Abstract: Method and apparatus are described for determining volumes of body fluids in a subject using bioelectrical response spectroscopy. The human body is represented using an electrical circuit. Intra-cellular water is represented by a resistor in series with a capacitor; extra-cellular water is represented by a resistor in series with two parallel inductors. The parallel inductors represent the resistance due to vascular fluids. An alternating, low amperage, multi-frequency signal is applied to determine a subject's impedance and resistance. From these data, statistical regression is used to determine a 1% impedance where the subject's impedance changes by no more than 1% over a 25 kHz interval. Circuit components of the human body circuit are determined based on the 1% impedance. Equations for calculating total body water, extra-cellular water, total blood volume, and plasma volume are developed based on the circuit components.

Abstract: A soft-sided air displacement volumometer includes a soft-sided bag, an air injector, a pressure transducer, and a recording device. The soft-sided volumometer is used to measure the total body volume of a person or the volume of one limb of a person. After the person enters the soft-sided bag, the bag is sealed and pressurized to a predetermined level using the air injector. The pressure inside the soft-sided bag is continuously monitored using the pressure transducer and recording device while the volume of the soft-side bag is incrementally reduced. The elasticity of the soft-sided bag is eliminated from the volume calculations by operating the soft-sided air displacement volumometer over a relatively small range of volumes and pressures in which the elasticity is substantially constant. The volume of the person inside the soft-sided bag is calculated as the volume of the empty bag (i.e. without a person) minus the initial volume of the bag with the person pressurized to an initial pressure level.