Control of Cardiac Output Flashcards
What is cardiac output?
Volume of blood ejected per minute
- Responsible for blood flow and perfusion of organs/tissues
What is stroke volume?
Volume of blood ejected per heartbeat
What is the equation linking heart rate, cardiac output and stroke volume?
CARDIAC OUTPUT = STROKE VOLUME x HEART RATE
Why does cardiac output increase during exercise?
Due to increases in heart rate and stroke volume
What is total peripheral resistance?
Resistance to blood flow through circulation
What is the equation linking cardiac output, blood pressure and TPR?
CARDIAC OUTPUT = BLOOD PRESSURE/TPR
Outline the importance of normal blood pressure
Provides drive for normal blood flow to maintain gas exchange and nutrient supply to tissues
What are the three factors that influence stroke volume?
- Force of contraction
- Afterload
- Preload
Briefly describe preload
- Stretching of heart at rest depending volume of blood that returns to heart
- Extent of stretching determines energy of contraction and therefore determines stroke volume
- Controlled by Starling’s Law
Briefly describe after load
- Opposes ejection - can reduce stroke volume and energy of contraction
- Controlled by Laplace’s Law
PRELOAD IN DETAIL
When a high volume of blood reaches the heart during diastole, what happens?
- Ventricles fill with blood - ventricular volume increases so walls stretch. This stretching is preload.
- Higher preload (i.e higher stretching) means greater force of contraction so greater volume of blood ejected from heart
RECAP : What is end-diastolic volume?
How does it influence preload?
Volume of blood left in ventricles after relaxation
- Increases preload due to stretching the heart more
How did Starling prove that end-diastolic volume increases preload?
- Injected a bolus of fluid increasing blood volume
- Increased end-diastolic volume
- Caused greater stretching therefore greater ejection (i.e greater stroke volume)
What does the Starling Curve show about the relationship between pressure and stroke volume?
- At normal filling pressure (5 mmHg), stroke volume is 70-80 ml
- Small changes in pressure lead to large changes in stroke volume
- If blood pressure were to increase suddenly during exercise, filling pressure and therefore stroke volume would rise
How is a Starling curve used with patients with low cardiac outputs?
- Low filling pressure - give fluids to raise cardiac output
- Heart failure - do not give fluids
How do the actin and myosin fibres work to cause cardiac contraction?
- Fibres move relative to each other
- Myosin heads interact with actin, bringing Z-bands close together
- Muscle fibre shortens i.e contraction
Why does stretching a muscle fibre cause an increase in energy of contraction?
- Less mechanical interference
- Reduced overlapping of actin and myosin
- Greater potential for cross bridge formation
- Greater number of exposed calcium binding sites
How is preload important in balancing the effects of the right and left sides of the heart?
- When blood returns to right side of heart, stretching of right ventricle occurs. Greater energy of contraction - greater volume of blood ejected into pulmonary circulation
- More blood then returns from lungs to left side of heart. Greater volume of blood ejected into systemic circulation
How does haemorrhage affect blood pressure?
- Reduced stretching
- Reduced cardiac output therefore reduced blood pressure
People can faint if they are standing for too long.
Suggest why.
- Blood pools in legs so reduced return to heart
- Reduced preload and reduced cardiac output
- Reduced blood pressure and perfusion to brain
Use Starling’s Law to suggest why blood flow increases during exercise.
- Blood vessels in legs contract - greater venous return to heart
- Increased cardiac output
- Increased blood flow and oxygen delivery to tissues
What is afterload equal to?
- Heart wall stress
- Opposes contraction needed for ejection of blood
What three factors is afterload affected by?
- Blood pressure
- Heart wall thickness
- Ventricle radius
Suggest why afterload is disadvantageous to humans.
- With high wall stress comes more energy during systole for contraction
- Heart becomes less efficient
What is the effect of a high blood pressure within the heart?
Greater wall tension
- Harder to contract
How does a large radius influence the opposition to contraction?
- Greater wall tension in heart directed across the walls
- Increased opposition to contraction
How does a low radius influence opposition to contraction?
- Tension directed across centre of chamber and less along the walls
- Reduced opposition to contraction
Mathematically outline the relationship between wall tension, pressure and the radius.
T ∝ Pr
What does T∝Pr tell you about the relationship between pressure, radius and wall tension.
- A high radius and pressure will give rise to a high tension
Why is it not completely correct to say that tension = pressure x radius?
This equation does not account for wall stress.
What is the equation linking wall stress, tension and wall thickness?
Stress = tension/wall thickness
APPLICATION QUESTION:
What would be the effect of applying tension over the left ventricle compared to the right ventricle?
- The walls of the LV are thicker than those of the RV
- In thicker walls, tension is distributed across more muscle fibres
- Less stress opposing contraction of each fibre
What is the equation linking wall stress, pressure, radius and wall width?
Stress = (Pressure x radius) / (2 x wall thickness)
*2 - because 2 directions of curvature
What factors increase afterload?
What factors decrease afterload?
INCREASE - increased pressure in ventricle and increased radius of chamber
DECREASE - increased thickness of heart wall
How would afterload influence a ventricle with a low radius?
