Red blood cells are made in the red bone marrow, which is the soft, spongy tissue found in the center of certain bones. In adults, the primary sites of red blood cell production include the vertebrae, ribs, sternum (breastbone), pelvis, and scapulas. This process of red blood cell formation is called erythropoiesis and takes about seven days from stem cell to mature red blood cell. The bone marrow continuously produces red blood cells to replace those that are aged or lost, with about 2.4 million new cells produced every second in a healthy adult. In embryos, red blood cells are initially produced in the liver, but after birth, production shifts to the bone marrow. The kidneys regulate this production by releasing the hormone erythropoietin in response to low oxygen levels, which stimulates the bone marrow to increase red blood cell production