- Free water, or solute-free water, is produced in the diluting segments of the kidney (i.e., thick ascending limb and early distal tubule), where NaCl is reabsorbed and free water is left behind in the tubular fluid.
- In the absence of ADH, this solute-free water is excreted and CH2o is positive.
- In the presence of ADH, this solute-free water is not excreted, but is reabsorbed by the late distal tubule and collecting ducts, and (Ch2o) is negative. 1. Calculation of CH2o
Ch2o = free-water clearance (ml/min)
V = urine flow rate (ml/min) Cosm = osmolar clearance (Uosm V/Posm) [ml/min]
- Example: If the urine flow rate is 10 ml/min, urine osmolarity is 100 mOsm/L, and plasma osmolarity is 300 mOsm/L, what is the free-water clearance?
2. Urine that is isosmotic to plasma (isosthenuric)
- can occur during treatment with a loop diuretic, which inhibits NaCl reabsorption in the thick ascending limb, inhibiting both dilution in the thick ascending limb and production of the corticopapillary osmotic gradient. Therefore, the urine cannot be diluted during water drinking (because a diluting segment is inhibited) or concentrated during water deprivation (because the corticopapillary gradient has been abolished).
3. Urine that is hyposmotic to plasma (low ADH)
- Ch2o is positive.
- occurs with excessive water intake (in which ADH release from the posterior pituitary is suppressed), central diabetes insipidus (in which pituitary ADH is insufficient), or nephrogenic diabetes insipidus (in which the collecting ducts are unresponsive to ADH).
4. Urine that is hyperosmotic to plasma (high ADH)
- Ch2o is negative [or positive free-water reabsorption (Tch2o)].
- occurs in water deprivation (ADH release from the pituitary is stimulated) or SIADH.
E. Clinical disorders related to the concentration or dilution of urine
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