Digestion

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.

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.

Every section of the colon and anus secretes mucous. This serves to lubricate the mucosal layer of the intestine to allow for the easy passage of stool. Although bacterial degradation of waste material occurs in the colon, this is not a secretory process.

Absorption is the process by which the nutrients are transported across the mucosal layer into the blood stream. Each part of the small intestine is anatomically different to absorb specific nutrients. The brush border membrane in each portion of the small intestine has been adapted to facilitate easy passage of the products of digestion that are preferentially absorbed there. The methods by which the molecules are transported vary according to the type of nutrient. Some nutrients are actively transported across the membrane. This requires the build- up of an electrochemical gradient whose energy can be used to transport larger nutrient molecules.

Passive transport is the most common mechanism of transport. This requires the presence of an electrolyte on the other side of the membrane to trade places with the nutrient that desires entry. As the electrolyte enters the lumen of the intestine, the nutrient crosses the barrier and moves into the blood stream. Some passive transport requires the help of a carrier molecule. In the case of fat, micelles carry the products of fat digestion across the border in a passive fashion. Micelles are complex carrier molecules that are made in the liver.

Diffusion is the other method by which nutrients can cross over the barrier. Because of different gradients on either side of the membrane, some molecules can move freely between the lumen and blood stream in a direction from highest to lowest concentration. Water is moved in and out of the lumen in this fashion. The first place that absorption occurs is in the mouth. Because of the enormous amount of blood vessels in the mouth, small molecules and water can diffuse directly into the blood stream at this level. There is minimal absorption in the stomach. The small intestine and colon are responsible for the remainder of absorption. In the small intestine, protein, carbohydrates, fats, vitamins, and minerals are absorbed. In the colon, large amounts of water are reabsorbed so that very little water is eliminated through the stool.

Assimilation is the process whereby the nutrients are delivered to the rest of the cells and organs in the body. Once the nutrients are transported into the blood stream, they are released from their transport molecules. Through efficient circulatory methods, the nutrients are delivered to the rest of the body. The awaiting cells respond by recognizing the nutrients they need and transporting them into the cell. The nutrients are then used appropriately. Waste products are transported into the blood stream and eliminated via the kidneys or the digestive tract.

Elimination is the removal of undigested food and waste products accumulated during digestion. They remain in the lumen during the process of digestion. Peristalsis serves a housekeeping function to keep the waste products moving through the gut toward the anus. The stool enters the colon in a liquid form. Once in the colon, bacteria assist in the final stages of digestion. The gaseous by-products of bacterial degradation are eliminated via the rectum and anus. As the stool moves towards the rectum, water is aggressively reabsorbed. The mucosa of the colon has an incredible absorptive capacity that enables almost complete reabsorption of all the water from the stool. The stool becomes firm and bulky and moves into the rectum, which acts as a reservoir until proper evacuation can be performed.

The rectum serves a very important function. Equipped with both voluntary and involuntary nerve inputs as well as several layers of highly specialized muscle, the rectum allows the storage of stool until it is appropriate for evacuation. Upon entering the rectum, the firm stool triggers an involuntary reflex that tightens the muscles of the anus. Only with voluntary input can these muscles be relaxed, thereby initiating the process of defecation and evacuation.