Dehydration occurs when the loss of water and salt is greater
than intake. Water and salt depletion never occur separately
although one or the other usually predominates.
ECF Deficit: When salt depletion is greater, the extracellular
compartments respond by excretion of an amount of fluid which
corresponds to the amount of salt lost. After this compensation
extracellar fluid and electrolytes are in balance.
Lab Test: Therefore, the hematocrit value is again
used to monitor this condition. If the normal %HCT is 40, than
a value of 45% HCT indicates an ECF deficit. The volume is less
therefore the red blood cell amount which is constant give a
higher % in a smaller volume.
ICF Deficit: When water depletion is predominant, the
greatest fluid loss is sustained by the intracellular compartment.
Initially, water loss leads to an increase in electrolytes in
the extracellular compartment. This leads to an osmotic flow
of water from the intracellular compartment. This is the reverse
of the normal situation at #4 in the figure.
Lab Test: The sodium concentration is an indirect measure
of the fluid conditions in the intracellular compartment. If
the normal value of sodium is 140 mmole/L, than what does a value
of 155 mmoles/L indicate? Again the higher value indicates that
for the same amount of sodium it is not in a more concentrated
condition, which means that the volume of water is less. So
a higher sodium value indicates an ICF Deficit.