The Gastrointestinal Tract

1. The Gastrointestinal Tract

The gastrointestinal tract begins with the mouth and ends with the anus. Disease symptoms may arise anywhere along that route if we do not follow the Laws of Life, i.e., if we live unhealthfully. A few of the most common “diseases” will be discussed and the reason for their occurrence. It is not necessary to elaborate upon every disease known, as all diseases stem from a common cause—toxicosis.
First of all, it is necessary for you to have a brief review
of the function and structure of the normal gastrointestinal tract.

1.1 The Mouth

The tongue, composed primarily of striated muscles and covered by mucous membrane, plays important roles in the mastication of food and in the act of swallowing. The teeth have an important role in the mechanical mastication of food prior to swallowing.

Solid food taken into the mouth is reduced by mastication into smaller particles to facilitate swallowing. Food in the mouth also is mixed with saliva, which moistens and lubricates the food mass. In addition, digestion of starches commences in the mouth by the action of ptyalin in the saliva.

The chief source of ptyalin (salivary a-amylase) is the glands in the mouth. Ptyalin acts most effectively at an optimal pH of 6.7, and it catalyzes hydrolysis of starch into two disaccharides, maltose and isomaltose. In le stomach, ptyalin may act for up to an hour in the center of the food mass before the fundic contents are mixed with acid gastric secretions.

Once pH of the food in the stomach declines below 4.0, the activity of ptyalin is inhibited. Before this inhibition takes place, up to 40 percent of ingested starches will have been converted into maltose by ptyalin. The activity of ptyalin is also inhibited by the presence of protein because he presence of any protein food in the stomach initiates the secretion of the hydrochloric acid for its digestion. Therefore we recommend that proteins and starches not be eaten together.

As digestive enzymes act solely at the surface of food particles, the rate of digestion is related directly to the extent to which food is masticated.

1.2 The Pharynx and Esophagus

The pharynx is the portion of the digestive tract serving is a passageway for both the respiratory and digestive systems.

The esophagus is a long, straight tube extending from the pharynx to the stomach. Passage of food is facilitated by ordinary gravitational forces, as well as by the type and arangement of muscles in the tube itself. It is located between the trachea and the vertebral column. Esophageal glands serve to lubricate food during its passage from the pharynx to the stomach.

1.3 The Stomach

The stomach is the most widely-dilated portion of the digestive tract. It functions to store and digest food. In the stomach, solid food ultimately is converted into a semifluid mass by contraction of the muscular wall combined with mixture of the food with the glandular secretions of the gastric mucous membrane. Although food in the upper region of the stomach may remain solid for relatively long periods, food becomes transformed into a pulpy fluid mass (chyme) in the lower part of the organ.

Chyme is then ejected into the small intestine in small quantities once a proper consistency has been achieved.

The stomach consists of three parts: the fundus, an upper portion ballooning toward the left; a body; the central portion; and the pyloric portion (antrum), a relatively constricted portion at the terminal end just before the entrance into the duodenum.

The cardia is the opening between the esophagus and the stomach. The pylorus is the opening between the stomach and the duodenum. The circular muscle layer is thickened in the pyloric region to form the pyloric sphincter.

1.3.1 Gastric Juice

Cells of the gastric glands secrete a total volume ranging between two and three liters per day. This digestive fluid contains a number of substances.  In addition, gastric mucous cells and glands secrete a thick alkaline mucus that forms a thin coating on the stomach wall. Thus, it is of great importance in protecting the epithelial lining of the stomach.

The gastric glands secrete digestive juices. Of particular significance are the chief cells that secrete pepsinogen and parietal cells that secrete hydrochloric acid.

Pepsinogen. The proteolytic enzyme pepsin, which degrades ingested proteins into polypeptides, is secreted by chief cells of the stomach in an inactive form, pepsinogen. When pepsinogen is secreted into the gastric lumen, it contacts hydrochloric acid and pepsin which had been formed earlier. Cleavage of the pepsinogen molecule now occurs so that more active pepsin is produced. Pepsin is active enzymatically only in a highly-acidic medium (optimum pH 2.0); it is inactivated above a pH of 5.0. Consequently, secretion of hydrochloric acid is essential to protein digestion in the stomach by pepsin.

Hydrochloric Acid. Parietal cells of the gastric glands secrete free hydrochloric acid into the lumen of the stomach. These cells can perform the osmotic work necessary to concentrate hydrogen ions to a level of over 4,000,000 times greater than in arterial blood.

Energy for hydrocholoric acid secretion is provided by aerobic glycolysis. That is, the conversion of glycogen into glucose. Energy is needed for the transport of hydrogen ions across the membrane of the parietal cell1. Chloride ion also is secreted actively by the parietal cells.

1.3.2 Histamine and Gastric Acid Secretion

Histamine is a powerful stimulant to gastric acid secretion, and the action of histamine is mediated by cyclic adenosine monophosphate. As the gastric mucosa normally has a high concentration of histamine, liberation of this compound has been implicated as the chemical mediator in stimulation of acid secretions.

Chemical agents in addition to histamine also appear to have a role in acid secretion by the stomach, for example, gastrin.

1.4 The Small Intestine

The small intestine extends from the pyloric sphincter to the cecum, the first portion of the large intestine. It is approximately 18 feet in length and is divided into three portions: the duodenum, jejunum and ileum. The duodenum is the shortest, widest and most fixed portion of the small intestine. It receives secretions of the liver and pancreas.

