Erythrocytes

The major function of erythrocytes is to carry oxygen to cells. This oxygen is bound to an iron-containing pigment within the cells called hemoglobin. Erythrocytes are small, disk-shaped cells with no nucleus. Their concentration of about 5 million per μL (cubic millimeter) of blood makes them by far the most numerous of the blood cells. The hemoglobin that they carry averages 15 g per deciliter (100 mL) of blood. A red blood cell gradually wears out and dies in about 120 days, so these cells must be constantly replaced. Production of red cells in the bone marrow is regulated by the hormone erythropoietin (EPO), which is made in the kidneys.