What is described about the horizontal segments of the nephron?

• A. They focus on what the body needs and lose what it doesn't need.
• B. They maintain a constant urine volume.
• C. They are the primary site of filtration.
• D. They reabsorb water via aquaporins.

Answer: (A)

The main idea is that these portions of the nephron are where the filtrate is adjusted to match what the body needs. As fluid moves through the tubule, cells actively reclaim substances the body requires—like nutrients and electrolytes—and add or remove other substances to keep balance. This selective reabsorption and secretion is how the kidney maintain homeostasis, shaping the final urine composition rather than simply filtering.

Filtration, the initial entry of plasma into the nephron, occurs at the glomerulus, not along these horizontal segments. The idea that they maintain a constant urine volume isn’t accurate because urine output varies with hydration and hormonal control. Water reabsorption via aquaporins happens primarily in the proximal tubule and collecting ducts, under specific regulation, rather than being the defining role of these horizontal segments.