Physiological basis of giving normal saline to a patient with isosmotic dehydration.

o   Dehydration is a decrease in ECF volume due to loss of H₂O and solutes such as Na⁺.

o   In isoosmotic dehydration loss of H₂O is equal to the loss of solutes.

o   Loss of isotonic fluid from plasma causes a loss of fluid from interstitium to plasma to maintain hydrostatic balance between the compartments.

o   But there is no change in the osmolality in the ECF compartment because the fluid lost is isotonic.

o   Thus there is no shift of fluid into or out of ICF because there is no change in ECF osmolality.

o   Causes for isotonic dehydration are blood loss diarrhea, vomiting and burns.

o   Normal saline(0.9%NaCl) is given as treatment in isotonic dehydration as it is an isotonic solution itself.

o   Infusion of a 1L will remain in the ECF ¼ in plasma ¾ in ISF.

o   In isotonic dehydration fluid is lost from the ECF and normal saline is given to replace the fluid loss as it will be distributed only in the ECF