Renal potassium handling. More than half of filtered potassium is passively reabsorbed by the end of the proximal convolted tubule (PCT). Potassium is then added to tubular fluid in the descending limb of Henle's loop (see below). The major site of active potassium reabsorption is the thick ascending limb of the loop of Henle (TAL), so that, by the end of the distal convoluted tubule (DCT), only 10% to 15% of filtered potassium remains in the tubule lumen. Potassium is secreted mainly by the principal cells of the cortical collecting duct (CCD) and outer medullary collecting duct (OMCD). Potassium reabsorption occurs via the intercalated cells of the medullary collecting duct (MCD). Urinary potassium represents the difference between potassium secreted and potassium reabsorbed [11]. During states of total body potassium depletion, potassium reabsorption is enhanced. Reabsorbed potassium initially enters the medullary interstitium, but then it is secreted into the pars recta (PR) and descending limb of the loop of Henle (TDL). The physiologic role of medullary potassium recycling may be to minimize potassium "backleak" out of the collecting tubule lumen or to enhance renal potassium secretion during states of excess total body potassium [12]. The percentage of filtered potassium remaining in the tubule lumen is indicated in the corresponding nephron segment.

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