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Cardiovascular System 3: Pump Function
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A detailed summary of Sterling's Law, factors affecting stroke volume and the extrinsic control of contractility.
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PUMP FUNCTIONCardiac Output (L/min) = HR (bpm) x Stroke Volume (litres)
CO = approx. 5L/min for a normal adult.
Cardiac Output must equal venous return.
Pulmonary Oedema
RV output > LV output.
Therefore, left sided heart failure.
Build up of pressure in pulmonary vein and pushes fluid into lungs.
Factors influencing stroke volume:
1. Preload – filling pressure of LV. (Pre-contraction), how much blood is coming in.
2. Afterload – after contraction, pressure to eject blood.
3. Contractility – effect of SNS action.
PRELOAD AFTERLOAD CONTRACTILITY
Intrinsic mechanism Intrinsic mechanism Extrinsic mechanism
Stretch on myocyte prior Afterload is the load Not due to changes in
to contraction. against which the heart fibre length but due to
has to work to eject changes in contractile
blood. energy.
Increase in number of An increase in afterload Sterling’s law is
potential cross bridges means you get a INTRINSIC ; therefore,
results in increase in decrease in stroke doesn’t change
contraction. volume as the heart can’t contractility.
Increase in filling get all of the blood out.
pressure results in As an increase in
increase in contraction afterload means that you
and increase in stroke get a transient increase
volume. in preload due to an
increase in stroke
volume.
Difference between systole and diastole pressure is known as Developed pressure.
Sterling’s Law
Intrinsic regulation of stroke volume.
Balances outputs of RV and LV.
Increase in CVP will normally increase CO.
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