The findings of macrothermodynamics (supramolecular thermodynamics) of quasi-closed systems and the published data about the variation of the chemical composition of living organisms in ontogeny confirm the thermodynamic tendency of aging processes. According to the thermodynamic theory, the specific value of the Gibbs function of the formation of supramolecular structures of the organism tends to a minimum. That tendency explains the variation of supramolecular and chemical composition and the morphology of tissues during aging. Thermodynamic theory makes it possible to define the principles upon which proper diets and medications can be devised to slow down aging. Such diets and medications are also useful in preventative care and in the treatment of various pathologies, including those related to old age. The chemical stability of the supramolecular structures of tissues makes it possible to understand the causes of the essentially continuous evolution of the biological world from the perspective of the second law of thermodynamics.