The spleen is our body's "blood" reserve It is this organ that can back up the body. Thus, if the bone marrow breaks down, the spleen takes over its role☝🏻 As in the intrauterine period, it begins to produce red blood cells again. Such "safety net" occurs not only in the presence of pathology, but also in the normal functioning of the body. As an example, the spleen begins to produce more red blood cells during physical activity - for this reason it hurts in the side.