I am trying to understand why changes in preload do not cause concomitant changes in end systolic pressure. If you have preload enhancement than this should lead to an increase in cardiac output..increased cardiac output at a constant heart rate I would think would lead to increased end systolic pressure. Why doesn't this occur?
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$\begingroup$ remember that blood pressure is not solely dependent on heart rate and stroke volume. Total peripheral vascular resistance is important too. If you increase your cardiac output, your body may have a transient increase in end systolic pressure (i.e. systolic blood pressure) - but this is going to be balanced and decrease total peripheral resistance to maintain what the body is perceiving as "normal" systolic blood pressure. $\endgroup$– Vance L AlbaughCommented Nov 25, 2015 at 15:58
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$\begingroup$ One more thing to keep in mind is when talking about increased preload, you should note which ventricle is concerned. The left ventricular preload would increase only if the pulmonary blood return increases. However, the pulmonary vasculature is extremely compliant and can accommodate vast amounts of blood without significantly affecting the preload to the LV. The blood pressure, on the other hand is measured in the systemic circualtion which might not be affected that much if the preload to the RV increases. $\endgroup$– One FaceCommented Mar 27, 2016 at 17:12
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Preload is defined as the volume of blood that stretches the ventricle at the end of diastole. Therefore, when preload is increased, the end diastolic volume is increased. In simpler terms, there is more blood in the heart just prior to contraction. Even though there is more blood being ejected from the ventricle during systole (that is, there is an increased stroke volume), the increase in preload alone will not affect the end systole volume or pressure.
When preload is increased while afterload and inotropy are held constant, the cardiac muscle fibres will still shorten to the same length. This means that the end systolic volume is unchanged, therefore the end systolic pressure is unchanged. This is best visualized by a pressure-volume loop. On the first example on the diagram below, you can see that the end systolic pressure and volume at point 3 are unchanged with increased preload:
Since cardiac output is equal to the product of stroke volume and heart rate, with increased preload the cardiac output is increased due to the increase in stroke volume. The reason why the end systolic pressure is unchanged with the increased cardiac output is because the end diastolic volume is increased.
