Tubular functions of PCT, loop of Henle, DCT and collecting duct
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Definition
The collecting duct is the final tubular segment, where aldosterone governs Na+ reabsorption and K+ secretion by principal cells and ADH governs water reabsorption, setting final urine composition and concentration.
With adequate ADH, urine osmolality can reach 1400 mosm/kg H2O; without ADH it falls to as low as 30 mosm/kg H2O.
Principal Cells: Na+ Reabsorption and K+ Secretion
Principal cells, the most abundant collecting-duct cell type, reabsorb Na+ and secrete K+; a basolateral Na+-K+ pump sets up the gradient for Na+ entry through apical ENaC (epithelial Na+ channels).
Aldosterone increases ENaC synthesis and insertion and increases Na+-K+ ATPase activity, so it increases Na+ reabsorption and facilitates K+ secretion, mainly in the cortical collecting duct.
Step 1 Aldosterone acts genomically, inducing sgk and related proteins that form ENaC and drive its insertion into the luminal membrane.
Step 2 Na+ entering the principal cell through ENaC is pumped into the interstitial space by the basolateral Na+-K+ pump, whose activity aldosterone also promotes.
Most urinary K+ derives from collecting-duct secretion; in K+ excess (e.g. high-K+ diet) the duct secretes more K+, while in K+ depletion it reabsorbs K+ instead.