A healthy human red blood cell has an approximate lifespan of 120 days.
Red blood cells, or erythrocytes, carry oxygen using haemoglobin and transport some carbon dioxide back toward the lungs. Mature human red blood cells lack a nucleus and most organelles, leaving more internal space for haemoglobin but preventing them from repairing themselves or dividing.
As cells age, their membranes become less flexible. They are removed mainly by macrophages in the spleen, liver, and bone marrow. Components such as iron are recovered and reused, while haemoglobin is broken down into products including bilirubin.
The bone marrow continuously produces replacement cells through erythropoiesis. The kidney hormone erythropoietin helps regulate this process in response to reduced oxygen availability. Blood loss, disease, or increased destruction can therefore alter red-cell numbers and produce anaemia, even though the usual lifespan remains about four months.