PM04.03.17 Properties of the Tracer and Compartment Measured

When assessing body fluid compartments using tracers, the characteristics of the tracer determine its distribution within the body. Tracers are typically introduced into the vascular system, where they spread throughout body water until they encounter a barrier they cannot cross. Two primary barriers encountered by tracers are capillary membranes and cell membranes.

Tracer Characteristics and the Compartments Measured

The properties of tracers used to measure different fluid compartments depend on their ability to permeate these barriers.

1. Plasma Volume

• Tracer Characteristic: The tracer should not be permeable to capillary membranes.
• Common Tracer: Albumin
• Reason: Albumin remains within the plasma because it cannot cross the capillary membranes, allowing for the measurement of plasma volume.

2. Extracellular Fluid (ECF) Volume

• Tracer Characteristic: The tracer should be permeable to capillary membranes but not to cell membranes.
• Common Tracers:
  • Inulin
  • Mannitol
  • Sodium
  • Sucrose
• Reason: These tracers can diffuse into the extracellular fluid but are restricted from entering cells, making them ideal for measuring the ECF compartment.

3. Total Body Water (TBW)

• Tracer Characteristic: The tracer should be permeable to both capillary and cell membranes.
• Common Tracers:
  • Tritiated water
  • Urea
• Reason: These tracers can freely move across both capillary and cell membranes, enabling them to distribute throughout the entire body water compartment.

Table of Tracers and Their Compartment Properties

Points to Remember

• Tracers must be carefully chosen based on their ability to permeate specific barriers within the body.
• Plasma volume is measured using tracers that cannot cross capillary membranes, ensuring they stay within the vascular compartment.
• ECF volume requires tracers that can cross capillary membranes but not enter cells.
• TBW requires tracers that can freely permeate both capillary and cell membranes to distribute throughout all body water.

Bibliography Reference Links

• Fluid Compartments and Tracers
• Principles of Body Water Measurement