David Monroe, Facebook asked:
How is bone marrow turned to blood and how is the marrow replaced once it’s used up?
Chris - Bone marrow is found in the bone marrow cavity of bones, so if you cut across a bone and have a look, you'll see there’s a sort of hole in the middle with lots of plates of bone which jut out into that hole, and those plates in healthy bone are covered in stem cells. These stem cells are dividing very, very fast to produce new blood cells. In fact, the body destroys something like 1011 cells every single day and makes another 1011 cells every single day, and that's just red blood cells. In fact, we worked out over the course of lifetime, that you make a quarter of a million kilograms –a quarter of a ton of new red blood cells over the course of a whole lifetime, so it’s a very fast process. And these cells are just dividing all the time and pumping out daughter cells which then slowly, over a course of maturations, turn themselves into the new blood cells. This includes platelets as well. You have big cells called megakaryocytes and those megakaryocytes bleb off little bits of their cytoplasm, the stuff inside the cell, which become these little bits that floats around inside the blood, and they’re what causes you to be able to clot if you have a hole in the blood vessel, the platelet would stick on, and it causes the blood to clot. And then you have white blood cells as well, lymphocytes. They're also made by stem cells in the bone marrow which then go out from the bone marrow, go around the body, and turn into mature immune cells.