Human Erythrocytes – What Is The Life Cycle Of Erythrocytes?
The human erythrocytes are produced by the process of ‘erythropoiesis’. The stem cells that are committed to develop as the blood cells, grow and develop into mature red blood cells. This takes about 7 days of time. The matured cells reside in the circulation for about 100–120 days. After their lifespan ends, they are removed from the circulation after becoming senescence.
What Is Erythropoiesis?
The developmental process by which new erythrocytes are produced is called Erythropoiesis. Each cell matures in about 7 days. The erythropoiesis, is a continuous process of production of erythrocytes from the red bone marrow present in the large bones. This forms at the rate of about 2 millions per second in a healthy adult. Liver is the major site for RBC production in the embryos. The stimulation of the production of is possible by the hormone erythropoietin (EPO). This hormone is synthesised by the kidney.
The functional phase lasts for about 100–120 days. During this phase, the human erythrocytes are found to be moving by the blood flow continuously; ‘push’ in arteries and ‘pull; in veins. These are then squeezed through the micro vessels (capillaries) when they are found compressed against each other to move.
Changes are seen in the plasma membrane of the aging erythrocyte. This makes it susceptible to recognition by macrophages and then going through phagocytosis. This occurs in the reticuloendothelial system that involves – spleen, bone marrow and liver. The old and defected cells are removed from the blood stream continuously. This process is called eryptosis, or erythrocyte programmed cell death. The rate of cell death is almost the same with the rate of production by erythropoiesis. This helps in maintaining the balance of the total count of circulating red blood cells. The resulting breakdown products are sometimes seen re-circulated in the blood.
The heme constituent present in the hemoglobin is broken down into Fe3+ and biliverdin. The biliverdin undergoes a reduction reaction to form bilirubin. This bilirubin is released into the plasma and bound to the albumin. Then it is recirculated to the liver. This is again released into the plasma from where it is recirculated by transferrin, a carrier protein. This is how all the erythrocytes are removed. The removal from the circulation is started before the erythrocytes are too old to get hemolyzed. The hemolyzed hemoglobin is found bound to haptoglobin, a protein found in the plasma. This is not excreted by the kidney.