The formation of blood cells in the living body; especially, in the bone marrow; also, hematopoiesis, haematopoiesis, hemopoiesis, haemopoiesis, sanguification.

In the embryo and fetus it takes place in a variety of sites including the liver, spleen, thymus, lymph nodes, and bone marrow; from birth throughout the rest of one's life it is mainly in the bone marrow with a small amount occurring in lymph nodes.