We report a new algorithm and measurement system that permits simultaneous monitoring of the hematocrit and plasma volume fraction of blood within the intravascular space of an optically probed volume of skin. The system involves probing with a near infrared laser and simultaneously collecting the Rayleigh and Mie scattered light as one raw signal and the undifferentiated Raman and fluorescence emission as the second raw signal. Those two physically independent raw signals and six parameters that can be obtained by either direct calculation or empirical calibration permit monitoring of the blood in rat paws. We tested a device based on the algorithm in the context of improving detection of blood loss for people with an early undiagnosed internal hemorrhage via real-time monitoring of signal changes with direct correlation to hematocrit. We performed experiments monitoring rat paw skin in vivo while removing blood, centrally or peripherally, and then adding replacement fluids such as Normocarb and blood. Blood removal itself elicits a predictable and consistent response, decreasing hematocrit and increasing relative plasma volume, that depends on the rate and location of removal, the total amount of blood removed, the location of monitoring, and possibly other factors as yet unknown. Similarly, replacing the blood with whole blood vs. saline consistently produces a rational range of responses. Calibration across subjects and the measurement of absolute hematocrit will also be discussed.