Cardiac output control Flashcards
What is cardiac output? [1 mark]
The amount of blood ejected from the heart per minute.
What is heart rate? [1 mark]
How often the heart beats per minute.
What is stroke volume? [1 mark]
How much blood (ml) is ejected per beat.
What is the formula for cardiac output? [3 marks]
Cardiac output = heart rate × stroke volume
What is the formula for blood pressure? [3 marks]
Blood pressure = cardiac output × total peripheral resistance
What can control heart rate & contractility? [2 marks]
- SA node
- Autonomic control
What can control the strength of contraction? [2 marks]
- Sympathetic nerves
- Adrenaline causing an increase in intracellular calcium
What happens in preload and what does it act in accordance to? [3 marks]
- Stretching of heart at rest
- Increase of stroke volume
- Acts according to Starling’s law
What happens in afterload and what does it act in accordance to? [3 marks]
- Opposing ejection
- Reduces stroke volume
- Acts according to Laplace’s law
What is the energy of contraction? [1 mark]
The amount of work required to generate stroke volume
Where does stroke volume come from? [2 marks]
- Contracts until ventricle pressure > aortic pressure (isovolumetric contraction)
- Ejection from ventricles
What does Starling’s law state? [2 marks]
The energy of contraction of cardiac muscle is relative to the muscle fibre length at rest
What happens if you stretch the heart further? [2 marks]
- There is a higher energy of contraction
- A greater stroke volume is achieved in systole
What is the formula for stroke volume? [3 marks]
Stroke volume = end diastolic volume - end systolic volume
What happens when the heart is overstretched? [1 mark]
The stroke volume is decreased
What is the molecular basis for Starling’s law? [4 marks]
- In unstretched fibre, actin and myosin overlap and so there’s more mechanical interference
- Less cross bridges are available for contraction
- In stretched fibres, there’s less overlapping
- More cross bridges are available and more sensitivity to calcium
What happens in orthostasis (standing for a long time)? [3 marks]
- Myocardium isn’t stretched as much
- Less blood reaches the brain
- This can leads to dizziness
What is afterload determined by? [1 mark]
Wall stress directed through the heart wall
What is needed to overcome wall stress? [1 mark]
More energy of contraction
What does overcoming wall stress do? [2 marks]
Produces cell shortening and ejection
What is the formula for wall tension (T) [4 marks]
Wall stress (T) = (Pressure x radius in a ventricle) / 2
Why is (pressure x radius) divided by 2? [1 mark]
The chamber has 2 directions of curvature
What increases afterload? [2 marks]
Increased pressure & radius
What decreased afterload and stress? [1 mark]
Increased wall thickness
What happens to afterload in a ventricle with a small radius? [4 marks]
- Greater wall curvature
- More wall stress directed towards centre of chamber
- Less afterload
- Better ejection
What happens to afterload in a ventricle with a large radius? [4 marks]
- Less wall curvature
- More wall stress directed through heart wall
- More afterload
- Less ejection
Which law overcomes the other in a healthy heart? [1 marks]
Starling’s Law > Laplace’s Law
What is an inotrope? [3 marks]
- An agent that alters the force or energy of muscular contractions
- Negative = weakens force of contraction
- Positive = strengthens force of contraction
What is the Anrep effect? [1 mark]
A sudden increase in afterload on the heart causes an increase in ventricular inotropy
Why does volume overload occur in heart failure? [2 marks]
- Weaker ejection
- Heart doesn’t fully empty before refilling again
What is the benefit of ventricular hypertrophy? [2 marks]
- Decreases wall stress
- Maintains stroke volume and cardiac output
What is the disadvantage of ventricular hypertrophy? [2 marks]
- Requires more energy
- Amount of energy needed will increase
What does an increased venous return lead to? [2 marks]
- An increase in end diastolic volume
- Increased preload and more stretch
What happens in Laplace’s law in terms of pressure and volume? [4 marks]
- Longer time spent in isovolumetric contraction
- To increase ventricular pressure
- Uses more energy
- Lowers force of contraction (reducing SV)
