In the human body, red blood cells (erythrocytes) are primarily produced in the bone marrow, specifically in a type of bone marrow known as “red bone marrow.” Red bone marrow is found in the spongy or trabecular bone of certain bones, such as: Flat Bones: Flat bones, such as the sternum (breastbone), ribs, and the flat bones of the skull (e.g., cranial bones), contain red bone marrow. Pelvic Bones: The pelvic bones, including the hip bones (ilium, ischium, and pubis), also contain red bone marrow. Long Bones: While long bones, like the femur (thigh bone) and humerus (upper arm bone), primarily contain yellow bone marrow (which stores fat), they also contain red bone marrow in the spongy bone tissue at the ends of the bones, particularly in the regions near the epiphyses (the ends of long bones). The process of red blood cell production is called erythropoiesis. It involves the differentiation and maturation of hematopoietic stem cells (multipotent stem cells) in the bone marrow into mature red blood cells. These newly formed red blood cells are released into the bloodstream to transport oxygen from the lungs to tissues throughout the body and carry carbon dioxide back to the lungs for exhalation. Erythropoiesis is tightly regulated by the hormone erythropoietin (EPO), which is produced by the kidneys in response to low oxygen levels in the blood. When oxygen levels are low, such as at high altitudes or in cases of anemia, the kidneys release EPO, which stimulates the bone marrow to increase the production of red blood cells to help restore oxygen balance in the body.