I was wondering if nearby is a physiological reason for have a higher heart rate when you are sitting(up) comparing to anyone supine/laid down?

Answer:
Gravity! the heart works harder to pump blood when you are standing!

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I'm not totally sure if I'm right, but I believe that this is a sign of dehydration. When I was surrounded by the military, that was one of the ways they checked to see if we be dehydrated.

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Well when you are sitting up your heart have to pump blood up. Compared to when you are laying down your heart doesn't hold to work as hard because it isn't pumping it up.

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when you are lying down your body's axis is within horizontal position parallel to your heart, there is smaller quantity gravitational pull of the blood and that`s why less pumping force for the heart to generate and to deliver blood to your tissues and organs.

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Yes. Gravity plays a roll on the the distribution of blood thru your body. So you would own blood presure and heart rate changes when you adaptation positions! It is normal and usually last 60-90 seconds in the past going back to your baseline!

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