Learning Objective: By the end of this section, the learner should be able to identify appropriate tracer substances for measuring plasma, extracellular fluid (ECF), and total body water (TBW), and understand how membrane barriers influence tracer distribution.
Properties of the Tracer and Compartment Measured
Tracers are usually introduced into the vascular compartment and distribute throughout body water until they reach a membrane barrier that they cannot cross. The two major barriers are:
- Capillary membranes
- Cell membranes
The choice of tracer depends on which compartment is being measured:
| Compartment | Tracer Characteristics | Example Tracers |
|---|---|---|
| Plasma | Cannot cross capillary membranes | Albumin |
| Extracellular Fluid (ECF) | Can cross capillary membranes but not cell membranes | Inulin, Mannitol, Sodium, Sucrose |
| Total Body Water (TBW) | Can cross both capillary and cell membranes | Tritiated Water, Urea |
