Blood stem cells, known as hematopoietic stem cells, reside primarily in marrow, the spongy interior of bones. These "starter" cells resupply three types of blood cells: erythrocytes, commonly known as red blood cells; platelets, also called the blood-clotting cells; and leukocytes, the white blood cells of the immune system.

When the body needs to replace red blood cells, platelets for blood clotting, or immune cells, stem cells located in the bone marrow mature in a process called hematopoiesis. Hematopoiesis goes on constantly in the human body, but certain conditions call for increased activity. For example, when a person moves to a high altitude or attempts to recover from serious bleeding, increased numbers of stem cells must move from the bone marrow into the bloodstream. 

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