The human body builds red blood cells through the mediation of a protein called erythropoietin, claims the American Society of Hematology. This protein is made largely in the kidneys.
Red blood cells are made in the bone marrow, says the American Society of Hematology. An immature red blood cell is large, round and has a nucleus. When a red blood cell is mature, it loses its nucleus and leaves the marrow for the bloodstream.
As the cell matures, it takes on hemoglobin, which is a pigment that gives it its red color, posits the American Society of Hematology. Hemoglobin is also a protein that lets the blood carry oxygen from the lungs to the other cells in the body, and lets it carry carbon dioxide from the cells back to the lungs. Erythropoietin also stimulates the production of hemoglobin, says MedicineNet.
The red blood cell is one of the few cells in the human body that doesn't have a nucleus, claims the American Society of Hematology. This allows it to change shape and squeeze through narrow capillaries.
It takes about seven days for a red blood cell to mature, and it has a life span of about 120 days, says the American Society of Hematology. After this, it leaves the blood cell and is destroyed. This short life span is the price the red blood cell pays for its flexibility.