How does RBCs comes out of Bone?
RBCs are produced in the bone marrow. Red cells go through a metamorphosis of about 6 different types of cells before becoming red cells. They make this transformation while attached to erythroblastic islets, which are comprised of multiple transforming red cells attached to a central macrophage.
Once the cells are mature, they are ejected into tubes called venous sinuses, which empty into blood vessels which are part of systemic circulation. There are holes in the bones through which these vessels exit.