Blood is broadly made up of red and white blood cells and plasma. Red blood cells contain hemoglobin, which gives blood its red color and more importantly: it facilitates the transportation oxygen. The total number of hemoglobin molecules within the body is referred to as hemoglobin mass (Hbmass). White blood cells possess an important immune function but only constitute a very small fraction (less than 1%) of total blood volume. Glucose and other important substances are transported in plasma. In total, the red blood cells and plasma constitute the greatest portions of blood by far.
The fraction of red blood cells to the total blood volume is referred to as the hematocrit. Standard values for females is 39.5% and 43.5% for males. Assuming 5 liters of blood in a healthy adult male, this means that 2.2 liters will be red blood cells while the remaining 2.8 liters are plasma. The concentration of hemoglobin in the blood in a similar adult will be around 130-gram pr liter. Hematocrit and hemoglobin concentrations are easy to assess routinely but none of these measurements provide any information on the total blood volume. Also, these measurements do not provide important information regarding why the hematocrit and/or hemoglobin concentration may have changed.
Hence, measurement of total blood volume is of importance when evaluating the impact of training as well as for treatment of a broad range of critical diseases.
For further scientific reading on the topic:
Montero D, Lundby C
“Regulation of Red Blood Cell Volume with Exercise Training.”
Comprehensive Physiology, 2018, link