Journey of the Digestive Juices

Digestive Juices - The glands that act first are in the mouth and are called the salivary glands. Saliva produced by these glands break down the starch from food into smaller molecules.

The next set of digestive glands is in the lining of the stomach. These produce stomach acids. Other acids come into the stomach from the pancreas. The mucosa in the stomach lining protects the stomach from these acids as digestive juices produced by the pancreas contain a wide array of enzymes to break down the carbohydrate, fat and protein in food. Further enzymes that are active in the process come from glands in the wall of the intestine or even a part of that wall.

The liver produces digestive juices known as bile. Bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in our food. The bile acids dissolve the fat into the watery contents of the intestine, much like detergents from a frying pan. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

Absorption and Transport of Nutrients
Digested molecules of food, as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. Most absorbed materials cross the mucosa into the blood and are carried off in the bloodstream to other parts of the body for storage or further chemical change. Of course, this part of the process varies with different types of nutrients.

Carbohydrates
Digestible carbohydrates are broken into simpler molecules by enzymes in the saliva, in digestive juices produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps. First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose; then an enzyme in the lining of the small intestine(maltese) splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.

Table sugar is another carbohydrate that must be digested to be useful. An enzyme in the lining of the small intestine digests table sugar into glucose and fructose, each of which can be absorbed from the intestinal cavity in the blood. Milk contains yet another type of sugar, lactose, which is changed into absorbable molecules by an enzyme called lactase, also found in the intestinal lining.

Proteins
Foods that consist of giant molecules of protein, such as meats, eggs and beans, must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the digestive juices of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells.

Fats
Fat molecules are a rich source of energy for the body. The first step in digestion of a fat such as butter is to dissolve it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.

Vitamins
Another vital part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins. The two different types of vitamins are classified by the fluid in which they can be dissolved: water-soluble vitamins (all the B vitamins and vitamin C) and the fat-soluble vitamins (vitamins A, D, and K).

Water and Salt
Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands.

What is the role of microbes in this process?
The microbes present in the GI tract have the potential to act in a favorable, deleterious or neutral manner. Microbes are not very prevalent in the stomach or upper small intestine. The high acid and bile concentrations coupled with the rapid transit time of contents are not favorable to microbial growth. However, toward the lower small intestine, microbes begin to attain higher populations; in the colon they constitute an even higher amount in the colon contents. This becomes a huge number of microbes. Considering their multitude in the intestinal tract, what are the effects of their presence?

It is known that microbes in the large intestine complete the digestion process on any food components that were not digested in the small intestine, such as lactose in lactose intolerant people or fibers resistant to the enzymes they encounter in the small intestine. But there is evidence of non-digestive microbial activities as well. Certain intestinal microbes are known to produce vitamins.

Also, in studies done with special microbe-free laboratory animals, evidence is strong that without normal microbial populations, the immune system functions poorly, and resistance to pathogenic bacteria is greatly reduced. Other evidence suggests that intestinal microbes might act on mutagenic or pre-carcinogenic compounds. Depending on the specific microbe, mutagenic or carcinogenic activity can be either increased or decreased.

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