Digestion

Stages of Digestion

Movement

The gastrointestinal tract is in a state of constant motion. The pace is set so that particles are moved in one direction from the mouth to the anus. The only thing that can reverse this flow is exposure of special receptors in the duodenum to toxins. This will then initiate reverse peristalsis that will result in the regurgitation of undigested material.

Neural networks from the brain control the migrating motor complex. It serves as a sweeping function to keep the digestive tract clean. It is interrupted by mass movements, which are initiated both by neural networks as well as local molecular factors. These mass movements occur during digestion proper and serve to move the bulky stool through the colon.

Secretion and Degradation

The process of secretion requires input from all compartments and accessory organs. It is the transport of digestive materials into the lumen of the intestine. Once the appropriate materials for digestion are present, degradation follows. Degradation is the process by which food particles are broken down into smaller constituents until they are in a basic molecular form that can be transported into the blood stream.

In the mouth, the salivary glands produce and release salivary amylase. This enzyme represents the first stage of chemical digestion. It breaks down starch and helps to moisten the food so it can be formed into a bolus that is then propelled through the swallowing process. The stomach secretes several important agents of digestion: mucous, to protect the lining of the stomach from the harsh acidic environment; and pepsin, an enzyme important for protein digestion and acid.

There are three mechanisms for gastric acid secretion. The first is the cephalic phase. During this phase, chewing and swallowing stimulate the vagus nerve, which in turn signals cells in the stomach to release acid. The second phase is the gastric phase. Distension of the stomach as food enters sends a signal to the local nerves, which in turn stimulate the acid-producing cells in the stomach to release acid. The last phase is the intestinal phase. As protein digestion occurs in the intestine, signals are sent via molecules to the stomach to produce more acid. Once the acid is secreted, it activates pepsin, which initiates protein digestion.

The small intestine secretes mucous from its mucosa to serve as a protective barrier. In this compartment, however, the accessory organs secrete their products into the lumen. The gall bladder secretes bile, which aids in the transportation of fat into the blood stream. The pancreas secretes bicarbonate, which neutralizes the acid that is being received from the stomach. The pancreas also secretes amylase, which continues the breakdown of carbohydrates started in the mouth; and lipase, which is responsible for breaking down fats.