The average life cycle of a red blood cell is about 120 days. These cells, also called erythrocytes, transport oxygen from the lungs to tissues and carry some carbon dioxide back toward the lungs.
Mature red blood cells have a distinctive flexible, biconcave shape and lack a nucleus and most organelles. That design leaves more room for hemoglobin, the protein that binds oxygen, but it also means the cell cannot repair itself or replace worn-out internal components. Over time, red blood cells become less flexible and are removed mainly by macrophages in the spleen, liver, and bone marrow.
The bone marrow continually produces replacements in a process called erythropoiesis. The hormone erythropoietin, produced primarily by the kidneys, increases red-cell production when tissues sense low oxygen. A healthy adult therefore maintains a balance between cell production and removal.
“120 days” is an average, not an exact expiration date for every cell. Disease, inherited blood disorders, immune reactions, medications, or mechanical damage can shorten red-cell survival. A common mix-up is applying this figure to all blood cells; platelets and white blood cells have very different lifespans.