Final answer:
Increased ADH from the posterior pituitary concentrates urine by allowing more water reabsorption into the bloodstream, thus producing less but more concentrated urine.
Step-by-step explanation:
Increased ADH (antidiuretic hormone) released from the posterior pituitary gland leads to a concentration of urine (less urine). This occurs because ADH increases the permeability of the kidney's collecting ducts to water, allowing more water reabsorption into the bloodstream, thus producing a more concentrated urine.
When plasma osmolarity rises, indicating a higher concentration of solutes in the blood, ADH is released, prompting the insertion of aquaporin channels into the collecting duct cells' membranes. Water is then reabsorbed out of the filtrate, resulting in a decreased volume but increased concentration of urine. If plasma osmolarity decreases, indicating a lower concentration of solutes in the blood, less ADH is released, leading to decreased water reabsorption, more dilute urine, and increased urine volume.