Explain how ADH helps to maintain water homeostasis in humans.

ADH (antidiuretic hormone) is a hormone that targets the kidney. It triggers the insertion of aquaporins into the walls of the collecting duct.

When the water concentration in the body is too low (high osmolarity), ADH is released from the posterior pituitary gland. This causes aquaporins to be inserted into the walls of the collecting duct, allowing water to move from the lumen of the collecting duct into the surrounding tissue. In this way, water is reabsorbed, and the water concentration increases. 

When the water concentration in the body is too high (low osmolarity), ADH is not released from the posterior pituitary gland. This means aquaporins are not  inserted into the walls of the collecting duct and water cannot move from the lumen of the collecting duct into the surrounding tissue. In this way, water is not reabsorbed and the water concentration is lowered.