Answer:
1. The decrease in the extracellular fluid upsurges the plasma osmolarity. 
2. This enhanced plasma osmolarity stimulates the osmoreceptors. 
3. These osmoreceptors further instigate the discharging of anti-diuretic hormone. 
4. The water is reabsorbed from the renal tubules and the urine turns more concentrated when the discharging of ADH takes place.