensures self regulation of stroke volume of left and right ventricles?

Answer 10

The proportional relationship between the volume of venous return and the stroke volume

Question 11

What is the afterload?

Answer 11

The load against which the muscle tries to contract

Question 12

What is the aortic pressure affected by?

Answer 12

How much blood is pushed into the aorta (cardia output)

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

Question 13

Why does an increase in TPR result in a reduction in stroke volume?

Answer 13

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.

Question 14

What is responsible for the variable portion of the total peripheral resistance?

Answer 14

The arterioles

Question 15

What is the total peripheral resistance set by?

Answer 15

The arterial pressure - what sets the afterload

Question 16

What affects preload?

Answer 16

Venules/veins and capacitance vessels

These all affect the venous return

Question 17

What affects afterload?

Answer 17

Arterioles and resistance vessels

Question 18

What does a bigger end diastolic volume mean?

Answer 18

Bigger pre-load and therefore more tension and more strength of contraction

Question 19

How does the sympathetic nervous system affect stroke volume?

Answer 19

Noradrenaline from nerves and the adrenal medulla act on beta 1 receptors of the myocytes - which increases contractility - stronger but shorter contraction

Question 20

What does ionotropic refer to?

Answer 20

The strength of contraction

Question 21

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

Answer 21

Little effect - vagus nerve does not innervate the ventricular muscle

Question 22

What is the effect of hypercalcaemia, hypocalcaemia and ischaemia on stroke volume?

Answer 22

•Hypercalcemia

–shifts curve up and left – more strength of contraction, more X bridges, more extracellular and therefore more intracellular calcium when the action potential reaches the muscle

•Hypocalcemia

–shifts curve down and right

•Ischaemia

–shifts curve down and right

Question 23

How does the heart compensate for a reduced pumping ability?

Answer 23

Works around a bigger end diastolic volume - lower ejection fraction and reduced excersize capacity.

Question 24

What is the effect of barbituates on stroke volume?

Answer 24

–shifts curve down and right

Question 25

Why does stroke volume decrease with an increased heart rate?

Answer 25

Shortened cardiac cycle cuts into rapid filling stage - smaller EDV (small length of muscle), less tension and therefore less stroke volume

Question 26

What are the combined mechanisms that increase heart rate?

Answer 26

Via decreased vagal tone and increased sympathetic tone

Question 27

What shortens systole?

Answer 27

Effect of increased sympathetic tone on the contractility - alters ionotropic state

Question 28

How does venous return increase?

Answer 28

Via venoconstriction

And skeletal / respiratory pumps

Maintains preload when we expect it to fall with a reduced EDV brought on with a fast heart rate

Question 29

Why does total peripheral resistance fall with a fast heart rate?

Answer 29

Arteriolar dilation in muscle, skin & heart

Reduced afterload - makes it easier for heart to pump out blood

Question 30

What are the 4 ways CO increases when workload increases?

Answer 30

HR increases

Contractility increases

Venous return increases

Total peripheral resistance falls