The collecting ducts, which consist of cortical and medullary segments, reabsorb the final 5 to 7% of the filtered Na+. The epithelium forming the collecting ducts consists of two distinct cell types: principal cells and intercalated cells. The relative preponderance of the two cell types varies along the length of the collecting duct and between nephron segments. Principal cells are responsible for the reabsorption of Na+ and the secretion of K+ (Fig. 21.5). Na+ enters the principal cell from the tubular fluid through a unique and highly selective epithelial Na+ channel, ENaC. Intercalated cells reabsorb HCO3~ and K+ and secrete H+. Normally, H+ is secreted into the urine and HCO3~ is reabsorbed, while
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