The process of synthesis of blood cells is known as hematopoiesis.
It occurs in the bone marrow (located in the center of most bones) by the cells known as hematopoietic stem cells.
The process of hematopoiesis begins during embryonic development and keeps on happening throughout adulthood.
Approximately, 1011 - 1012 cells are synthesized via the process of hematopoiesis regularly to maintain steady-state levels in the peripheral circulation.