The elimination of substances from peritubular capillaries (or surrounding hemolymph in invertebrates) into the renal tubules to be incorporated subsequently into the urine. Substances that are secreted include organic anions, ammonia, potassium and drugs. [ GOC:rph http://www.ncbi.nlm.nih.gov/pubmed/25287933 http://en.wikipedia.org/wiki/Renal_secretion#Secretion ]