Hemopoiesis, or hematopoiesis, is the process by which new blood cells are formed. Bone marrow, the tissue inside bones, is one of the most active organs in the body, and is the site where red blood cells, the majority of white blood cells, and platelets are produced. In children, the marrow inside all of the bones makes blood cells, while in adults the marrow in the long bones of the arms and legs is mostly inactive. Only about a quarter of the marrow is involved in making red blood cells because they have a longer life span than white blood cells and require replacing less often. Alternative spellings of hemopoiesis and hematopoiesis, used mainly in UK English, are haemopoiesis and haematopoiesis.
Inside the bone marrow, cells called hematopoietic stem cells are able to produce all the different types of blood cells. Initially, they form either lymphoid stem cells or myeloid stem cells. Lymphoid stem cells migrate to the spleen, lymph nodes and thymus and go on to produce lymphocytes, which are white blood cells involved in the immune system's response to infection. Myeloid stem cells develop into red blood cells, which carry oxygen, and white blood cells known as granulocytes, megakaryocytes and monocytes. Granulocytes and monocytes help fight off infection, while megakaryocytes break into fragments to form platelets, which are involved in blood clotting.
Regulation of hemopoiesis is normally very precise, in order to maintain normal levels of blood cells in the circulation. Red blood cells generally live for around 120 days, and platelets for about ten, while white blood cells may survive for only a few days or even several hours. When infection occurs, the production of white blood cells increases dramatically, while an episode of bleeding causes more platelets to be formed. Red blood cell production is regulated by a hormone called erythropoietin, which is produced in the kidneys. There are normally around 5 million red blood cells per microliter in the average adult circulation, and more will be formed in conditions of low oxygen.
Sometimes bone marrow can fail, affecting hemopoiesis and resulting in decreased production of all blood cells or only certain lines. Damage to hematopoietic cells may be caused by genetic conditions present at birth, viruses such as hepatitis B, exposure to radiation and certain drugs. Deficiencies in vitamin B12 and folate can prevent blood cells from maturing properly, and some cancers, such as lymphoma, may infiltrate the bone marrow. Common symptoms of bone marrow failure include tiredness, weakness, recurrent infections and excessive bruising and bleeding. Treatments vary according to the cause, but often a bone marrow transplant is required from a suitable donor.