The concept of “free water clearance”:
- Assessment of renal function includes the handling of water of kidney.
- The “free water clearance” quantifies how much water is gained (by excreting a concentrated urine) or lost (by excreting a dilute urine) by renal handling of water; it involves measurement of urine osmolality and the volume of urine excreted.
- Kidney has the ability to separate solute and water; this ability is essential to form a concentrated or dilute urine. [Note:- Separation of solute and water occurs in the thick ascending limb (TAL) of loop of Henle. This part removes solutes from the tubular fluids, but is impermeable to water.]
- Solutes will be excreted in an iso-osmotic fluid (i.e., that part of the urine will have the water needed to dissolve those solutes). And, the remaining ‘separated’ water will be the “solute-free water”. When body needs water to be conserved, this “solute-free” water will be returned to the body, and the urine will be concentrated. When the urine is dilute, the “solute-free” water is excreted from the body.
- The concept of “free water clearance” quantifies the amount of solute-free water generated by the kidneys; this is the water that is gained or lost from the body, by excretion of a concentrated or dilute urine.
How is “free water clearance” determined?
a) The total solute excreted by the kidneys is calculated first. It is termed as ‘osmolar clearance’ (Cosm).
[The clearance equation, for a substance ‘x’ ~ Cx = (Ux X V)/Px; in which: Ux is urinary concentration of ‘x’ (mg/mL), V is volume of urine (mL/min), and Px is plasma concentration of ‘x’ (mg%).]
b) Free water clearance (CH2O) is then calculated as:-
CH2O = V – Cosm
Note:- Cosm represents the amount of water necessary to excrete the osmotic load (solutes) into a urine that is isotonic to plasma. Thus, from the total volume of urine (V), if we subtract the water that removes solute in isotonic urine, the remaining water is the ‘solute-free’ water and its clearance is free water clearance.
• When urine is concentrated (hypertonic), CH2O is negative, which indicates that solute-free water is being retained in the body. And, the value of CH2O is positive when urine is dilute (hypotonic), indicating that solute-free water is being excreted.
• Values for free water clearance:-
(1) With maximum action of ADH ~ 1.3 mL/min (or, 1.9 L/day)
(2) In the absence of ADH ~ 14.5 mL/min (20.9 L/day)