Blood is made in the bone marrow.
All the cells in the blood, red blood cells, white blood cells and platelets are made in the bone marrow. This occurs in the large flat bones in your body like the pelvis, the sternum and the skull.

This microscopic view of the bone marrow shows the immature and mature red blood cells, the cells that produce platelets and the immature and mature white blood cells.

All of the blood cells begin from a stem cell. The stem cells become whatever type of blood cell the body needs. Pluripotent (simplest) stem cells form the lymphoid stem cells which develop into the B and T-cells, or the lymphocytes. The myeloid stem cells differentiate into red blood cells, platelets or other white blood cells.

Blood cell cycle

Since all blood cells live for only a short time, the human body produces new blood cells on a regularly basis.