Why does systole decrease when standing up?

[Lothric]

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Hey,

Upon standing up more blood accumulates in lower body --> hypotension due to decreased preload --> increased sympathetic firing and decreased parasympathetic stimulation --> increased HR, vasoconstriction and DIASTOLIC blood pressure.

But apparently, the systolic blood pressure decreases. Why is that?

 

[Fruit Plant]

Hey,

Upon standing up more blood accumulates in lower body --> hypotension due to decreased preload --> increased sympathetic firing and decreased parasympathetic stimulation --> increased HR, vasoconstriction and DIASTOLIC blood pressure.

But apparently, the systolic blood pressure decreases. Why is that?

I think one thing to remember in problems like this is that neurohormonal compensation typically never completely corrects any problem (usually no overcompensation). The increased sympathetic firing and decreased parasympathetic stimulation you mentioned compensates for the decreased preload and prevents it from falling even further, but it does not compensate to the point that systolic blood pressure increases or even remains unchanged. Ultimately the effect of blood accumulating in the lower extremities is greater than what the neurohormonal system can compensate for and so you see decreased systolic blood pressure - the neurohormonal compensation simply prevents it from decreasing too much.