Final answer:
Water uptake or loss in the collecting duct is regulated by ADH, hormones, and aquaporin channels. ADH stimulates aquaporin channels to recover more water, resulting in more concentrated urine. On the other hand, less ADH leads to the recovery of less water and the production of dilute urine.
Step-by-step explanation:
The regulation of water uptake or loss in the collecting duct is crucial for maintaining the body's normal osmolarity. When the blood becomes hyperosmotic, more water is recovered from the filtrate to dilute the blood. Conversely, when the blood becomes hyposmotic, less water is recovered, leading to concentration of the blood. This regulation is controlled by the hormone ADH (vasopressin) released from the posterior pituitary.
Under the influence of ADH, aquaporin channels are inserted into the apical membrane of the principal cells in the collecting ducts. As the ducts descend through the medulla, the surrounding osmolarity increases. If aquaporin channels are present, water is pulled from the collecting duct into the interstitial space and peritubular capillaries, resulting in more concentrated urine. If less ADH is secreted, fewer aquaporin channels are inserted, leading to the recovery of less water and the production of dilute urine.
The volume of water recovered or lost is determined by the number of aquaporin channels present, which regulates blood osmolarity, blood pressure, and the osmolarity of the urine.