Proximal convoluted tubule brush border and reabsorptive capacity
A dense brush border of microvilli (tiny finger-like projections) allows the proximal convoluted tubule (the first part of the tubule after the glomerulus) to reclaim most of the filtered fluid.
Remember
The brush border of microvilli gives the proximal tubule the large surface area and the transporters it needs for bulk reabsorption, so losing it sharply reduces sodium and water reabsorption.
The proximal convoluted tubule is the segment where the bulk of reabsorption happens. About 65% of the sodium and water filtered at the glomerulus is taken back here, along with many other solutes.
A dense brush border lines the lumen side of its cells. This carpet of microvilli greatly increases the surface area and carries most of the apical (lumen-facing) transporters.
A big surface, plenty of transporters and plenty of energy explain why most of the filtrate is reclaimed in this segment. Sodium gets in by several routes at once, including the sodium-glucose cotransporters (sodium-coupled carriers) and the amino acid carriers. A sodium pump on the blood-facing side pushes sodium out of the cell and keeps the level inside low, which drives that entry, and the many mitochondria supply the energy. Water follows sodium passively because the leaky epithelium is highly water permeable. The tubular fluid therefore shrinks by about two-thirds yet remains iso-osmotic, keeping the same osmolarity as plasma.
Without the brush border there are fewer transporters and less surface, so much less sodium and water is reabsorbed in the proximal tubule. A larger load then flows on to the loop of Henle (the next part of the nephron, which builds the concentration gradient) and beyond, and this may produce a large volume of urine.
None of the later segments named next has a prominent brush border. In the loop of Henle, the thin descending limb has thin, flat cells that let water leave and the thick ascending limb pumps ions actively; the distal convoluted tubule fine-tunes sodium, chloride and calcium, and the collecting duct sets the final amount of water and salt in the urine under hormonal control.
Key facts
- Site and share
- the proximal convoluted tubule is the first part of the tubule after the glomerulus and reclaims about 65% of filtered sodium and water.
- Brush border
- a carpet of microvilli on the lumen side that gives the cells a huge area for transporters.
- Energy
- many mitochondria supply the energy for the sodium pump on the blood-facing side.
- Water route
- water follows sodium through a leaky epithelium that water crosses easily, so the fluid stays iso-osmotic.
- If the brush border is lost
- less sodium and water is reabsorbed in the proximal tubule, so more is delivered to the loop of Henle and beyond.
- Later segments
- they do more selective, regulated work than the proximal tubule; the loop of Henle builds the concentration gradient, the distal convoluted tubule fine-tunes ions and the collecting duct sets the final amount of water and salt.
Exam traps
- Few mitochondria and mostly passive work do not describe the proximal tubule, because its cells contain many mitochondria to power the sodium pump and the sodium-coupled transport that depends on it.
- Motile cilia (moving projections on a cell's surface that mix fluid, as in the airway) are not how the proximal tubule increases reabsorption, because its cells amplify the apical surface with a brush border of microvilli.
- Losing the proximal brush border does not reduce potassium secretion in the collecting duct, because principal cells (the duct cells that have their own channels) do not depend on it; if anything, the extra fluid arriving downstream tends to increase potassium loss.
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