A solute component containing all of the solute at the same osmolality as plasma.
Loss of this component does not change plasma osmolality
Ladle of soup
A free water component providing the balance of the volume.
Loss of this solute free water will change serum osmolality.
Boiling off water from soup
In regards to sodium all that matters is the free water component
Free water clearance 0.5 liter free water Solute component (plasma osmolality 284 mOsm/kg) Use clearance to calculate the osmolar clearance 1 liter 142 mOsm/Kg 0.5 liter 284 mOsm/Kg
Free water clearance Solute component (Solute Clearance) ? 0.5 liter Zero mOsm/Kg Free water component (Free water Clearance) The free water component equals urine volume minus the solute component 1 liter 142 mOsm/Kg 0.5 liter 284 mOsm/Kg