Making blood cells is called hematopoiesis. In adults it happens in the red bone marrow, which makes billions of new cells every day.
Where the marrow is
In children, red marrow fills most bones. In adults it is found mainly in the pelvis, sternum, vertebrae, ribs, skull and the ends of the long bones.
Stem cells
All blood cells come from one type of stem cell in the marrow. It splits into a myeloid line, which makes red cells, platelets and most white cells, and a lymphoid line, which makes lymphocytes.
Growth signals
Erythropoietin from the kidney drives red cell production. Thrombopoietin from the liver drives platelets, and colony-stimulating factors boost white cells during infection.
Life span
The spleen removes old and damaged cells. A high reticulocyte count shows that the marrow is responding to blood loss or anemia.
| Cell | Line | Life span |
|---|---|---|
| Red cell | Myeloid | About 120 days |
| Platelet | Myeloid | 7 to 10 days |
| Neutrophil | Myeloid | Hours to a few days |
| Lymphocyte | Lymphoid | Weeks to years |
At the bedside
Marrow drains straight into the circulation, which is why an intraosseous line in the tibia or humerus works when no vein can be found. Chemotherapy suppresses the marrow, so neutrophil and platelet counts drop in the weeks after treatment.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).