How does bone marrow make blood cells?

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Internal structure of a human long bone — bone A long bone of the human body, showing a magnified cross section of the interior.

The process of blood cell formation, known as hematopoiesis, occurs within the bone marrow, which serves as the primary site for producing red blood cells, platelets, and most white blood cells in adults.1

The Mechanism of Production

Blood cells originate from primitive stem cells that possess the capacity to transform into various blood cell precursors. Through continuous division and differentiation, these precursor cells commit to specific lineages, eventually maturing into red cells, phagocytic cells, or megakaryocytes (which produce platelets).1

Regulatory Control

This production is tightly regulated by colony-stimulating factors (CSFs), which are hormonal glycoproteins produced throughout the body. These factors act as signals that respond to the body's changing needs:
* Oxygen Demand: A decrease in blood oxygen triggers the kidney to release erythropoietin, stimulating red cell production.1
* Infection: Bacterial components can accelerate the formation of granulocyte CSF, increasing the output of phagocytic granulocytes to combat infection.1

Once mature, these cells enter the general circulation by penetrating the walls of the sinusoids, which are specialized venous sacs within the marrow.1