In a healthy adult, red blood cells die at a rate of 2 million cells per second. This is balanced by the rate at which new blood cells are born, which is also 2 million cells per second. Therefore, there are always roughly the same amount of red blood cells in a human body. This may seem like a large number, but in an adult, there are about 30 trillion red blood cells which is about a quarter of all the cells in a human body. A lot of cells die because they get old and need to be replaced by new cells. A red blood cell lives about 3 months before it gets old and signals for the immune system to eat it.

Blood cells, or erythrocytes, are major oxygen carriers in the body. They are formed from a process called erythropoiesis where red bone marrow cells (only cells which have erythropoietin receptor) respond to the secretion of erythropoietin by the kidney due to low oxygen levels. In short, the hormone stimulates the production of new blood cells in the bone marrow. Blood cells are flexible and contain a lot of hemoglobin, the protein which binds to oxygen. As they mature in the bone marrow, they also lose their nucleus and organelles in order to increase space for oxygen. Due to this loss of a nucleus and other organelles, blood cells cannot repair themselves when damaged; this limits their lifespan to about 120 days. 2-3 million erythrocytes die per second and the spleen removes these old, damaged, and dying cells. One important factor of red blood cells life, is that their rate of formation must equal their rate of death in order to maintain a steady concentration of blood cells in the body.