How Does it Exit into the Bloodstream Through Solid Bone? - Quora

The middle of the bone is filled with red bone marrow. Blood vessels enter the bone from its outer surface, travel through microscopic canals in the bone.
In the bone marrow, there are large, thin-walled, permeable spaces (modified capillaries) called Sinusoids, blood cells can pass through the walls of the sinusoids into the blood stream as they mature, thus entering the general circulation.
Relevant Context:
- Bone Marrow
- Cell Specialization