Which segment reabsorbs about 15 percent of the filtered water, and by what mechanism?
AThe thick ascending limb, by active transport
BThe descending limb of the loop, by osmosis
CThe distal convoluted tubule, by active pumps
DThe proximal tubule, by simple filtration
Answer & Solution
Correct answer: B. The descending limb of the loop, by osmosis
1. The proximal tubule takes about 67 percent of filtered water osmotically alongside the solutes.
2. The descending limb of the loop takes a further 15 percent, again by osmosis.
3. It can do so because its membranes carry permanent water channels and the surrounding medulla is increasingly concentrated.
4. The distal tubule recovers about 8 percent, and only when ADH is present.
5. The thick ascending limb recovers no water at all, since it has no water channels, so it cannot be the answer.
6. Filtration happens only at the glomerulus, so no tubule segment reabsorbs water by filtering.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 25 "The Urinary System", section 25.6 Tubular Reabsorption_
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