Hello! Could anybody help me? My wondering seems so trivial, but I can't skip it. They say that since u and d quarks are much lighter than QCD scale(~200MeV), in reality we can consider the QCD Lagrangian has an approximate global chiral symmetry with respect to these two flavors. At first, it sounded plausible, but now I wonder what relation between the QCD scale and the quark masses there is and how. Is this problem related to the renormalization scale?