Blood cells are formed inside bone marrow, the soft, spongy tissue found in the center of your bones. A healthy adult body produces between 200 billion and 500 billion blood cells every single day, and the vast majority of that production happens in this one tissue type. The process starts with a single kind of parent cell, called a hematopoietic stem cell, that can copy itself and mature into any type of blood cell the body needs.
Which Bones Produce Blood Cells
Not every bone in your body is actively making blood cells. In adults, the production sites are concentrated in flat bones and the central skeleton: the skull, collarbones, breastbone, ribs, shoulder blades, vertebrae, and pelvis. The upper ends of your two largest limb bones, the thighbone (femur) and upper arm bone (humerus), also retain active marrow. These locations contain red bone marrow, which gets its color from high concentrations of iron.
This wasn’t always the case. When you were born, virtually every bone in your body contained red marrow. As you grew, much of that red marrow gradually converted into yellow bone marrow, which is mostly fat cells. By around age seven, roughly half of your bone marrow has already turned yellow. In adults, the long bones of your arms and legs are largely filled with this yellow marrow, which doesn’t normally produce blood cells. It does, however, serve as a reserve: in cases of severe blood loss, yellow marrow can convert back into red marrow and start generating blood cells again.
What Gets Made Inside Bone Marrow
Red bone marrow produces all three major types of blood cells. Red blood cells carry oxygen from your lungs to every tissue in your body and shuttle carbon dioxide back for you to exhale. White blood cells are the backbone of your immune system, identifying and fighting infections. Platelets are small cell fragments that clump together at wound sites to stop bleeding.
The daily output is staggering. Each day your marrow releases roughly 400 billion red blood cells, 100 billion white blood cells, and 100 billion platelets. That pace exists because blood cells don’t last forever. Red blood cells survive about 115 days in circulation, though individual cells may last anywhere from 70 to 140 days. White blood cells are replaced every one to three days. Platelets circulate for about seven to ten days before they’re cleared and replaced. Your marrow has to constantly replenish the supply just to keep you at baseline, and it can ramp production up 10 to 20 times higher when the body is under stress, such as during a serious infection or after significant blood loss.
How Stem Cells Become Blood Cells
Every blood cell traces back to hematopoietic stem cells sitting in the bone marrow. These are true multipotent cells, meaning each one can produce any blood cell type. When a stem cell divides, it can either copy itself (maintaining the stem cell pool) or begin maturing into a more specialized cell. Early on, a dividing stem cell creates progenitor cells that are already leaning toward a particular destiny. Some progenitors are destined for the red blood cell and certain immune cell lineages. Others are primed to become lymphocytes, the white blood cells responsible for targeted immune responses.
Hormones and signaling molecules guide which cell types get produced and in what quantities. One key hormone, erythropoietin, is released by the kidneys when oxygen levels in the blood drop, and it tells the marrow to ramp up red blood cell production. Another hormone, thrombopoietin, drives platelet production. Knocking out thrombopoietin signaling in animal studies caused an 80% drop in circulating platelets. Additional signaling molecules steer production of specific white blood cell types, like granulocytes and macrophages, depending on what the body needs at any given moment.
Where Blood Cells Form Before Birth
Before a baby is born, blood cell production follows a shifting map through the developing body. The earliest blood cells appear in the yolk sac, the structure that nourishes the embryo in its first weeks. These early cells are primitive versions of red blood cells and immune cells. Around weeks four to five of development, a region near the developing kidneys and reproductive organs begins producing the first true stem cells capable of sustaining blood production long-term.
Those stem cells then migrate to the fetal liver around weeks six to seven, and the liver becomes the dominant blood-forming organ for most of pregnancy. It supports a massive expansion of both red blood cells and immune cells. Only near the end of fetal development do stem cells finally settle into the bone marrow, which will remain the primary production site from birth onward.
When Blood Cells Form Outside Bone Marrow
In certain situations, the body reverts to producing blood cells in organs outside the marrow. This is called extramedullary hematopoiesis, and it most often occurs in the spleen and liver, echoing the pattern seen during fetal development. It can happen during acute infections or significant inflammation, when the demand for new blood cells temporarily exceeds what the marrow can supply.
More concerning is when it becomes a chronic state. Diseases like myelofibrosis (scarring of the bone marrow), leukemia, sickle cell disease, and thalassemia can impair the marrow’s ability to function, forcing the spleen, liver, and occasionally even the lungs to pick up the slack. In these cases, the organs begin producing their own growth factors to support local blood cell production. Small numbers of blood-forming progenitor cells actually circulate in extramedullary organs even under normal conditions, but they only become clinically significant when the marrow is compromised or overwhelmed.