[Campbell Biology P.1037] Some cells lining tubules in the kidney maintain normal cell volume... | Practice Question

Some cells lining tubules in the kidney maintain normal cell volume by synthesizing organic solutes. Where in the kidney would you expect to find these cells, and why?

  • A: Proximal tubule, as it is the primary site of reabsorption of most filtered substances.
  • B: Descending limb of the loop of Henle, as it is highly permeable to water and loses water to the interstitial fluid.
  • C: Thick segment of the ascending limb of the loop of Henle, as it actively pumps out salt, creating a dilute filtrate.
  • D: Collecting duct, especially in the inner medulla, where cells are exposed to extremely high and variable interstitial solute concentrations.

Explanation

The inner medulla has an extremely high and increasing osmotic gradient, primarily established by the loop of Henle. The collecting duct passes through this region, and its cells are therefore exposed to a hyperosmotic environment. To prevent excessive water loss from themselves and maintain cell volume, these cells synthesize and accumulate organic solutes osmolytes like urea, betaine, and sorbitol, which counteract the osmotic stress without interfering with cellular functions. This is a crucial adaptation for survival in the kidney's hypertonic environment.