True about free-water clearance is

Free-water clearance denotes the volume of solute-free water that must be removed from, or added to, the flow of urine (in ml/min.) to make it isosmotic with plasma. CH20 = V (1 - Uosm/Posm) V is urine volume per unit time (ml/min.); Uosm and Posm are osmolalities of urine and plasma respectively. During Diuresis (when ADH is absent), Uosm < Posm, and their ratio is less than 1. CH2O will be positive. The maximum CH2O is 15-20 L/day or 10-15 ml/min. During Anti-diuresis (when ADH is present), Uosm > Posm, and their ratio is more than 1. CH2O will be negative; water is being conserved. Thus, option A is correct. * Aldosterone regulates total body Na+ content. It causes Na+ and water retention. * Furosemide or loop diuretics inhibit NKCC transpoer (Na+/K+/2 Cl- cotranspoer) in thick ascending limb. This increases Na+ delivered to collecting ducts. Na+ reabsorption at this site will cause K+ secretion; K+ is lost in the urine. Thus, it is not a free-water clearance. * In hea failure or renal failure, very little free water can be generated even if the urine is dilute because the GFR is decreased. Hence option D is not correct.