Quantifying Renal Urine Concentration and Dilution: “Free Water” and Osmolar Clearances
The process of concentrating or diluting the urine requires the kidneys to excrete water and solutes some-what independently. When the urine is dilute, water is excreted in excess of solutes. Conversely, when the urine is concentrated, solutes are excreted in excess of water.
The total clearance of solutes from the blood can be expressed as the osmolar clearance (Cosm); this is the volume of plasma cleared of solutes each minute, in the same way that clearance of a single substance is calculated:
where Uosm is the urine osmolarity, V is the urine flow rate, and Posm is the plasma osmolarity. For example, if plasma osmolarity is 300 mOsm/L, urine osmolarity is 600 mOsm/L, and urine flow rate is 1 ml/min (0.001 L/ min), the rate of osmolar excretion is 0.6 mOsm/min (600 mOsm/L x 0.001 L/min) and osmolar clearance is 0.6 mOsm/min divided by 300 mOsm/L, or 0.002 L/min (2.0 ml/min). This means that 2 milliliters of plasma are being cleared of solute each minute.
Relative Rates at Which Solutes and Water Are Excreted Can Be Assessed Using the Concept of “Free-Water Clearance.” Free-water clearance (CH2O) is calculated as the differencebetween water excretion (urine flow rate) and osmolar clearance:
Thus, the rate of free-water clearance represents the rate at which solute-free water is excreted by the kidneys. When free-water clearance is positive, excess water is being excreted by the kidneys; when free-water clearance is negative, excess solutes are being removed from the blood by the kidneys and water is being conserved.
Using the example discussed earlier, if urine flow rate is 1 ml/min and osmolar clearance is 2 ml/min, free-water clearance would be -1 ml/min. This means that instead of water being cleared from the kidneys in excess of solutes, the kidneys are actually returning water back to the systemic circulation, as occurs during water deficits. Thus, whenever urine osmolarity is greaterthan plasma osmolarity, free-water clearance will be neg-ative, indicating water conservation.
When the kidneys are forming a dilute urine (that is, urine osmolarity is less than plasma osmolarity), free-water clearance will be a positive value, denoting that water is being removed from the plasma by the kidneys in excess of solutes. Thus, water free of solutes, called “free water,” is being lost from the body and the plasma is being concentrated when free-water clearance is positive.
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