The cellular portion of blood contains red blood cells (RBCs), white blood cells (WBCs) and platelets. The RBCs carry oxygen from the lungs ; the WBCs help to fight infection; and platelets are parts of cells that the body uses for clotting. All blood cells are produced in the bone marrow . As children, most of our bones produce blood. As we age this gradually diminishes to just the bones of the spine (vertebrae), breastbone (sternum), ribs, pelvis and small parts of the upper arm and leg. Bone marrow that actively produces blood cells is called red marrow, and bone marrow that no longer produces blood cells is called yellow marrow. The process by which the body produces blood is called hematopoiesis . All blood cells (RBCs, WBCs and platelets) come from the same type of cell, called the pluripotential hematopoietic stem cell . This group of cells has the potential to form any of the different types of blood cells and also to reproduce itself. This cell then forms committed stem cells that will form specific types of blood cells.