Now. Isn't it possible that we have perfectly adequate physiology and
don't need to take any more than normal precautions against dehydration?

Just Yes or No will do. Simple answer--Yes
but incomplete. Hydration is one parameter. Renal flow is another.

Hartley Falbaum

Hartley,
Could you comment on the thoughts below.
On a few flights, I would be peeing constantly but not sweating. I
was hydrated at the beginning of the flight, as I am very
conscientious about it. The legs were very cold, to the point of not
having very good control of my legs and feet when stepping out the
cockpit. The rest of the body was reasonably comfortable during the
flight. I would assume that due to the constrictions of the vessels in
the legs and lower body
extra fluids had to be expelled.
Can an acute dehydration occur due to that process in flight, should
one fly shortly there after in much warmer and lower altitude with
sunny conditions?
Regards
Udo

Udo