Control of Cardiac Output Flashcards
Equation for blood flow
Total peripheral resistance
Stroke volume
Volume of blood ejected per heart beat
What is preload
- stretching of heart at rest
- increases stroke volume
- due to Starlings law
- stretching of left ventricle
What is afterload
- opposes ejection
- reduces stroke volume
- due to Laplaces law
Define energy of contraction
Amount of work required to generate stroke volume
2 functions of stroke work
1) isovolumetric contraction- chamber pressure greater than aortic pressure
2) ejection from ventricle
Explain Starlings law
- ventricle is stretched by pushing blood into it
- heart is able to fill more
- contracts harder (greater energy of contraction)
- greater stroke volume
How do you calculate stroke volume
SV = EDV - ESV
Central venous pressure (CVP)
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
Molecular basis of Starlings law
Stretched fibre
- less actin myosin interference
- more cross bridge formation
- contraction
- increased sensitivity to Ca2+
Effects of Starlings law
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
Describe what is meant by afterload
Afterload is wall stress
Wall stress prevents muscle contraction
No muscle contraction means no blood is ejected
Afterload reduces stroke volume
What 2 factors increase afterload and why
1) pressure- increases wall stress
2) radius- more stress directed through heart wall
Importance of Laplaces law in a failing heart
- 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
Name 3 factors which counter acute rises in blood pressure
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
How does laplaces law facilitate ejection during contraction
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)
Chronic increases in arterial blood pressure
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
What causes mitral valve regurgitation and cardiomyopathy?
- 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
What causes aortic stenosis?
Pressure overload causes hypertension
This high blood pressure damages aorta
Aorta becomes narrow and blocked
Why is ventricular hypertrophy bad
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
Laplaces law reduces stroke volume
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
