The red cells are physiologically destroyed in spleen. To pass through the narrow gaps in splenic sinusoids need the deformability (flexibility, elasticity) of the red cells. When the nucleus is extruded out in the late normoblast stage in bone marrow, new synthesis of RNA is stopped. Still in reticulocyte stage the remaining RNA can synthesize all the proteins including enzymes of anaerobic glycolysis. What ever enzymes are there after that will slowly decay. The most important function of end products of glycolysis is synthesis of ATP and 2:3 DPG. The latter is useful for optimum oxygen delivery to the tissues, while ATP is is needed to operate sodium:potassium ATPase enzyme to pump out extra sodium (with water) from the red cells, so that the biconcave shape, essential for deformability is maintained. In the old cells, there is loss of this function due to decreased levels of ATP below critical levels, which is function of time (age). At this stage the red cell becomes more rigid because of accumulation of water (with sodium) due to failure of the Na:K ATPase pump. These red cells experience difficulty in negotiating the narrow pores in splenic sinusoids. These cells get trapped and engulfed by splenic macrophages. The average life of a normal human red cell is found to be 120 +/- 20 days.