Living in south Florida makes you aware of how easy it is to become dehydrated. If more water is required in the blood stream, what is released by the pituitary gland?

antidiuretic hormone(ADH)

In case of dehydration the Osmoreceptor  in the hypothalamus  detects the changes in blood solute potential and send input signals to the ant. pituitary gland under the brain mediation. This gland secret home ADH;( anti diuretic hormone)

This travels in the blood,to the walls of the Collecting ducts(CD). It binds to the receptor proteins of the CD membranes;this activates the enzymes  inside the cells which have  abundant aquaporins . Therefore the enzymes activation causes the aquaporins to migrate to the cell membranes fused with the wall of the CD, therefore increasing their permeability. As the fluid(urine) flows through the CD  it enters  through the aquaporins, out of  and passed out of CD to reach the blood. This gradient flow is set up because the bloods  solute potential is higher than that of the fluid in the CD.

Therefore, hypertonic urine is produced, dehydration is mitigated, until access to water is available.