A method of measuring total vascular hemoglobin mass. One step of the method is to store a volume of fluid to be injected using a processor. A hemoglobin value is measured in vivo on a substantially continuous basis. The method also measures a total circulating vascular volume based on the volume of fluid to be injected by detecting a change in blood concentration over a period of time upon injection of a fluid into a body. The total hemoglobin mass is calculated based on the measured hemoglobin value and the measured total circulating vascular volume.
