Proximal convoluted tubule
The entire renal tubule is involved in reabsorption to some degree, but the proximal convoluted tubule cells are by far the most active "reabsorbers" and the events just described occur mainly in this tubular segement. Normally, the proximal convoluted tubule reabsorbs all of the glucose, lactate and amino acids in the filtrate and 65% of the Na+ and water. Additionally, 80% of the filtered bicarbonate (HCO3-), 60% of Cl- and about 55% of the K+ are reclaimed in the proximal convoluted tubule. The bulk of the reabsorption of electrolytes is accomplished by the time the filtrate reaches the loop of Henle. Nearly all of the uric acid and about half of the urea are reabsorbed in the proximal tubule, but both are later secreted back into the filtrate.
Loop of Henle
Beyond the proximal convoluted tubule, the permeability of the tubule epithelium changes dramatically. Here, for the first time, water reabsorption is not coupled to solute reabsorption. Water can leave the descending limb of the loop of Henle but not the ascending limb, where aquaporins are scarce or absent in the tubule membrane. These permeability differences play a vital role in the kidneys ability to form dilute and concentrated urine.
The rule for water is that it leaves the descending (but not the ascending) limb of Henle's loop, and the opposite is true for solutes. Virtually no solute reabsorption occurs in the descending limb. but both active and passive reabsorption of solute occurs in the ascending limb. In the thin portion of the ascending limb, Na+ moves passively down the concentration gradient created by water reabsorption. A Na+K+2Cl- symporter is the main means of Na+ entry at the luminal surface in the thick portion of the ascending limb . A Na+K+ATPase operates at the basolateral membrane to create the ionic gradient that drives the symporters. The thick asceding limb also has Na+H+ antiporters. In addition, some of 50% of Na+ passes via the paracellular route in this region.
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