3. Control of Cardiac Output Flashcards
What is cardiac output?
The amount of blood ejected from the heart per minute
It is proportional to how often the heart beats per minute (heart rate, HR) and how much blood (ml) is ejected per beat (Stroke volume, SV).
CO = HR x SV
Blood flow (CO) = BP/TPR (BP = Blood Pressure, TPR = Total peripheral resistance)
What is preload?
The resistance to left ventricular filling (stretching of heart at rest). It increases stroke volume due to Starling’s law.
What is afterload?
The resistance to ejection. It reduces stroke volume due to Laplace’s law.
What is contractility?
The strength of contraction due to sympathetic nerves, circulating adrenaline and increasing intracellular calcium.
What is the energy of contraction?
The amount of work required to generate stroke volume.
Properties of un-stretched fibre?
Overlapping actin/myosin
More mechanical inference
Less cross-bridge formation available for contraction
Properties of stretched fibre?
Less overlapping actin/myosin
Less mechanical interference
Potential for more cross-formation
Increased sensitivity to Ca2+ ions
What are the roles and effect of Starling’s law?
- Balances outputs of the right ventricle and left ventricle – important.
- Responsible for fall in CO during a drop in blood volume or vasodilation (e.g. haemorrhage, sepsis).
- Restores CO in response to intravenous fluid transfusions.
- Responsible for fall in CO during orthostasis (standing for a long time) leading to postural hypotension & dizziness as blood pools in legs.
- Contributes to increased SV & CO during upright exercise.
How does a small ventricular radius affect stress/afterload?
Greater wall curvature
More wall stress directed towards the centre of the chamber
Less afterload
Better ejection
How does a large ventricular radius affect stress/afterload?
Less wall curvature
More wall stress directed through the heart wall
More afterload
Less ejection
What does Laplace’s law do?
-Opposes Starling’s law at rest
Increased preload gives an increased stretch of the chamber (Starling’s law). This increases chamber radius (decreases curvature) which increases afterload.
In a healthy heart, Starling’s Law overcomes Laplace’s – so good ejection
-Facilitates ejection during contraction
Contraction reduces chamber radius so less afterload in ‘emptying’ chamber. This aids expulsion and increases stroke volume.
-Contributes to a failing heart at rest and during contraction. In a failing heart, the chambers are often dilated - so increased afterload opposing ejection.
According to Laplace’s law, what ill increased blood pressure do?
Increase wall stress, which in turn increases afterload and reduces ejection.
What are acute rises in blood pressure offset by?
Starling’s law - increased stretch give increased contraction and increased SV
Local positive inotropes (noradrenaline) or Anrep effect (accumulation of Ca2+)
Baroreflex - decreased sympathetic tone, decreasing blood pressure.
According to Laplace’s law, what will happen with increased radius (r)?
Heart failure where the heart does not contract properly (MI, cardiomyopathies, mitral valve regurgitation) with the blood left in the ventricle leading to eventual volume overload.
According to Laplace’s law, what will happen with increased pressure (P)?
Pressure-overload heart failure due to increased pressure/afterload in the chamber (hypertension, aortic stenosis). This opposes ejection.
