a physiologist is probably better qualified to answer this question than a physicist.... but here's my input anyway for what it's worth....
i experience this occasionally too, and i think my BP is about normal. keeping well hydrated should help maintain your BP. try drinking lots of water (to start with), i mean about 1-2 litres over the course of an hour, and then see if you get the same symptoms. you should feel noticably less dizzy when standing up.
lately, i've been going to the gym in the morning. initially, i didnt bother drinking at all, because i was only doing weights (my last drink was the night before). i would get a little light-headed when getting up from a machine - i was dehyrated. now, i drink fluids at the gym (only 1 litre) and i dont get the problem any more.
i always drink 2 litres before diving to make sure i'm well very hydrated. basically, i aim to drink until i pee quite a lot.
![Eek! :eek: :eek:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
there's no need to drink loads in normal day-to-day life, just a moderate amount, but it could be a good idea before diving, or any exercise for that matter.
if you think the dizziness is severe, then it may be a good idea to talk to your doctor for some advice and get checked out.
HTH
alun