Digestive juices chemically break food down into components that are usable in fueling the body's metabolism. The exact chemical reaction that ensues is largely dependent on the specific juice. Stimuli from food, such as sight, smell and taste, triggers increased production of digestive juices. As food is processed in the gastrointestinal tract, juice production increases even more from the organs in proximity and combines with the food. The juices then separate the various components of food, such as sugar and protein, from each other and prepare them for absorption into the system.
Saliva is the first of the digestive juices to act upon food. Amylase—also referred to as "ptyalin"—, an enzyme found in saliva, begins digestion by catalyzing the dissolution of starch into simpler sugars. As an individual chews, saliva is mixed thoroughly into the food, acting upon the starch present and lubricating the food in preparation for other digestive processes.
Food is then swallowed and transported to the stomach, which contains gastric juice. This is considered one of the most volatile digestive juices, with hydrochloric acid being one of its primary components. The powerful acid serves to dissolve the food, while the enzymes pepsin and rennin break protein down into simpler amino acids. Potassium chloride and sodium chloride present in the juice help neutralize the acid, allowing for the safe transfer of food from the stomach into the small intestine, or duodenum.
The duodenum contains two digestive juices: pancreatic juice and bile. Several enzymes act upon food in pancreatic juice; namely, amylase, lipase, and trypsin. The amylase in pancreatic juice, as in saliva, catalyzes the breakdown of complex sugars into simpler sugars. Lipase, on the other hand, catalyzes the breakdown of lipids through hydrolysis. In the same fashion, trypsin catalyzes the dissolution of chemical bonds in peptides to release simpler amino acids.
Bile, the second of the duodenum's digestive juices, is composed primarily of water. Roughly 10 percent of bile contains bile salts, however, which serve to emulsify droplets of fat from partially digested food into micelles. These fats, triglycerides and phospholipids are bound together to form structures known as micelles. The increased surface area created by the emulsification allows lipase in pancreatic juice to act upon the fat, breaking triglycerides down into simpler fatty acids and monoglyceride. These substances are then absorbed through the villi in the intestinal tract, to be used for the body's different metabolic processes.