The principal cell mediates the collecting duct's influence on sodium and potassium balance via sodium channels and potassium channels located on the cell's apical membrane. Aldosterone determines expression of sodium channels (especially the ENaC). Increases in aldosterone increase expression of luminal sodium channels. Aldosterone also increases the number of Na⁺/K⁺-ATPase pumps that allow increased sodium reabsorption and potassium excretion. Vasopressin determines the expression of aquaporin channels that provide a physical pathway for water to pass through the principal cells. Together, aldosterone and vasopressin let the principal cell control the quantity of water that is reabsorbed.