The digestive juices are the secretions of the digestive tract that break down food. They include saliva, gastric juice, pancreatic juice, bile, and intestinal juice. The digestive juices are secreted by different organs, vary widely in chemical composition, and play different roles in the digestive process. Each is constantly produced by the body in small amounts, but the presence of food as it passes through the digestive tract causes increased production and secretion.
Digestion begins in the mouth, where the mechanical action of the teeth and tongue and the chemical action of saliva begin to break down food. Saliva is produced by the salivary glands in the mouth. It is composed primarily of water, mucus, various mineral electrolytes, and digestive enzymes including amylase, which begins the breakdown of food starches. Saliva also serves to moisten and lubricate the mouth, provide minerals to maintain tooth enamel, and reduce the level of bacteria in the mouth.
Upon being swallowed, food heads to the stomach, where it is bathed in gastric juice, the second of the digestive juices. Gastric juice is a nearly colorless, strongly acidic liquid secreted by the gastric glands. Its active food-dissolving ingredients are the digestive enzymes pepsin and rennin, which break down proteins, and hydrochloric acid. Gastric juice also contains mucus to protect the stomach lining from being dissolved by the acid.
The next stop for partly-dissolved food is the duodenum, the first section of the small intestine, where it is acted upon by two digestive juices. The first is pancreatic juice, a clear fluid secreted by the pancreas, which contains a plethora of digestive enzymes including tripsin, lypase, and amylase. Tripsin breaks down protein; lypase breaks down fats. Amylase, in the duodenum as in the mouth, works by turning starch into sugar.
The second digestive juice released in the duodenum is bile, also known as gall, a yellow-green fluid produced by the liver and stored in the gall bladder. Bile contains salts which emulsify the fats in the food and allow them to be absorbed through the lining of the small intestine. Bile also serves to carry waste products from the liver into the intestinal tract, where they will eventually pass from the body.
Unlike gastric juice, pancreatic juice and bile are both alkaline. This helps to neutralize stomach acids as the food moves into the lower portions of the small intestine, where glands lining the walls secrete intestinal juice. Also known as succus entericus, intestinal juice is a clear fluid containing a soup of enzymes. It combines with pancreatic juice and bile to complete the digestion of proteins and fats. The remaining nutrients are then absorbed into the intestine walls, and waste products pass to the large intestine to leave the body as feces.
Several disorders of the digestive tract can be traced to having too much or too little of one of the digestive juices. In particular, the production of too much gastric juice, usually as the result of a bacterial infection, can lead to stomach ulcers. Too little bile can lead to jaundice or to the inability to digest fats. Too much cholesterol in bile can lead to gall stones. Too little saliva results in dry mouth and increased tooth decay; it can also be a symptom of any of a number of diseases.