Intestinal Digestion
The food passes from the stomach, through the pylorus into the small intestine. In this condition of partial digestion it is called chyme.
The first twelve inches of the small intestine is known as the duodenum. In the duodenum the food is acted on by the pancreatic juice, the bile, and the intestinal juices. These juices act on proteins, fats, and carbohydrates. The bile acts on the fats, while the pancreatic and intestinal juices act on the proteins and the carbohydrates. The starches, or dextrin, not fully digested by the saliva, are changed to maltose and glucose, while the trypsin from the pancreatic juice, together with the intestinal juices, change into peptone the protein not fully digested in the stomach. The pancreatic juice also digests the starch found in raw fruits and in such raw vegetables as radishes and lettuce.
Fats are almost entirely digested in the small intestine. The presence of fat stimulates the flow of pancreatic juice, which, in turn, stimulates the flow of bile. For this reason, in some conditions, if the liver is sluggish, fatty foods in moderation are desirable. When bile is not present in sufficient amount the fatty foods ferment and cause gases and foul odors.
The fats are absorbed almost entirely in the small intestine—mostly in the duodenum. Some of the fat may be absorbed directly without undergoing the process of emulsification. Some oils, as paraffin oil, are not absorbed at all but act only as a lubricant of the intestines.
When the food enters the intestine its reaction is acid. Mixed with the bile, pancreatic and intestinal juices, which are alkaline, its reaction becomes alkaline.
The pancreatic juice splits up the fats into glycerin and fatty acids and enables the bile to exert its important emulsifying power. The bile markedly aids this action of the pancreatic juice though it has no fat-splitting power in itself.
Steapsin, another ferment of the pancreatic juice, acts on both fats and carbohydrates in either an acid or alkaline medium.
The sodium in the bile unites with the fatty acid, forming a soap which coats the tiny particles of fat and emulsifies them. The bile thus aids in the absorption of the fats. It also lubricates the intestinal mass, facilitating its passage through the entire length of the intestines. Thus it is a very potent agent in regulating the bowel movements.