Your bones may just seem like scaffolding, but they do far more than simply hold up the weight of your body. Most people know that bone marrow actively produces crucial red and white blood cells and platelets. But less attention goes to the special blood supply for the bones.

The vessels that provide blood enter into the bone through the thin membrane that covers it, which is known as the periosteum. Without this circulation, you'd lack any immune cells--and without an immune response, any exposure to everyday germs would prove fatal.