Aldosterone causes an upregulation in the Na/K ATPase in the principal cells of the collecting duct. This pumps Na out of the cell towards the bloodstream, and K into the cell. The exit of Na creates a gradient that passively draws Na from within the collecting duct lumen into the cell, and then into the bloodstream. Epithelial sodium channels (ENaC) on the lumenal surface allow this passive movement of Na from the lumen down the gradient into the cell. Aldosterone upregulates ENaC production as well.
Vasopressin, also called ADH, or antidiuretic hormone, affects the expression of aquaporin channels on the surface of principal cells, which control the amount of water reabsorbed osmotically.
The collecting duct helps concentrate urine by reabsorbing important electrolytes and water, thus concentrating the urine as it goes to the ureter.
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