The hemoglobin concentration and the hematocrit percentage are usually
used to diagnose anemias and erythrocytoses and to monitor their trea
tment and progress. However, they may be misleading because of dehydra
tion or dilution and it is imperative to keep in mind their relationsh
ip to the size of the red-cell mass, This size is determined by the ce
llular kinetics of four distinct compartments which make up the erythr
on, the stem cells, the progenitor cells, the precursor cells and the
mature cells. Under normal physiologic conditions, the kinetics of the
progenitor-cell compartment determines the size of the red-cell mass
but, under abnormal conditions, the kinetics of each of the compartmen
ts may affect the size. In this review, normal and abnormal kinetics o
f the four compartments are defined and related to the size of the red
-cell mass and the pathogenesis of anemias and erythrocytoses.