An increased blood osmotic pressure would trigger an increased production of which would result in production.

Increase osmotic pressure would trigger production of Aldosterone which end up causing increase fluid absorption resulting to decrease urine production.

The whole system is called the Renin-Angiotensin-System (RAS) or sometimes Renin-Angiotensin-Aldosterone-System (RAAS)

Her parents start noticing changes about her. she's probably just competive though not on drugs

Ibelieve this would be an obstacle.