Secreted:
Urea
K+
For the first time, reabsorption of water via osmosis is NOT automatically coupled to the reabsorption of filtered solutes because the ascending limb of the loop of Henle is relatively impermeable to water. The filtered water is reabsorbed in the descending limb of the loop of Henle. The apical membranes of the cells in the thick ascending limb of the loop of Henle contain Na+/K+/2Cl- Symporters . This means that there is simultaneous reclaim of 1 Na+, 1 K+, and 2 Cl- from the fluid in the tubular lumen. Many K+ leakage channels are present in the apical membrane and as a result, most K+ brought in by the symporters moves down its concentration gradient back into the tubular fluid. Therefore, the main effect of these symporters is Na+ and Cl- reabsorption.
Reabsorption in the Early Distal Convoluted Tubule (DCT)
Reabsorbed:
10-15% of the filtered water
5% of filtered Na+ 5% of filtered Cl-
Ca++
Reabsorption of Na+ and Cl- occur by Na+/Cl- Symporters in the apical membranes. Na+/K+ pumps and Cl- leakage channels in the basolateral membranes then permit reabsorption of Na+ and Cl- into peritubular capillaries.
This area is also a major site where Parathyroid Hormone (PTH) stimulates reabsorption of Ca++.
Reabsorption and Secretion in the Late DCT and Collecting Duct
By the time fluid reaches this level, 90-95% of filtered solutes and water have returned to the bloodstream .
Reabsorption of water, Na+ and bicarbonate occurs while urea, K+ & H+ are secreted.
There are 2 types of cells present at the late or terminal part of the DCT and throughout the collecting duct:
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