The tracer (indicator-dilution) technique is often used to measure the volume of interstitial water in the lung tissue. The basic hypotheses required for the validity of the formula: volume = flow × (mean transit time) are reviewed. One of the conditions is that, at a steady state, the average concentration of the tracer over the entire system be equal to the concentration at the sampling site. It is shown that if a tracer (such as THO) is permeable through the membrane that separates the blood from the tissue space, which in turn is limited by an impermeable wall, then, at a steady state, the concentration of that tracer is uniform in both compartments. If a tracer is confined to the vascular space by a semipermeable membrane, then its concentration is nonuniform, but the average value meets the requirement of the tracer-volume formula.

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