The body fluid spaces consist of the plasma, the interstitial fluid volume, and the intracellular fluid volume. The sizes of these spaces are tightly controlled by hormonal and neuronal mechanisms, but their size may be of interest to assess by scientific methods, as disturbances often occur in the wake of trauma and surgery.

A key approach is to use a tracer, by which the volume of distribution of an injected substance is measured after full distribution. Useful tracers must solely occupy a specific body fluid space. The volume effect of an infusion fluid can be calculated by applying a tracer method before and after the administration.

Guiding estimates of the sizes of the body volumes can be obtained by bioimpedance measurements and anthropometric equations.

The Hb concentration is a frequently used endogenous tracer of changes in blood volume. Hb is the inverse of the blood water concentration and Hb changes indicate the volume of distribution of an infused water volume. Volume kinetics is based on mathematical modeling of Hb changes over time, which, together with measurements of the urinary excretion, can be used to analyze and simulate the distribution and elimination of infusion fluids over time.