Orthostatic hypotension is the term for that.

G13, the elevated pulse is the first indication you are dehydrated and your bodies first attempt to keep your cells in your body healthy and well oxygenated. After the pulse can no longer keep up, the BP starts to fall.

As a paramedic the first thing I looked for in a patient that could be bleeding on the inside from a car wreck was what their pulse was doing (among other stuff). It was a quick indicator that only took moments to feel and get my info.
(Damn I miss that career, it was sooo cool)

I suggest you work on getting your dehydration issues corrected. Drink more fluids. A normal resting pulse in a healthy person is 60-80