CVS S6 - Control of cardiac output

Question 1

How would increasing or decreasing total peripheral resistance at fixed cardiac output impact on arterial and venous pressures?

Answer 1

Increasing TPR- increase arterial pressure, decrease venous pressure

Decreasing TPR- decrease arterial pressure, increase venous pressure

Question 2

How would increasing or decreasing cardiac output at fixed TPR on arterial and venous pressures?

Answer 2

Increasing CO- increase arterial pressure, decrease venous pressure

Decreasing CO- decrease arterial pressure, increase venous pressure

Question 3

What is stroke volume?

Answer 3

Difference between end diastolic and end systolic volume

Question 4

What does a ventricular compliance curve show?

Answer 4

• Relationship between venous pressure and ventricular volume
• Higher venous pressure the more the ventricles fill during diastole

Question 5

Explain the difference between ‘pre-load’ and ‘after-load’ on the ventricular myocardium

Answer 5

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

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

Question 6

How would increasing end diastolic volume effect the force of contraction during systole?

Answer 6

Increasing volume means the myocardium is more stretched and so contracts harder

Question 7

Describe the relationship between venous pressure and stroke volume and what is the name of this relationship?

Answer 7

Increasing venous pressure increases ventricular filling, this in turn increases force of contraction which will increase stroke volume- Starling’s Law

Question 8

What is contractility?

Answer 8

Slope of the relationship between stroke volume and venous pressure, intrinsic ability of myocardium to contract

Question 9

What would cause an increase in heart contractility?

Answer 9

Noradrenaline and adrenaline from the sympathetic nervous system

Question 10

How would after-load be decreased and what effect would this have on stroke volume?

Answer 10

Decrease in TPR would decrease after-load and thereby increase stroke volume

Question 11

What effect would a fall in arterial pressure have on heart rate and contractility?

Answer 11

Increase both

Question 12

What is the Bainbridge reflex?

Answer 12

• High venous pressure is sensed in the right atrium

- This causes reduced parasympathetic activity which causes an increase in heart rate

Question 13

What is the effect of eating a meal on venous and arterial pressure?

Answer 13

• Increased activity of the gut causes local vasodilatation

- Local TPR falls causing venous pressure to rise and arterial pressure to fall

Question 14

How are the changes in arterial and venous pressure when eating a meal returned to normal by the CVS?

Answer 14

Increase in venous pressure causes an increase in cardiac output which in turn increases arterial pressure

This thereby causes a decrease in venous pressure

Question 15

If heart rate alone is increased how is cardiac output returned to normal?

Answer 15

• Cardiac output increases causing a rise in arterial pressure yet TPR remains constant
• This decreases venous pressure resulting in a decrease in stroke volume
• Cardiac output thereby returns to normal value

Question 16

Why does standing up cause a decrease in both venous and arterial pressure?

Answer 16

• Blood ‘pools’ form in superficial veins of the legs due to gravity
• Central venous pressure falls
• This decreases stroke volume causing a subsequent decrease in cardiac output
• Arterial pressure thereby falls as a result

Question 17

What is a possible complication of high venous pressure left uncorrected in exercise and how is this prevented?

Answer 17

• More blood returns to the right side of the heart
• Therefore more blood is pumped by the right side of the heart
• Causing fluid accumulation in the lungs- pulmonary oedema

Rise in heart rate before exercise commences which is driven by the brain ensures heart rate is already high before venous pressure rises and so stroke volume is kept low

Question 18

How is the problem of low venous pressure worsened in haemorrhage?

Answer 18

Baroreceptors detect low arterial pressure caused by the decreases cardiac output due to lack of blood return from the veins

This stimulates an increase in heart rate which lowers venous pressure further

Question 19

How is haemorrhage corrected?

Answer 19

Veno-constriction, auto-transfusion and replacement of fluid lost (matching plasma concentrations of electrolytes and proteins)

Question 20

What is responsible for control of blood volume and how?

Answer 20

Kidneys, by balance of sodium excretion and water re-absorption

Question 21

What changes occur in response to long term increase in blood volume?

Answer 21

• Venous pressure rises
• Results in increased stroke volume
• Increases cardiac output
• Arterial pressure rises as a result
• Auto-regulation means TPR increases
• Arterial pressure rises further and then remains high