In adults, the bone marrow is the organ that makes red blood cells. More specifically, it is the red bone marrow, the spongy, blood-forming tissue inside the pelvis, sternum, ribs, vertebrae, skull, and the upper ends of the femur and humerus. It turns out an enormous number of new red cells every day, replacing the ones that wear out after about 120 days in circulation. The kidneys control how fast this happens, using a hormone called erythropoietin.
As a doctor specializing in haematology, I get asked about this all the time, often by patients who have just been told their red cell count is low. Knowing how the marrow makes red cells helps explain why so many different problems, from iron deficiency to haematologic cancers, end up showing as anemia.
Where Red Blood Cells Are Made
Bone marrow is the main site of hematopoiesis, the process that forms all types of blood cells: red cells, white cells, and platelets. The marrow sits in the hollow centers (medullary cavities) of bones and has two forms. Red bone marrow is the active, blood-forming kind. Yellow marrow is mostly fat and does not make blood under normal conditions.
Where the marrow is active changes with age. Before birth, blood is made first in the yolk sac, then in the liver and spleen, and finally in the marrow. In young children, almost every bone has red marrow. By adulthood, most of the long bones have turned to yellow marrow, and blood production is concentrated in the central skeleton.
| Life stage | Main site of red cell production |
|---|---|
| Early embryo | Yolk sac |
| Mid-gestation fetus | Liver and spleen |
| Late fetus and infant | Bone marrow of nearly all bones |
| Adult | Red marrow of the pelvis, sternum, ribs, vertebrae, skull, and proximal femur and humerus |
This shift matters clinically. When the marrow is severely damaged or replaced by scar tissue, the liver and spleen can sometimes start making blood again. Doctors call this extramedullary hematopoiesis, and it is one reason the spleen can enlarge in some marrow disorders.
How the Marrow Produces Red Blood Cells
Red cell production, called erythropoiesis, starts with hematopoietic stem cells in the red bone marrow. These stem cells can renew themselves and can also give rise to every blood cell line. Once a cell commits to the red cell line, it passes through a series of well-defined stages.
- Committed progenitors (BFU-E and CFU-E), which respond strongly to erythropoietin.
- Proerythroblast, the first cell recognizable under the microscope as a red cell precursor.
- Erythroblasts (normoblasts), which divide, shrink, and fill up with hemoglobin.
- Reticulocyte, formed when the erythroblast pushes out its nucleus. It leaves the marrow and finishes maturing in the blood over about a day.
- Mature red blood cell, a flexible biconcave disc without a nucleus.
Going from committed progenitor to circulating cell takes roughly a week. Along the way, the cell builds up hemoglobin, the iron-containing protein that carries oxygen. The mature erythrocytes then circulate for about 120 days before macrophages in the spleen and liver remove them and recycle their iron.
What the Marrow Needs to Do Its Job
The marrow can only work as well as its supply chain. Erythropoietin (EPO) is the main signal. The kidneys make it when they sense that oxygen delivery to tissues has dropped, whether from blood loss, living at high altitude, or lung disease. Higher EPO levels speed up production, and after significant bleeding a healthy marrow can raise its output several-fold.
The marrow also needs raw materials:
- Iron to build the heme part of hemoglobin.
- Vitamin B12 and folate for DNA synthesis in dividing precursor cells. When either one is lacking, the cells grow large and fragile, which is called megaloblastic anemia.
- Protein, along with smaller amounts of other vitamins and trace elements.
- Normal thyroid and hormonal function, which sets the body’s overall demand for oxygen.
This is why people with chronic kidney disease so often become anemic. The marrow itself may be healthy, but without enough EPO it has no signal to produce.
Conditions That Disrupt Red Cell Production
Knowing how the marrow makes red cells is especially useful for patients with anemia, leukemia, and other blood disorders. Caregivers and students benefit too, because the causes group neatly by where the problem lies.
| Problem | Examples | Typical effect |
|---|---|---|
| Missing raw materials | Iron, B12, or folate deficiency | Small (iron) or large (B12/folate) red cells |
| Too little EPO signal | Chronic kidney disease | Normal-sized cells, low reticulocyte count |
| Marrow failure | Aplastic anemia, drug or radiation injury | Low red cells, white cells, and platelets |
| Marrow replaced | Leukemia, lymphoma, myeloma, metastatic cancer, myelofibrosis | Anemia with other abnormal counts |
| Faulty hemoglobin genes | Thalassemia, sickle cell disease | Ineffective production or early destruction |
In aplastic anemia, the stem cells themselves are damaged, often by an autoimmune attack or, less often, by certain medications or toxins. In blood cancers, abnormal cells multiply out of control and crowd out normal blood production.
How Doctors Assess Bone Marrow Function
A complete blood count (CBC) is the first step. It measures hemoglobin, hematocrit, the red cell count, and red cell size (MCV). A reticulocyte count is the simplest way to check whether the marrow is responding. A high count means the marrow is working hard, usually to replace cells lost to bleeding or destruction. A low count in someone with anemia points to a production problem.
Iron studies, B12 and folate levels, kidney function tests, and a blood smear usually narrow down the cause. When the answer is still unclear, or when more than one cell line is abnormal, a bone marrow aspiration and biopsy lets us look directly at the marrow’s cellularity and structure and check for disease such as leukemia or marrow aplasia.
Treatment and Keeping Healthy Marrow Function
Treatment depends on the cause. Good bone marrow function keeps oxygen reaching the tissues, and low red cell levels can cause fatigue, breathlessness (dyspnea), and pallor that affect everyday life.
- Replacing nutrients: oral or intravenous iron, B12 injections or tablets, and folic acid for deficiency anemias, along with diet changes.
- Erythropoiesis-stimulating agents (ESAs): synthetic EPO for selected patients, such as those with kidney disease.
- Blood transfusion: quick support in severe or symptomatic anemia.
- Immunosuppression or stem cell transplantation: for aplastic anemia and some blood cancers, to restore normal hematopoiesis.
For prevention, I encourage patients to manage chronic conditions, eat a varied diet with enough iron and vitamins, and ask their doctor before taking supplements. More background is in our red blood cells guide.
When to See a Doctor
See a healthcare provider if you have ongoing fatigue that rest does not fix, shortness of breath with light activity, dizziness, a racing heartbeat, pale skin, or yellowing of the skin or eyes (jaundice). Get checked sooner if you also bruise easily, have frequent infections, or have unexplained weight loss, because these can mean more than one blood cell line is affected.
Frequently Asked Questions
What organ makes red blood cells?
After birth, the red bone marrow makes red blood cells. Before birth, the yolk sac, liver, and spleen do this job at different stages of development.
How long do red blood cells live?
A normal red blood cell circulates for about 120 days. After that, the spleen and liver remove it and recycle its iron to make new hemoglobin.
Does the kidney make red blood cells?
No. The kidney makes erythropoietin, the hormone that tells the marrow to produce more red cells. That is why kidney disease often causes anemia even when the marrow is healthy.
Can bone marrow recover after damage?
Often it can. Marrow suppressed by chemotherapy usually recovers within weeks. Severe marrow failure may need immunosuppressive treatment or a stem cell transplant, and your hematologist will explain which approach fits your situation.
