CVS S6 - Control of the CVS

Question 1

Describe the effect on the CVS if metabolic demand of the body changes and cardiac output remains the same

Answer 1

If metabolic demand falls:

• TPR rises
• Arterial pressure rises
• Venous pressure falls

If metabolic demand rises:

• TPR falls
• Arterial pressure falls
• Venous pressure rises

Question 2

Describe the effect on pressure in the veins and arteries if cardiac output is changed and TPR remains the same

Answer 2

If CO rises:

• Arterial pressure will rise
• Venous pressure will fall

If CO falls:

• Arterial pressure will fall
• Venous pressure will rise

Question 3

How does the heart supply enough blood to the body at all times if cardiac output can fall or rise?

Answer 3

System is demand led

TPR changes in response to metabolic demand altering arterial and venous pressures and hence cardiac output

Cardiac output is therefore dependent on metabolic demand

Question 4

How do changes in arterial and venous pressures form ‘signals’ to the heart?

Answer 4

Heart responds to increased venous and decrease arterial pressure by increasing cardiac output

Heart responds to decreased venous and increased arterial pressure by lowering cardiac output

In this way, changes in pressure lead to the heart being ‘signalled’ to meet demand

Question 5

What is cardiac output the product of?

Answer 5

Stroke volume and heart rate

Question 6

What is end diastolic volume?

Answer 6

The volume of blood in the ventricle at the end of diastole

Question 7

What is end systolic volume?

Answer 7

The volume of blood left in the ventricle at the end of systole

Question 8

What is stroke volume?

Answer 8

The difference between end diastolic and end systolic volume?

Question 9

Describe the connections to the ventricle during diastole

What does this mean for ventricular filling?

Answer 9

During diastole the ventricle is connected to the veins and isolated from the arteries

Therefore the ventricle will fill until the walls stretch to the point that ventricular pressure matches venous pressure

Question 10

Describe the relationship between venous pressure and ventricular filling

Answer 10

Higher the venous pressure the more the heart fills in diastole

Question 11

What is the ventricular compliance curve?

Can you draw it?

Answer 11

Curve showing how end diastolic volume (x) is related to venous pressure (y)

Drawn as a shallow curve showing a positive relationship

Question 12

Define the terms:

Pre-load

After-load

Answer 12

Pre-load is the end diastolic stretch in the myocardium, determined by venous pressure

After-load is the force needed to expel blood in the ventricles into the arteries

Question 13

What is end systolic volume dependent on?

Answer 13

How hard the heart contracts

How hard it is to eject blood

Question 14

Describe how end diastolic volume and stroke volume are related

Answer 14

The more the heart fills (the greater the end diastolic volume) the harder the ventricle will contract (up to a limit) and the bigger the stroke volume

This is Starlings’ law of the heart and can be summarised simply as:
- More in = more out

Question 15

Can you draw the Starling curve?

What does it show us?

Answer 15

X = Venous pressure
Y = Stroke volume
Gradient = Contractility

Curve is a steep rise followed by a short fall after the apex

This shows us that the more the heart fills, the harder it contracts (greater stroke volume)

There is however a limit where the heart becomes over-filled and the myocardium is overstretched, resulting in a fall in stroke volume after a certain venous pressure

Question 16

Define the term ‘contractility’

How is it determined?

Answer 16

The intrinsic ability of the myocardium to contract (ionotropic state)

The gradient of the Starling curve shows contractility

Delta stroke volume / Delta venous pressure

Question 17

What effect does the sympathetic nervous system have on contractility?

Answer 17

Sympathetic activity makes the heart more susceptible to venous pressure, raising the gradient of the Starling curve

Question 18

What effect does increased arterial pressure have on contractility and contractile force?

Answer 18

Raises contractility and hence contractile force by increasing sympathetic stimulation to the heart (via baroreceptors)

Question 19

What is aortic impedance?

What effect does this have on arterial pressure?

Answer 19

The difficulty of ejecting blood from the ventricle, this mainly depends on TPR (rise in TPR raises aortic impedance)

This means the harder it is to eject blood the higher the pressure will rise in the arteries

Question 20

What is the effect of a drop/rise in arterial pressure on the stroke volume?

Answer 20

If arterial pressure drops:
- End systolic volume will fall as there is decreased after-load, therefore stroke volume has risen

If arterial pressure rises:
- End systolic volume will rise as there is increased after-load, therefore stroke volume has fallen

Question 21

What is the effect of a drop/rise in venous pressure on the stroke volume?

