Control of Cardiac Output

Question 1

Equation for blood flow

Answer 1

Total peripheral resistance

Question 2

Stroke volume

Answer 2

Volume of blood ejected per heart beat

Question 3

What is preload

Answer 3

• stretching of heart at rest
• increases stroke volume
• due to Starlings law
• stretching of left ventricle

Question 4

What is afterload

Answer 4

• opposes ejection
• reduces stroke volume
• due to Laplaces law

Question 5

Define energy of contraction

Answer 5

Amount of work required to generate stroke volume

Question 6

2 functions of stroke work

Answer 6

1) isovolumetric contraction- chamber pressure greater than aortic pressure
2) ejection from ventricle

Question 7

Explain Starlings law

Answer 7

• ventricle is stretched by pushing blood into it
• heart is able to fill more
• contracts harder (greater energy of contraction)
• greater stroke volume

Question 8

How do you calculate stroke volume

Answer 8

SV = EDV - ESV

Question 9

Central venous pressure (CVP)

Answer 9

Diameter of vena cava carrying deoxygenated blood to right atrium

When vena cava becomes narrow/constricts it increases CVP

CVP reflects amount of blood returning to the heart

Question 10

Molecular basis of Starlings law

Answer 10

Stretched fibre

• less actin myosin interference
• more cross bridge formation
• contraction
• increased sensitivity to Ca2+

Question 11

Effects of Starlings law

Answer 11

1) balances output of right and left ventricle that can be distorted by abnormal increase or decrease in blood pressure e.g. Orthostasis; postural hypotension
2) decrease in blood volume results in decrease in cardiac output
3) restores cardiac output after intravenous transfusion
4) increases SV and CO during exercise

Question 12

Describe what is meant by afterload

Answer 12

Afterload is wall stress

Wall stress prevents muscle contraction

No muscle contraction means no blood is ejected

Afterload reduces stroke volume

Question 13

What 2 factors increase afterload and why

Answer 13

1) pressure- increases wall stress

2) radius- more stress directed through heart wall

Question 14

Importance of Laplaces law in a failing heart

Answer 14

• In a failing heart, the chambers are dilated
• this increases radius
• which increases afterload and prevents ejection
• ejection can only occur with a small radius

Question 15

Name 3 factors which counter acute rises in blood pressure

Answer 15

1) Starlings law increases stroke volume
2) Inotropes which increase energy of muscular contraction
3) Baroreflex decreases blood pressure by watering down the sympathetic response

Question 16

How does laplaces law facilitate ejection during contraction

Answer 16

During contraction the chamber radius reduces

So there is less stress directed through heart wall

So less afterload in emptying chamber

Ejection can occur

(Helps eject last drops of blood and also increases stroke volume)

Question 17

Chronic increases in arterial blood pressure

Answer 17

Energy is required to maintain stroke volume

Without energy, less blood is ejected per heart beat

Therefore stroke volume is reduced and so is blood pressure

Question 18

What causes mitral valve regurgitation and cardiomyopathy?

Answer 18

• volume overload of blood increases radius too much
• Laplaces law is now in overdrive
• heart unable to contract
• valve damaged and blood flows freely between chambers
• cardiomyopathy harder for heart to pump blood to rest of body

Question 19

What causes aortic stenosis?

Answer 19

Pressure overload causes hypertension

This high blood pressure damages aorta

Aorta becomes narrow and blocked

Question 20

Why is ventricular hypertrophy bad

Answer 20

Thickness of heart walls increase to cope with stress

Thicker walls means more sarcomeres/muscle fibres

More sarcomeres means more energy required

O2 demand cannot be met

Contractility will decrease

Question 21

Laplaces law reduces stroke volume

Answer 21

Isovolumetric contraction uses up all the energy

Not enough energy left for muscle contraction

Less blood is ejected

Reduces stroke volume

More left over blood in heart post ejection