Erythrocytes are cellular components of blood, also called red blood cells (RBC). "Mature erythrocytes are non-nucleated, biconcave disks containing hemoglobin whose function is to transport oxygen." Reticulocytes are the immediate immature but circulating precursors of erythrocytes. Erythroblasts are among the more distant precursors, in bone marrow, within the process of erythropoesis.
A basic way to characterize the functioning of the mass of erythrocytes in a living creature is to determine the hemoglobin content per unit volume of blood, the number of erythrocytes, or the percentage of blood made up of cells (i.e., the hematocrit; the bulk of blood cells are erythrocytes). Given these measurements, it is possible to compute erythrocyte indices (i.e., red blood cell or RBC indices), which characterize the properties of the cells. Anemia is a result of an inadequate number of normal erythrocytes, or ineffective erythrocyte function because the erythrocytes do not contain enough hemoglobin or are easily destroyed.
It is possible to have too many erythrocytes, as in polycythemia vera.