The chemical reactions and pathways resulting in the breakdown of heparin, any member of a group of glycosaminoglycans of average Mr (6000-20000), consisting predominantly of alternating alpha-(1->4)-linked D-galactose and N-acetyl-D-glucosamine-6-sulfate residues. [ GOC:mah http://openisbn.com/search.php?q=0198506732&isbn=1/ ]