stroke volume and heart rate regulation

Question 1

explain the effects of the sympathetic nervous system in increasing heart rate:

Answer 1

• Nerves release noradrenaline
• Plus, the already circulating adrenaline from the adrenal medulla
• Act on ß1 receptors (adrenergic) on SA node
• Increase slope of the pacemaker potential so reaches threshold quicker
• Increase heart rate

Question 2

explain the effects of the parasympathetic nervous system in decreasing heart rate:

Answer 2

• Cell hyperpolarises so starts off lower down
• Vagus nerves release acetylcholine
• Act on muscarinic receptors on SA node
• Decreases slope of the pacemaker potential so reaches threshold slower
• Decreases heart rate

Question 3

what is the law that relates to the preload of stroke volume?

Answer 3

Starling’s law: the energy of contraction (of cardiac muscle fibres) is proportional to the initial length of the cardiac muscle fibres

Question 4

what is preload?

Answer 4

Preload is the stretch put on the muscle before it contracts.

Question 5

what effect does increased venous return have on stroke volume?

Answer 5

• Increased venous return, increased end diastolic volume, & therefore increased stroke volume

Question 6

what effect does decreased venous return have on stroke volume?

Answer 6

• Decreased venous return, decreased EDV, & therefore decreased stroke volume

Question 7

what is afterload?

Answer 7

Afterload is the force that the muscle is trying to work against once it has started contracting (the pressure inside the aorta)

Question 8

what is afterload set by?

Answer 8

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

Question 9

explain why if TPR increases, stroke volume will go down:

Answer 9

if TPR increases, aortic pressure will increase, the ventricles will have to work harder to push open the aortic valve, and it will have less energy to eject blood i.e. SV will decrease (more energy is being wasted building up sufficient pressure to open the aortic valve)

Question 10

what happens to the stroke volume when you constrict the arterioles?

Answer 10

If you constrict arterioles, you affect the afterload, the afterload goes up making it more difficult for the heart to pump the blood out and SV goes down.

Question 10

what happens to the stroke volume when you constrict the venules and veins?

Answer 10

If you constrict the venules and veins, blood will be squeezed towards the heart and increases the venous return (which increases EDV and in turn preload, and that increases SV).

Question 11

explain the effects of parasympathetic system on stroke volume

Answer 11

• little effect, this is probably because the vagus does not innervate the ventricular muscle (only the top and the nodes)

Question 11

explain the effects of sympathetic system on stroke volume

Answer 11

• sympathetic nerves releasing noradrenaline
• plus, circulating adrenaline from adrenal medulla
• both act on ß1-receptors on the myocytes
• increases contractility (an inotropic effect) – Inos[strength] tronos[time] ie positive inotropic agents increase force of contraction
• gives stronger (because more Ca2+ is released), but shorter contraction

Question 12

what effect does hypercalcemia have on the curve?

Answer 12

shifts curve up and left, has the same effect as sympathetic stimulation

Question 13

what effect does hypocalcaemia have on the curve?

Answer 13

shifts curve down and right

Question 14

what effect does ischaemia have on the curve?

Answer 14

(that side of the heart will pump less strongly) – shifts curve down and right

Question 15

what effect does barbiturates (or other drugs) have on the curve?

Answer 15

shifts curve down and right. (if used too much)

Question 16

why do you have a low stroke volume if you had a myocardial infarction?

Answer 16

the curve will be shift down and you will end up with a smaller SV for any given EDV

Question 17

what is the equation for cardiac output?

Answer 17

CO = HR x SV

Question 18

what effect does heart rate have on stroke volume and cardiac output?

Answer 18

increasing HR higher and higher, decreases SV lower and lower = CO stays the same.