Basic Concepts of Fluid and Electrolyte Therapy

11. Disorders of Sodium, Potassium, Calcium,
Magnesium and Phosphate
Introduction
It is impossible to give a detailed account of all aspects of these electrolytes
in a brief chapter such as this. The authors have therefore
confined themselves to a short summary of some common aspects.
For more detailed treatment the reader is referred to the easily available
review articles and books listed under ‘Further Reading’.
Sodium (Na + )
The total body sodium is 3000-4000 mmol, of which only 60% is
exchangeable, the remainder being locked mainly in bone. Short-term
changes in the serum sodium concentration are usually due to
changes in water balance, although, in some cases, salt balance may
contribute. This reflects the fact that salt balance is about maintenance
of volume, whereas water balance is more concerned with
osmolality. Hyponatraemia and hypernatraemia may therefore occur
in the presence of positive, negative or zero salt balance. The serum
Na concentration on its own, therefore, cannot be used to diagnose
the state of Na balance, although if change in water balance is known
from serial weighing, then the day to day balance of Na can be
inferred from the change in serum Na concentration over the same
period (Chapter 3). The principle is best illustrated by some examples:
Hyponatraemia
In severe cases with serum Na +