The Bone Marrow

bonemarrow1
http://www.nlm.nih.gov/medlineplus/magazine/issues/summer11/articles/summer11pg15.html

The bone marrow is inside the bones of the body and is the site where blood cells are made. There are 3 main types of blood cells- red blood cells, whose function is to carry oxygen around the body, white blood cells, which are specialised at fighting infections, and platelets, which stop bleeding. Red blood cells are made by a process called erythropoiesis. This means that they start off as immature cells and over the process of a few days become more and more mature and specialised until they are ready, at which point they leave the bone marrow and enter the bloodstream. Once in the bloodstream, they carry out their function for about 3 months at which point they die a natural death.