- Greater wall curvature
- Greater wall stress directed towards centre of chamber
- Less afterload
- Greater ejection of blood
How would afterload influence a ventricle with a high radius?
- Less wall curvature
- Greater wall stress directed across heart wall
- Greater afterload
- Reduced ejection
How do Laplace’s Law and Starling’s Law oppose one another? PART 1
- As preload increase, greater stretching of the chamber
- Greater radius of the chamber - increased afterload
How do Laplace’s Law and Starling’s Law oppose one another? PART 1
- Healthy hearts will have greater forces of contraction when stretched
- Therefore, here preload will overcome effects of wall stress
RECAP: How does the rate of ejection of blood during systole change over time?
- Rapid initially
- Slower towards the end
How does Laplace’s Law influence the slow step of ejection?
- Reduced pressure so decrease in radius
- Curvature increases so more of force directed inwards
- Reduced afterload - allows any remaining blood to be ejected
When does Laplace’s Law overcome Starling’s Law?
During heart failure
- Chambers become dilated
- Their radius increases
- Greater afterload opposing ejection
According to Laplace’s Law, how will an increase in blood pressure influence stroke volume?
- Increase wall stress
- Increases afterload
- Reduced ejection so reduced stroke volume
RECAP: What is isovolumetric contraction?
- Period of time where aortic pressure > ventricular pressure
During isovolumetric contraction, why can ejection of blood sometimes be difficult?
- Greater wall stress
- Greater afterload
- Reduced stroke volume
How can Starling’s Law overcome the effects of high blood pressure on stroke volume?
- Greater pressures give rise to increased muscular stretching
- Increased force of contraction
- Increased ejection volume - increased stroke volume
How does noradrenaline overcome the effects of high blood pressure on stroke volume?
- Noradrenaline is a positive inotrope which binds to β1 adrenergic receptors
- This increases force of contraction
- Increased volume of blood ejected into systemic circulation
In chronic hypertension, why is there a high energy expenditure?
- Constant afterload
- Energy expenditure needed to compensate and maintain stroke volume
- This requires high energy consumption - inefficient
What is heart failure?
- Heart does not contract properly
- Heart does not relax or fill properly
How can heart failure affect stroke volume and cardiac output?
Reduced stroke volume and cardiac output
What is volume overload?
- Typically after myocardial infarction where heart cannot contract properly
- Reduced volume of blood ejected
- Blood remains in ventricle after systole
What is pressure overload?
- Occurs with increased aortic pressure e.g in hypertension/aortic stenosis
- Greater pressure required for rejection
- Afterload overcomes preload
- Reduced cardiac output
Using Laplace’s Law, how does pressure-overload increase afterload?
- Pressure is greater
- Greater wall stress
- Greater afterload
- Greater opposition to ejection
How does the heart compensate to volume and pressure overloads?
Ventricular hypertrophy
Using Laplace’s Law, suggest how ventricular hypertrophy can maintain cardiac output and stroke volume?
- Wall stress is distributed across more sarcomeres
- Reduced afterload
- Reduced opposition to ejection
What is the main disadvantage of ventricular hypertrophy?
- Greater energy requirements for contraction
- Greater oxygen and nutrient demands
RECAP: Refer back to the ventricular pressure-volume loop (CAN BE FOUND ON SLIDES). What does the area within the loop relate to?
Amount of energy consumed to produce stroke volume
What are the two functions of the work done by the heart?
- Isovolumetric contraction
- Ejection
Why does isovolumetric contraction require high amounts of energy?
- Energy required to raise left ventricular pressure above aortic pressure
- Energy required to open aortic valve
Why does ejection require energy?
- To push blood out
- For contraction of ventricles
How does increasing the energy requirement for isovolumetric contraction affect cardiac output?
- Less energy available for ejection
- Reduced cardiac output
How does decreasing the energy requirement for isovolumetric contraction affect cardiac output?
- Greater energy available for ejection
- Greater cardiac output
Refer to the slide labelled ‘Preload & Ventricular Pressure Volume Loop’
Why is the loop for increased preload wider compared to the normal baseline?
- Veins contract slightly
- Greater venous return to heart
- Increased EDV
- Greater stretching and therefore greater preload - therefore greater volume of blood ejected
Refer to the slide labelled ‘Preload & Ventricular Pressure Volume Loop’
Why is the loop for increased preload have a shorter isovolumetric portion?
- Increased preload means greater force of contraction
- Greater pressure
- Reduced difference between ventricular and aortic pressure
Refer to the slide labelled ‘Afterload & Ventricular Pressure Volume Loop’
Why does the loop for increased afterload have a greater isovolumetric segment?
- Greater afterload
- Greater opposition to ejection
- Greater wall stress
- Greater isovolumetric contraction
Refer to the slide labelled ‘Afterload & Ventricular Pressure Volume Loop’
Why does the loop for increased afterload have a shorter ejection phase?
- Isovolumetric contraction requires high amounts of energy
- Isovolumetric contraction is increased
- Less energy for ejection
