Control of cardiac output Flashcards
Describe cardiac output
The amount of blood ejected from the heart per minute
Cardiac output equation
Cardiac output (CO) = Heart rate (HR) x Stroke volume (SV)
HR: how often the heart beats per minute
SV: how much blood (ml) is ejected per beat
Blood flow (CO) equation
Blood flow (CO)= BP / TPR
BP: blood pressure
TPR: total peripheral resistance
Blood pressure equation
BP: CO x TPR
Cardiac output at rest
70 bpm x 70 ml = approx 5 litres/min
Cardiac output during exercise
180 bpm x 120 ml = 22 litres/min
Define Preload and the Law governing it
STRETCHING OF LEFT VENTRICLE ON FILLING:
- The end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demands
- Stretching at rest
- Increases Stroke Volume
- Governed by Starling’s law
Define Afterload and the Law governing it
RESISTANCE TO EJECTION
- The pressure against which the heart must work to eject blood during systole
- The end load against which the heart contracts to eject blood
- Governed by Laplace’s law
What controls heart rate?
- SA node pacemaker
- Sympathetic and Parasympathetic nerves
What is contractility?
Strength of contraction due to sympathetic nerves and circulating adrenaline increasing intracellular calcium
What is meant by energy of contraction?
Energy of contraction is the amount of work required to generate stroke volume
What two factors does energy of contraction depends on?
- Starling’s law
2. Contractility
What two functions does stoke work carry out?
- Contracts until chamber pressure is greater than aortic pressure (isovolumetric contraction)
- Ejection from ventricle
Relationship between preload and stroke volume
Preload increases the stroke volume
Relationships between afterload and stroke volume
Afterload opposes the stroke volume
What does starling’s law state?
- “energy of contraction of cardiac muscle is relative to the length of the muscle fibre at rest”
- This means that the greater the stretch of the ventricles in diastole (blood entering), the greater the energy of contraction and the greater the stroke volume achieved in systole (blood leaving)
Intrinsic property of cardiac muscle
More blood in = more blood out
Describe the graph that represents Starling’s law (Stroke Volume against Central Venous Pressure)
- Throughout the graph, the CVP increases, the SV increases
- At the end of the graph, with an increasing CVP, the SV starts falling
- Indicates the point at which Laplace’s law takes over, so the heart pumps less blood out
Why does Starling’s Law work (in the sense of contractile fibres)?
- With an unstretched fibre, there is overlapping actin and myosin, not much room for Ca2+
- Means that there is mechanical interference, so there is less cross-bridge formation available for contraction
- With a stretched fibre, there is less overlapping actin and myosin
- Means that there is less mechanical interference, so there is potential for more cross-bridge formation
- There is also an increased sensitivity to Ca2+ ions and more space for Ca2+ to interact
- Therefore contracts harder when stretched beforehand
What are some of the roles and effects of Starling’s law in cardiac physiology?
- Balances outputs of the right and left ventricle
- Responsible for fall in Cardiac output during a drop in blood volume/vasodilation (e.g. sepsis)
- Restores Cardiac output in response to intravenous fluid transfusions
- Responsible for fall in Cardiac output during orthostasis (standing for a long time): leading to postural hypotension and dizziness as blood pools in legs
- Contributes to increased stroke volume and cardiac output during upright exercise
Define Laplace’s law
Afterload opposes ejection of blood from the heart and is determined by wall stress directed through the heart wall
What are some of the implications of Laplace’s law?
- Stress through the wall of the heart prevents muscle contraction
- More energy of contraction is needed to overcome wall stress: producing cell shortening and ejection
Wall tension equation (in terms of pressure and radius)
T = P x r / 2
T= Wall tension P= Pressure r= radius in a chamber (ventricle)
Why is the wall tension calculated by diving (P x r) by two?
Divided by two because the chamber has two directions of curvature
Wall tension equation (in terms of stress and thickness)
T= S x w
T= wall tension S= wall stress w= wall thickness
Wall stress equation
S= (P x r)/2w
S= wall stress P= pressure r= radius w= wall thickness
How is afterload increased?
Increasing pressure and radius
How is afterload reduced?
Increasing wall thickness
how does a small radius effect wall stress/afterload?
- Small ventricle radius
- Greater wall curvature
- More wall stress is directed towards centre of chamber
- Less afterload
- Better ejection
how does a big radius effect wall stress/afterload?
- Larger ventricle radius
- Less wall curvature
- More wall stress directed through heart wall
- More afterload
- Less ejection
How does Laplace’s law oppose Starling’s law at rest?
Increased preload gives an increased stretch of the chamber
This increased chamber radius decreases curvature: therefore increased afterload
How does Laplace’s law facilitate ejection during contraction?
Contraction reduces chamber radius so less afterload in ‘emptying’ chamber. This helps expulsion and increases stroke volume
How does Laplace’s law contribute to a failing heart at rest and during contraction?
In a failing heart the chambers are often dilated: so increased afterload opposing ejection
When does Laplace’s law result in good ejection?
with a small radius
When does Laplace’s law result in bad ejection?
with a large radius
Relationship between Laplace’s law and afterload
Laplace’s law states that increased blood pressure (P) will increase wall stress. This will increase afterload and reduce ejection.
What does increased radius result in (in terms of Laplace’s law )
Heart failure, where the heart does not contract properly (MI, cardiomyopathies, mitral valve re-gurgitation). Blood left in the ventricle leads to eventual volume overload
What does increased pressure result in (in terms of Laplace’s law)
Pressure-overload heart failure due to increased pressure/afterload in chamber (hypertension, aortic stenosis)
Relationship between Laplace’s law and hypertrophy in heart failure
- Increase in radius or pressure will increase wall stress (afterload) which opposes ejection
- Heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres) increasing wall thickness. This decreases wall stress per sarcomere and therefore afterload (SV and CO is maintained)
- This requires more energy (more sarcomeres used): greater O2. The amount of energy required increases and so contractility will decrease and produce more heart failure
Describe relationship between Starling’s law and ventricular pressure-volume loop
- During exercise, increased venous return leads to an increase in EDV: increased preload and more stretch
- This causes shorter isovolumetric contraction phase, and increase in SV due to Starling’s law
- More blood back to the heart and more blood ejected from the heart
Describe the relationship between Laplace’s law and ventricular pressure-volume loop
- High blood pressure leads to increased afterload
- Longer time spent in isovolumetric contraction to increase the pressure in the chamber above that in the aorta: opens the valve
- This uses more energy and lowers the force of contraction, reducing Stroke Volume
- Therefore end systolic volume increases
