All blood cells (white, red, and platelets) are produced from stem cells called hemocytoblasts located in bone marrow. It takes about 7 days for a stem cell to fully mature into a red blood cell that is prepared to be released into the bloodstream.
During this process, the stem cell becomes an immature red blood cell called an erythroblast. Then, the nucleus and mitochondria of the erythroblast disappear, and the immature cell is gradually filled with hemoglobin. At this point, the cell is called a reticulocyte. Finally, the cell becomes a full mature red blood cell and enters the blood, ready to transport oxygen throughout the body.