Control Of Cardiac Output Flashcards
As a recap, describe cardiac output, including the two equations involving it.
Cardiac Output (CO) is the amount of blood ejected from the heart per minute.
It’s proportional to how often the heart beats per minute (heart rate, HR) and how much blood is ejected per beat (stroke volume, SV).
CO = HR x SV
BP = CO x TPR (total peripheral resistance)
Define preload and afterload, including the laws governing them.
PRELOAD: the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand. It is governed by Starling’s Law.
AFTERLOAD: the pressure against which the heart must work to eject blood during systole. In other words, it is the end load against which the heart contracts to eject blood. It is governed by Laplace’s Law.
What does Starling’s Law state?
It states that the “energy of contraction of cardiac muscle is relative to the muscle fibre length 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.
This is not a control system, it is just a property of the heart muscle.
Starling’s Law can be represented by a graph showing SV against CVP (central venous pressure).
Describe what the graph presents, and what the last part of the graph means.
Throughout the graph, as the CVP increases, the SV increases. However, at the end of the graph, with an increasing CVP, the SV starts falling. This indicates the point at which the Laplace’s Law takes over, so the heart pumps less blood out.
This emphasises the importance of care with fluid replacement, as giving too much may have an opposing effect.
Why does Starling’s Law work (in the sense of contractile fibres)?
With an unstretched fibre, there is overlapping actin and myosin. This 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. This 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.
List some roles of Starling’s Law in cardiac physiology.
it balances outputs of the right and left ventricle (which is important = isovolumetric)
it restores CO in response to intravenous fluid transfusions
it is responsible for the fall in CO during a drop in blood volume or vasodilation (eg. haemorrhage, sepsis)
it is also responsible for the fall in CO during orthostasis (standing), leading to postural hypotension and dizziness
it contributes to increased SV and CO during upright exercise
Define and describe the implications of Laplace’s Law.
Afterload opposes the ejection of blood from the heart and is determined by wall stress directed through the heart wall.
Stress through the heart wall prevents muscle contraction. More energy of contraction is needed to overcome this wall stress to produce cell shortening and blood ejection.
Laplace’s Law describes the parameters that determine afterload.
What is the equation associated with wall stress?
This describes the relationship between, wall stress (S), pressure (P), radius (r), wall thickness (w) and wall tension (T).
P = 2T/r combined with T = Sw gives us:
S = Pr / 2w
Why does the radius determine wall stress/afterload?
A
With a SMALLER ventricle RADIUS, there is greater wall curvature. This means that more wall stress is directed to the centre of the chamber. This, in turn, means that there is less afterload, so there is BETTER EJECTION.
With a LARGER ventricular RADIUS, there is less wall curvature. This means that more wall stress is directed to the heart wall. This, in turn, means that there is more afterload, so there is LESS EJECTION.
