We all know that red blood cells are produced through a process named erythropoiesis, developing from committed stem cells to mature erythrocytes in about 7 days. When matured, these cells live in blood circulation for about 100 to 120 days. At the end of their lifespan, they become senescent, and are removed from circulation. But why 120 days?
The answer lies in the red blood cell itself! Erythrocytes in mammals are anucleate when mature, meaning that they lack a cell nucleus (only known exceptions are salamanders of the Batrachoseps genus and fish of the Maurolicus genus, they have nucleus in their red blood cells). Because of the lack of nucleus in humans the red blood cells cannot produce necessary enzymes for energy producing processess like glycolysis for longer period of time. Thus whatever the enzymes that were present during erythrogenesis, these enzymes remain in the cells and help the cell to produce energy for 100-120 days. Not only does these enzymes help produce energy in the red blood cell they also help maintain the shape and flexibility of the cell such that the cells will squeeze there way through arteries and viens. when the cell runs out of these enzymes , the cell can no longer maintain its shape and producing thus making it vulnerable to shear and stress of the viens and arteries.
It is also believed that lack of these enzymes evokes factors in Red blood cells that signal a process called apoptosis(Programmed cell death!). The death of the cell is preprogrammed such that the cell is killed as soon as it becomes absolete! and the cell is recycled, just as soda cans are recycled, the remnants of cell death are used again to rebuild newer cells! The greenest technology lives right inside us! We can learn a thing or two by studying ourselves! The human body has got lot of things to teach us. If we can harness the essence and spirit of human body and use them for the development of our society then the immediate problems faced by us on the face of this planet like global warming, pollution , waste management etc., can be tackled intelligently and efficiently.