Week 6 - Control of Cardiac Output

Question 1

What are the effects on arterial and venous pressure if total peripheral resistance falls? (Cardiac output stays the same)

Answer 1

• Arterial pressure: falls

- Venous pressure: rises

Question 2

What are the effects on arterial and venous pressure if total peripheral resistance rises? (Cardiac output stays the same)

Answer 2

• Arterial pressure: rises

- Venous pressure: falls

Question 3

What are the effects on arterial and venous pressure if cardiac output rises? (total peripheral resistance stays the same)

Answer 3

• Arterial pressure: rises

- Venous pressure: falls

Question 4

What are the effects on arterial and venous pressure if cardiac output falls? (total peripheral resistance stays the same)

Answer 4

• Arterial pressure: falls

- Venous pressure: rises

Question 5

What controls cardiac output?

Answer 5

• Stroke volume x heart rate = cardiac output

- Arterial and venous pressures affect both, so hence control cardiac output

Question 6

What is demand-led pumping?

Answer 6

If the body needs more blood, then the heart needs to pump more to meet the demand

• Demand is expressed as changes in arterial and venous pressure
• If the heart responds to falls in arterial pressure and rises in venous pressure by pumping more blood, then it will meet the demand

Question 7

How does the total peripheral resistance relate to the body’s need for blood?

Answer 7

Total peripheral resistance is inversely proportional to the body’s need for blood

Question 8

What is stroke volume?

Answer 8

The difference between end diastolic volume and end systolic volume

Question 9

What determines the filling of the ventricles?

Answer 9

Venous pressure

• The ventricles fill until the walls stretch enough to produce an intraventricular pressure equal to venous pressure
• The relationship between venous pressure and ventricular volume is known as the ventricular compliance curve

Question 10

What is the relationship between stroke volume and venous pressure?

Answer 10

Rises in venous pressure automatically leads to rises in stroke volume

• The more the heart fills the harder it contracts (up to a limit)
• The harder it contracts the bigger the stroke volume

Question 11

What is contractility?

Answer 11

The nature of the relationship between stroke volume and venous pressure

• (Force of contraction)/(fibre length)
• Can be affected by neurotransmitters, hormones or drugs

Question 12

What is the end systolic volume?

Answer 12

How much the ventricle empties

- Depends on: how hard it contracts, how hard it is to eject blood

Question 13

What determines the force of contraction?

Answer 13

• End diastolic volume

- Contractility

Question 14

What is pre-load?

Answer 14

The end-diastolic volume at the beginning of systole (end-diastolic stretch)

Question 15

What is after-load?

Answer 15

The force necessary to expel blood into the arteries

• It determines the effect of a given force of contraction during systole
• If it is easy to eject blood, then the volume in the ventricle will fall a lot in systole, but the pressure will only rise a little
• So falls in total peripheral resistance increase stroke volume by decreasing after-load

Question 16

How are changes in arterial blood pressure corrected?

Answer 16

• The carotid sinus walls and aortic walls contain stretch receptors which detect changes in arterial blood pressure
• This information is released to the medulla in the brain
• Collections of neurones can then modify the behaviour of the heart and circulation via the autonomic nervous system
• Baroreceptors ensure that if arterial pressure falls, both heart rate and stroke volume tend to rise

Question 17

What are the effects of rises in venous pressure on heart rate?

Answer 17

• Leads to reduced parasympathetic activity
• So heart rate rises
• Not very important

Question 18

How can baroreceptors increase the heart rate?

Answer 18

By reducing parasympathetic activity and increasing sympathetic activity

Question 19

How can baroreceptors increase contractility?

Answer 19

By increasing sympathetic activity

Question 20

How can changes to arterial pressure affect flow resistance?

Answer 20

Falls in arterial pressure increases the flow resistance to certain tissues

Question 21

How can changes to arterial pressure affect venous capacitance?

Answer 21

Falls in arterial pressure reduces venous capacitance by venous constriction

Question 22

How can eating a meal affect the whole CVS?

Answer 22

• Increased metabolic activity in the gut produces vasodilator mediators and metabolites
• These cause local vasodilatation
• Total peripheral resistance falls and hence venous pressure rises and arterial pressure falls
• Rises in venous pressure leads to increased stroke volume
• Falls in arterial pressure triggers rise in heart rate and stroke volume
• So cardiac output rises
• This brings venous pressure back down to normal and arterial pressure back up to normal
• Demand has been met

Question 23

What effect does increasing the heart rate alone have on the cardiac output?

Answer 23

• Initially cardiac output will tend to rise, and total peripheral resistance remains the same
• This reduces venous pressure
• So stroke volume falls
• So as the heart rises, stroke volume will tend to fall, keeping cardiac output the same

Question 24

How can exercising affect the whole CVS?

Answer 24

Following an enormous increase in metabolic demand:

• Venous pressure would rise greatly, causing total peripheral resistance to fall (due to massive metabolic decompensation)
• Arterial pressure would fall greatly
• These changes may be too big to cope with
• It tends to overfill the heart, limiting the capacity of the vesicles to control cardiac output

Question 25

What is a risk associated with CVS and exercising?

Answer 25

Pulmonary oedema

• Because the outputs of the right and left ventricle cannot be matched
• Normally prevented due to a rise in heart rate at the onset of exercising
• – Triggered by activation of the sympathetic system
• – Occurs before large changes in arterial and venous pressure

Question 26

How can standing up affect the whole CVS?

Answer 26

The effects of gravity increase transmural pressure of the superficial veins in the lower limb

• Blood tends to ‘pool’ in the legs
• This produces a transient fall in central venous pressure
• This causes cardiac outlet to fall
• So arterial pressure falls
• – Triggers: rises in heart rate, vasoconstriction in skin and gut to increase total peripheral resistance and veno-constriction
• Total peripheral resistance rises, tending to stabilise arterial pressure and maintain perfusion to the vital organs

Question 27

How can a haemorrhage affect the whole CVS?

Answer 27

• Reduced blood volume, so less blood flow in the venous capacitance and hence venous pressure falls
• Cardiac output then falls
• Arterial pressure then falls
• – Detected by baroreceptors
• – Reflex rise in heart rate and rise in TPR
• The low venous pressure is exacerbated both by attempts to increase cardiac output and the rise in TPR

Question 28

How can sustained increases in blood volume affect the whole CVS?

Answer 28

Occasionally because of changes in the kidney or diet, blood volume tends to increase
-This produces a sustained increase in venous pressure
- TPR has not changed
- This causes an increased cardiac cycle
- Leads to an increased arterial pressure
More blood is forced through tissues
- The tissues that ‘autoregulate’ increase arteriolar tone to return flow to normal
- This increases TPR
- So arterial pressure rises further and stay up
- Partial solution = reversal of the rise in blood volume will, particulary in the early stages, reduce blood pressure back towards normal

Question 29

What is a problem with the sustained increase in arteriolar ligaments in resistance vessel walls?

Answer 29

May lead to long-term damage in the resistance vessel walls

• So the TPR rise is more or less permanent
• Arterial pressure is now persistently elevated

Question 30

How can you resolve low venous pressure?

Answer 30

Increase it by veto-constriction and auto-transfusion

• Movement of fluid from extra-cellular space into the circulation
• Eventually replace blood volume lost (water, electrolytes and RBCs) eventually replaced by homeostatic mechanisms