The process by which blood is created is called hematopoiesis. It starts in the bone marrow where the blood cell is first created. Other parts of the body that are involved in the process include the spleen, the thymus, and the lymph nodes.
In brief, besides the function of protecting and providing strength to our body , bones have another very important function. namely, to be the factory where blood is created. Blood is made up of millions of tiny blood cells. that live only for a few months. For this reason, the blood supply must be regenerated continuously.
The creator of new blood cells is located into the center of our bones, it has a spongy appearence and it is called bone marrow. If bleeding occures, or our blood cells are aging and finally die, they are replaced by new blood cells made by the bone marrow located inside our bones.