<P> Because of the lack of nuclei and organelles, mature red blood cells do not contain DNA and cannot synthesize any RNA, and consequently cannot divide and have limited repair capabilities . The inability to carry out protein synthesis means that no virus can evolve to target mammalian red blood cells . However, infection with parvoviruses (such as human parvovirus B19) can affect erythroid precursors, as recognized by the presence of giant pronormoblasts with viral particles and inclusion bodies, thus temporarily depleting the blood of reticulocytes and causing anemia . </P> <P> Human red blood cells are produced through a process named erythropoiesis, developing from committed stem cells to mature red blood cells in about 7 days . When matured, in a healthy individual these cells live in blood circulation for about 100 to 120 days (and 80 to 90 days in a full term infant). At the end of their lifespan, they are removed from circulation . In many chronic diseases, the lifespan of the red blood cells is reduced . </P> <P> Erythropoiesis is the process by which new red blood cells are produced; it lasts about 7 days . Through this process red blood cells are continuously produced in the red bone marrow of large bones . (In the embryo, the liver is the main site of red blood cell production .) The production can be stimulated by the hormone erythropoietin (EPO), synthesised by the kidney . Just before and after leaving the bone marrow, the developing cells are known as reticulocytes; these comprise about 1% of circulating red blood cells . </P> <P> The functional lifetime of an red blood cell is about 100--120 days, during which time the red blood cells are continually moved by the blood flow push (in arteries), pull (in veins) and a combination of the two as they squeeze through microvessels such as capillaries . They are also recycled in the bone marrow . </P>

Where are red blood cells produced in the body