Answer 21

Drops:
- End diastolic volume will fall and stroke volume will fall due to decreased pre-load

Rises:
- End diastolic volume will fall and stroke volume will rise due to increased pre-load

Question 22

Describe how falls and rises in arterial pressure affect the signals sent by the ANS to the heart and the changes this leads to

Answer 22

Baroreceptors sense the arterial pressure

Rise leads to:

• Heart rate falls by decreasing sympathetic and increasing parasympathetic activity
• Contractility falls due to reduced sympathetic activity

Fall leads to:

• Heart rate rises by increasing sympathetic and decreasing parasympathetic activity
• Contractility rises due to increased sympathetic activty

Question 23

What is the Bainbridge reflex?

Answer 23

If venous pressure as sensed by baroceptors in the right atrium rises this will lead to reduced parasympathetic activity and a rise in heart rate

Question 24

Give a detailed list of effects of an increase or decrease in arterial pressure on the heart

Hint: This summarises all the smaller cards in this deck related to the effects of arterial pressure on the heart

Answer 24

Increase:

• Increased end systolic volume and after-load
• Increased contractility
• Decreased heart rate and stroke volume
• Decreased cardiac output

Decrease:

• Decreased end systolic volume and after-load
• Decreased contractility
• Increased heart rate and stroke volume
• Increased cardiac output

Question 25

Give a detailed list of effects of an increase or decrease in venous pressure on the heart Hint: This summarises all the smaller cards in this deck related to the effects of venous pressure on the heart

Answer 25

Increase: - Increased venous return - Increased end diastolic volume and pre-load - Increased contractile force (up to a limit) - Increased heart rate and stroke volume - Increased cardiac output Decrease: - Decreased venous return - Decreased end systolic volume and pre-load - Decreased contractile force - Decreased heart rate and stroke volume - Decreased cardiac output

Question 26

Describe in detail the response of the CVS to eating a meal

Answer 26

Increased gut activity leads to a fall in TPR and subsequent fall in arterial pressure and rise in venous pressure Rise in venous pressure triggers rise in cardiac output Fall in arterial pressure triggers baroreceptors which signal the brain to increase sympathetic stimulation to the heart, raising heart rate and contractility and hence cardiac output rises Extra pumping of the heart reduces venous pressure and raises arterial pressure Demand met, system stable

Question 27

Describe what would happen if heart rate were to increase with no other change What does this tell us?

Answer 27

Initially cardiac output would rise, total peripheral resistance would however remain the same The rise in cardiac output decreases venous pressure so stroke volume will fall The cardiac output is back to its original value This tells us the heart is driven by the circulation, not the other way around

Question 28

What is the major effect of movement during exercise on the CVS?

Answer 28

Muscle pumping, which forces extra blood back to the heart

Question 29

If muscle pumping was the only change during exercise (no response from the CVS) what would be the effect?

Answer 29

Venous pressure would rise greatly Arterial pressure would fall greatly These changes may be too big to cope with

Question 30

What is the main problem the CVS must deal with during exercise? This leads to the risk of?

Answer 30

The great increase in venous pressure that will lead to overfilling of the heart and push the heart towards the peak of the Starling curve At the top of the Starling curve the left ventricular output cannot be further increased if the right ventricular output increases Therefore there is a risk of blood accumulating in the lungs and pulmonary oedema may result

Question 31

How does the CVS respond during exercise? Why is this necessary?

Answer 31

An increase in heart rate, driven by the brain at the start of exercise This means when venous pressure starts to rise heart rate is already high and stroke volume is kept down so the left and right cardiac outputs can be matched

Question 32

What happens in the CVS when we stand up?

Answer 32

Blood pools in the superficial veins of the legs (great/short saphenous veins) due to gravity Central venous pressure falls Cardiac output falls as a result Arterial pressure falls Baroreceptors detect fall in arterial pressure Increased sympathetic activity raises heart rate to raise arterial pressure Venous pressure still low so TPR increases (skin/gut) to both defend arterial pressure and lower venous pressure

Question 33

What is postural hypotension?

Answer 33

Sometimes the baroreceptor reflex which raises heart rate when arterial pressure falls as we stand up doesn't work Arterial pressure remains low, this is postural hypotension

Question 34

What is the response of the CVS to haemorrhage?

Answer 34

Reduced blood volume lowers venous pressure Cardiac output falls Arterial pressure falls Baroreceptors detect fall in arterial pressure and increase in heart rate results (this can become very high) TPR is increased Rise in heart rate lowers venous pressure further Problem is now worse, not better

Question 35

What is needed to restore the CVS to normal function following haemorrhage?

Answer 35

Veno-constriction to raise the venous pressure Blood transfusion to replace lost volume

Question 36

What is the response of the CVS to a long term increase in total blood volume?

Answer 36

Blood volume controlled by kidney Increase for days = an increase in venous pressure Cardiac output rises Arterial pressure rises More blood is forced through tissues, which auto-regulates and raises TPR Arterial pressure rises further and stays up