Biological assimilation, is the combination of two processes to supply cells with nutrients. The first is the process of absorbing vitamins, minerals, and other chemicals from food within the gastrointestinal tract. In humans this is done with a chemical breakdown (enzymes and acids) and physical breakdown (oral mastication and stomach churning.) The second process of bio assimilation is the chemical alteration of substances in the bloodstream by the liver or cellular secretions. Although a few similar compounds can be absorbed in digestion bio assimilation, the bio-availability of many compounds is dictated by this second process since both the liver and cellular secretions can be very specific in their metabolic action (see chirality.) This second process is where the absorbed food reaches the cells via the liver.