i know i should know this by now, but can anyone explain to me why in dehydration BUN and Scr increase? i know individually what they are and what they measure, protein metabolism and muscle catabolism, but why do they increse in dehydration? and why would they increase in pre-renal azotemia?

It's a concentration measure (mg/dl), therefore as you decrease the volume (dehydration), the concentration will increase.

But is it just a matter of concentration? It was taught to me that if BUN:Scr > 20:1 = hypovolemia... when they're dehydrated does [BUN] increase more than [Scr]? If they're BOTH high but less than the ratio of 20:1 then its kidney problems and not necessarily hypovolemia?