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The leading preventable cause of death, world-wide, civilian or military, for all people between the ages of 18-45 is
undetected internal hemorrhage. Autonomic compensation mechanisms mask changes such as e.g. hematocrit
fluctuations that could give early warning if only they could be monitored continuously with reasonable degrees of
precision and relative accuracy. Probing tissue with near infrared radiation (NIR) simultaneously produces remitted
fluorescence and Raman scattering (IE) plus Rayleigh/Mie light scattering (EE) that noninvasively give chemical
and physical information about the materials and objects within. We model tissue as a three-phase system: plasma
and red blood cell (RBC) phases that are mobile and a static tissue phase. In vivo, any volume of tissue naturally
experiences spatial and temporal fluctuations of blood plasma and RBC content. Plasma and RBC fractions may be
discriminated from each other on the basis of their physical, chemical and optical properties. Thus IE and EE from
NIR probing yield information about these fractions. Assuming there is no void volume in viable tissue, or that void
volume is constant, changes in plasma and RBC volume fractions may be calculated from simultaneous
measurements of the two observables, EE and IE. In a previously published analysis we showed the underlying
phenomenology but did not provide an algorithm for calculating volume fractions from experimental data. Here we
present a simple analysis that allows continuous monitoring of fluid fraction and hematocrit (Hct) changes by
measuring IE and EE, and apply it to some experimental in vivo measurements.
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