Brain cells try and defend their intracellular volume in the face of changes in the tonicity of the ECF. This helps prevent shrinkage in the face of increased tonicity of the ECF and swelling in the face of decreased tonicity of the ECF. This latter situation is particularly important, as the brain is enclosed within a rigid box.
To decrease cell volume, K+ along with Cl- is pumped out of the cell. The concentration of Cl- in the ICF varies between cells.
To increase cell volume e.g. during hypernatraemia, a number of mechanisms are employed. An important adaptation is to increase the [Na+] of the ICF. Normally Na+ is being pumped out of cells and therefore a decrease in this activity will allow the intracellular [Na+] to rise. It is also possible to increase the number of organic osmoles (amino acids, taurine).
These actions must be recognised when correcting hypo- or hyper- osmolar states, in order to allow the compensatory mechanisms to resolve and prevent rapid fluid shifts which occur if the treatment regimes are too aggressive.