The small intestine has three major functions: 1) to transport chyme onward from the stomach; 2) to continue digestion of chyme by means of special digestive juices elaborated by intrinsic and accessory glands; and 3) to absorb nutrients produced by the digestion of various foodstuffs.

This organ exhibits two important structural modifications that greatly enlarge the total surface area for absorption of nutrients, but without increasing its total length. These modifications are the grossly visible plicae circulares and the microscopic intestinal villi.

Plicae Circulares. Plicae circulares are permanent ridge-like folds that extend into the lumen of the intestine. The plicae not only increase the absorptive area of the intestine, but also mix chyme and digestive juices and slow the rate of transport of chyme so that more thorough absorption of
nutrients can occur.

Intestinal Villi. Intestinal villi are minute flattened (in the duodenum) or fingerlike (in the ileum) projections of the mucous membrane that cover the entire surface of the intestinal mucosa.

1.4.1 Enzymes of the Small Intestine

Many enzymes are found in the small intestine:

1. A number of peptidases are present. These substances are proteolytic enzymes that cleave polypeptides into their constituent amino acids.
2. A small quantity of intestinal amylase is present. This enzyme converts polysaccharides into disaccharides.
3. Four enzymes are present in the intestinal fluids that split disaccharides into monosaccharides. These include sucrose, maltase, isomaltase and lactose (in children).
4. An intestinal lipase is also present, and this enzyme degrades neutral fats into fatty acids and glycerol.

1.4.2 Accessory Digestion Secretions Pancreatic Secretion

Pancreatic secretion

The pancreas secretes between 1,200 and 2,000 ml/day of digestive fluid rich in bicarbonate and a number of enzymes. The pH of pancreatic juice is about 8.0. This alkalinity, together with the neutrality or slight alkalinity of the bile and intestinal juices, neutralizes acidity of the gastric chyme as it enters the duodenum. The pH of duodenal chyme is raised to between 6.0 and 7.0. Therefore, when chyme reaches the jejunum it is approximately neutral. Consequently, the intestinal contents almost never exhibit an acidic reaction.

Pancreatic juice contains a number of potent enzymes for digestion of proteins, carbohydrates, fats and other compounds. The proteolytic enzymes secreted by the pancreas include trypsin and two chymotrypsins. These enzymes cleave whole and partially-digested proteins. Carboxypeptidase is a pancreatic enzyme that attacks peptide chains at their ends, thereby liberating the terminal amino acid with its free carboxyl group. In addition, a ribonuclease and deoxyribonuclease are present in pancreatic juice. These enzymes split ribonucleic acid and deoxyribonucleic acid, respectively. Pancreatic a-amylase hydrolyzes starches, glycogen and many other carbohydrates into disaccharides. However, this enzyme does not hydrolyze cellulose, an important polysaccharide found in plant material. Pancreatic lipase hydrolyzes neutral fats into glycerol and fatty acids.

Bile

Bile is secreted continuously by hepatic cells and excreted via a system of ducts into the bile duct and eventually passes into the duodenum.

Bile is a complex fluid containing a number of components. It contains no digestive enzymes, but is of importance in digestion because of the bile salts it contains. Bile salts perform the important task of emulsifying fats in the intestine, thereby increasing enormously the total surface area of these substances exposed to the action of pancreatic and intestinal lipases. Exclusion of bile from the intestine results in a loss of up to 25 percent of ingested fat in the feces.

1.5 The Large Intestine

The large intestine differs from the small intestine in several ways, including its greater width and the following characteristics:

1. There are no villi on the surface of the mucosa.
2. The glands are of greater depth, are more closely packed, and contain many goblet cells.
3. The longitudinal muscle layer of the cecum and colon is limited to three bands, visible on the surface, called teniae coli.
4. Many extensions of fat-filled peritoneum are apparent along the free border of the colon.

The cecum, or first portion of the large intestine, is an elongated pouch situated in the right lower portion of the abdomen. Attached to its base is a slender tube, the appendix.

The ascending colon extends upward from the cecum on the right posterior abdominal wall to the undersurface of the liver just anterior to the right kidney. The transverse colon overlies the coils of the small intestine and crosses the abdominal cavity from right to left below the stomach.

The descending colon begins near the spleen, passing downward on the left side of the abdomen to the iliac crest to become the pelvic colon. The descending colon is six inches in length and does not possess a mesentery. The pelvic, or sigmoid, colon is so called because of its S-shaped course within the pelvic cavity.

1.5.1 Large Intestine Secretions

Mucus

The large intestine is provided with enormous numbers of goblet cells both in the glands as well as on the mucosal surface. These cells secrete quantities of a viscous mucus having a pH around 8.0. This is the only major secretion of the large intestine. The mucus serves not only to protect and lubricate the intestinal wall, but to bind fecal material together. The mucus also serves to protect the colon from acids formed by the enormous amount of bacterial activity that takes place in the fecal matter itself.

1.5.2 Water and Electrolytes

Irritation of the intestinal mucosa (e.g., when a drug is taken such as a cathartic) results in secretion of large quantities of water and electrolytes in addition to mucus. This water and electrolyte secretion serves not only to dilute the irritant, but the colonic distension also stimulates rapid movement of the watery feces to the anus, causing diarrhea.

Water and electrolyte loss from a patient can result in dehydration of the body tissues and a severe electrolyte imbalance that can have rapidly fatal consequences, especially in infants.