In normal human blood, the total amount of hemoglobin contained in RBCs ranges from 12 to 16 g/dL. The presence of a certain amount of free hemoglobin in plasma of healthy human blood is a physiological phenomenon associated with destruction of senescent RBCs. In adults the mature RBCs (about 2–3 · 1012 cells) are circulating for 100–120 days. The estimated mean RBC lifespan in newborns is 60 to 80 days. Reasons for this shortened lifespan are not clear, and it is perhaps due to differences in mechanical properties of newborn and adult RBCs. Mechanical and physicochemical properties of RBCs change as a result of the cellular aging process. RBCs become more fragile, less deformable and their surface becomes depleted of sialic acid molecules that are responsible for RBC surface charge.
