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Tubular reabsorption
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Urine is not made all at once. The first step is glomerular filtration: the kidney lets fluid and substances pass from the blood into the nephron, its working unit. Tubular reabsorption is the next step, the one that recovers whatever is still useful from that filtrate. The kidney throws out almost everything first and then decides what to pick back up.
The tubule, the duct the filtrate travels through, has four segments with different capacities. The one with the greatest capacity is the proximal convoluted tubule, the first: under normal conditions it recovers nearly all the glucose and nearly all the amino acids that were filtered, and around 65% of the sodium and water.
The work is done by transport. The cells of that segment move sodium by primary active transport, with a sodium potassium pump that uses energy, and water needs no pump of its own: it follows the solute by osmosis. At the end of the route, the collecting duct does the fine tuning, letting more or less water through depending on the activity of antidiuretic hormone, and that is what sets how concentrated the urine comes out.
In brief
Glomerular filtration is the first step in making urine; tubular reabsorption recovers what is useful across the tubule's four segments.
The kidney throws out almost everything first and then decides what to pick back up.
The proximal convoluted tubule has the greatest capacity: it recovers nearly all glucose and amino acids and about 65% of sodium and water.
Its cells move sodium with the sodium potassium pump and water follows the solute by osmosis; the collecting duct adjusts according to antidiuretic hormone.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Renal, StatPearlsPublished
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