Question 2c

Water Reabsorption via Aquaporins:

• Collecting duct cells contain water channels called aquaporins. When these channels are present, water can be reabsorbed from the filtrate in the collecting duct back into the blood, which increases urine concentration.
• The permeability of these cells to water is influenced by the hormone antidiuretic hormone (ADH).

Role of ADH (Antidiuretic Hormone):

• When the body needs to conserve water (e.g., during dehydration), the pituitary gland releases ADH. ADH binds to receptors on collecting duct cells, triggering the insertion of aquaporins into the cell membranes.
• This increased water permeability allows more water to be reabsorbed, concentrating the urine. Conversely, in the absence of ADH, aquaporins are removed, reducing water reabsorption and resulting in more dilute urine.

Medullary Osmotic Gradient:

• The renal medulla has a high osmotic gradient due to the reabsorption of salts in the loop of Henle. This gradient draws water out of the collecting duct by osmosis when aquaporins are present.
• The further down the collecting duct extends into the medulla, the higher the osmolarity of the surrounding tissue, allowing even more water reabsorption as needed.

Urea Recycling:

• Some cells in the inner medullary collecting ducts are also permeable to urea, which diffuses out into the medulla. This recycling of urea contributes to the osmotic gradient and further enhances water reabsorption.

Regulation of Urine Concentration:

The combined effects of ADH-regulated water permeability, the osmotic gradient in the medulla, and urea recycling allow the collecting duct cells to fine-tune the final urine concentration. This mechanism ensures that the kidneys can produce either concentrated or dilute urine depending on the body’s hydration status and overall water balance needs.