Practising The Manual
The challenge in guiding mana to his bone marrow production sites was not in the movement of mana from his heart to his femur, but rather, in what came next.
Just like how the body had large blood vessels like veins and arteries and then much smaller and finer blood vessels like capillaries, the same also held true for a body's mana distribution system.
The body had a main pathway to move mana, known as the mana circuit, which was like a smooth highway that moved mana to each major organ, however, the part where this highway ended, the mana circuit branched into several dozen smaller and finer pathways which could handle a very limited amount of mana load at a time.
The bone marrow was a specific organ that was connected to the main mana circuit via hundreds of these fine, small, mana passages that could only transport a very limited amount of mana at once, which proved to be a major challenge for Leo.