Regulation of Stroke Volume and Heart Rate

Question 1

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

Answer 1

Increases HR - tachycardia

Question 2

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

Answer 2

Decreases HR - bradycardia

Question 3

How does the sympathetic nervous system increase HR?

Answer 3

Nerves release noradrenaline plus circulating adrenaline from adrenal medulla
Acts on B1 receptors on sinoatrial node and increases slop of pacemaker potential and this decreases Cardiac interval

Question 4

How is there an increase in slope of pacemaker potential?

Answer 4

Increase ion flow through funny Na+ channels and Ca2+ channels

Question 5

How does parasympathetic nervous system decrease HR?

Answer 5

Vagus nerve releases acetylcholine and acts on muscarinic receptors on sinoatrial node
This hyperpolarises cells and decreases slope of pacemaker potential so decrease HR as increase of cardiac interval

Question 6

What does Starlings Law state?

Answer 6

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

Question 7

What is preload?

Answer 7

Initial stretching of cardiac myocytes prior to contraction

Question 8

What is peak tension on starling curve?

Answer 8

Optimal no. of cross-bridges forming from thin actin and thick myosin filaments
Means really strong muscle contraction

Question 9

What is preload affected by?

Answer 9

End Diastolic Volume - volume of blood filled into ventricle before contraction

Question 10

What is the effect of increased venous return?

Answer 10

Increase in EDV and therefore stroke volume
Ventricles are more stretched so bigger EDV and stronger contraction so bigger stroke volume

Question 11

What is the effect of decreased venous return?

Answer 11

Decreased EDV and therefore decreased stroke volume
This is because ventricles are not really stretched and there is weak contraction

Question 12

What is afterload?

Answer 12

Load against which the muscle tries to contract

Question 13

What is stopping the left ventricle shortening?

Answer 13

Aortic valve being closed so enough pressure in LV so it can exceed aorta pressure and then the aortic valve will open
Also mitral valve

Question 14

What is aortic pressure affected by?

Answer 14

Total Peripheral Resistance

Question 15

Explain total peripheral resistance

Answer 15

How easy it is for blood to get out through the arterioles

Question 16

What happens when TPR increases?

Answer 16

Aortic pressure will increase and the ventricle will have to work harder to push open the aortic valve so less energy left to do the useful bit of ejecting blood - stroke volume decreases

Question 17

How does the sympathetic nervous system affect stroke volume?

Answer 17

Nerves release noradrenaline plus circulating adrenaline from adrenal medulla
Both act on B1 receptors on myocytes which increased contractibility - gives stronger but shorter contraction so stroke volume increased

Question 18

What is the ionotropic effect that causes an increase in contractibility?

Answer 18

More release of Ca2+ within cardiomyocytes causing more cross-bridges to form and stronger contraction as Ca2+ taken up quicker

Question 19

What is the effect of parasympathetic nervous system on stroke volume?

Answer 19

Little effect on contractibility as vagus nerve does not innervate the ventricular muscle

Question 20

What vessels affect preload?

Answer 20

Venules and veins as capacitance vessels

Question 21

What vessels affect afterload?

Answer 21

Arterioles as resistance vessels

Question 22

What causes an increase diastolic volume?

Answer 22

If more blood is squeezed out of venules and veins

Question 23

What does cardiac output determine?

Answer 23

How much blood and 02 to tissues

Question 24

What is cardiac output equal to?

Answer 24

Heart rate x stroke volume

Question 25

What effect does increasing HR have on cardiac output?

Answer 25

Causes small increase in cardiac output but stroke volume starts decreasing

Question 26

How does increasing HR cause increase of CO and decrease of SV?

Answer 26

Shortened cardiac interval cuts into rapid filling phase and the reduced EDV reduces preload so cardiac muscle stretches less so SV will fall

Question 27

What are serval things that offset physiological increases in HR - exercise?

Answer 27

HR increases
Contractibility increases
Venous return increases
Total peripheral resistance falls
Causes CO increase x 4-6 times

Question 28

What causes increase of HR in control of CO?

Answer 28

Decreased vagal tone and increased sympathetic tone

Question 29

What causes increase of contractibility in control of CO?

Answer 29

Increased sympathetic tone and alters ionotropic state and shortens cycle

Question 30

What causes increase in venous return in control of CO?

Answer 30

Via vasoconstriction and skeletal/ resp. pumps which maintains preload

Question 31

What causes decrease in total peripheral resistance in control of CO?

Answer 31

Due to arteriolar dilation in muscle, skin and heart which reduces afterload - making heart easier to pump