The Kidney Water Clearance calculator computes the water excretion during urine production.

INSTRUCTIONS: Choose units and enter the following:

• (V_U) Volumetric flow of urine
• (U_Osm) Osmolarity of urine
• (P_Osm) Osmolarity of plasma

Kidney Water Clearance (C H₂0): The calculator returns clearance in liters per second.  However, this can be automatically converted to compatible units via the pull-down menu.

The Math / Science

This equation computes the water clearance, `C_(H_2O)`, of the kidneys.  The computed water clearance value is a volumetric flow of water  and is used to characterize water excretion during diuresis, urine production.  The formula for kidney water clearance is:

   C H₂O = V_U - (U_osm/P_osm) * V_U

where:

• C H₂0 - kidney water clearance
• V_U - volumetric flow of urine
• U_Osm - osmolarity of the urine
• P_Osm - osmolarity of the plasma

Notes

The concept of water clearance is used to quantify the volume of water excreted per unit time in the process of diuresis  If urine output increases because of increased plasma volume, sodium is excreted (natriuresis) and water is also excreted (diuresis). The key thing that is being examined in the use of the water clearance calculation is the ability to excrete hypotonic urine. Free water clearance is calculated by subtracting the osmolar clearance from the urine flow rate.