The digestive system comprises the mouth, esophagus, pharynx, stomach, large intestine, small intestine and rectum and is also known as the gastrointestinal or GI tract. The digestive system also includes a number of glands and organs, such as the salivary glands, gallbladder, liver and pancreas, which discharge various substances into the GI tract. The gastrointestinal tract plays a vital role in processing the foods that are consumed by us and transforms them into a form that can be absorbed by the body and distributed by the circulatory system to the cells in different parts of the body.
Basically, the gastrointestinal tract is a tubular structure that passes right from the mouth and ends up in the anus. In the case of adults, the length of this tubular structure or the gastrointestinal tract is about 15 feet. The interior part of the gastrointestinal tract (GI) is known as the lumen and, technically speaking, its contents are external to the body. For instance, several million bacteria thrive inside the large intestine. Nearly all these bacteria are beneficial for our health. However, these bacteria can not only turn out to be detrimental, but also dangerous if exit the intestine and go into the body.
When we eat anything, the food combines with saliva, which makes the food wet and lubricates it to enable us to swallow the food without difficulty. An enzyme known as amylase is present in saliva and it helps in digesting the carbohydrates present in the food.
The pharynx and the esophagus form the channel through which ingested food passes from the mouth to reach the stomach. In fact, the movement of the pharynx and the esophagus regulates the process involved in swallowing foods. In the stomach, the food particles are mixed with several substances, such as hydrochloric acid, gastrin, pepsin and mucus. While gastrin works to promote the secretion of hydrochloric acid, pepsin processes proteins. Together, hydrochloric acid and pepsin work to break down the food particles producing a mixture that is called chyme. In addition, hydrochloric acid also eliminates majority of the harmful bacteria that enter the body via the gastrointestinal tract along with the food we consume. However, some of the bacteria that enter our body with the ingested foods manage to stay alive and subsequently they not only continue to thrive, but also reproduce inside the large intestine. The stomach also serves as a store house for food, while it is being digested partially. Subsequently, the stomach delivers the partly digested food as well as fluid into the small intestine in little amounts, thereby allowing its utmost digestion and absorption into the body.
The small intestine is the place where the final phase of digestion takes place and from where the body absorbs the nutrients contained in the foods we consume. In fact, the small intestine is the longest part of the gastrointestinal tract. The pancreas releases enzymes that disintegrate chime into simpler substances like monosaccharides, amino acids and fatty acids. Subsequently, these substances pass through the epithelial cell layer lining the walls of the small intestine and go into the blood and lymph, a water-like fluid present in the lymph vessels.
In addition to the nutriments, a little water, some minerals as well as undigested substances also enter the large intestine. These substances are stored in the large intestine provisionally allowing the intestinal bacteria to act on them. Subsequently, the water is removed and the solid materials are made into a concentrated substance, which is eventually excreted from the body by means of defecation following the distension of the rectum. The substance that is eliminated from the body in this form is called feces and it comprises a variety of things including some food particles that the body was unable to digest and absorb, unwanted or discarded cells, toxic substances and even bacteria. All these substances comprise the major volume of the feces.
There are several different ways by which toxic substances reach the stomach. For instance, they may enter the stomach along with the food or water consumed by us or inhaled through the nose or mouth. They may also be swallowed. Several factors also influence the amount of toxins that are absorbed in the stomach - the quantity of toxins that have been ingested, the extent to which they are water or lipid soluble, the extent of their ionization, their molecular size as well as the pH of the toxic substances. In addition, the absorption as well as metabolism of toxins is influenced by various other substances like bile acids and hydrochloric acid.
Apart from foods and various plant elements, the small as well as the large intestine is susceptible to various harmful substances like viruses, bacteria, yeasts and parasites. In addition, they are also exposed to the toxic substances present in water, foods we consume and the environment. Our body may also assimilate foreign chemicals from the intestines. In order to assimilate into our body, these chemicals ought to be made soluble prior to the intestinal mucosa comes in contact with them. It is worth mentioning here that the utmost concentrations of the chemicals that are not absorbed by the body are found in the colon, a part of the large intestine found between the cecum and the rectum.
Generally, foreign chemicals are gradually assimilated into the body from the gastrointestinal tract. However, the quantity of chemical that is absorbed by our body is actually subject to the pace at which it travels through the digestive tract. When faster the pace of a chemical passing through the gastrointestinal tract, the lesser amount of it is absorbed by the body. In addition, the extent to which these chemicals are absorbed by the intestines also depends on the intestinal mobility, the gastric emptying time, the dimension as well as the condition of the small intestine's surface, diet, blood circulation to the intestine, age as well as genetic factors.
The gastrointestinal tract is endowed with a range of protective mechanisms to deal with viruses, bacteria, parasites, yeas and toxic chemicals. Several substances, such as enzymes, intestinal secretions, beneficial intestinal bacteria, mucus as well as the deepest stratum of epithelial cells protect the digestive tract from such harmful elements.
It is interesting to note that the unstirred layer of water in the intestines is the first major obstacle towards absorbing foreign chemicals into the body. Actually, this motionless fluid layer serves as a coating for the intestinal mucosa or the mucous membrane of the intestinal wall. The unstirred water layer comprises a mucus layer as well as a micro acid layer with high proton - atomic particles having a positive charge, content. It serves as a barrier for foreign chemicals and prevents them from entering the mucosa. However, non-polar chemicals soluble in lipids do manage to pass through this immobile water layer, but very slowly compared to the pace with which they would go through a cell membrane. Some of the common non-polar chemicals include food additives, dyes, and pesticides.
The gastrointestinal mucus is the second barrier that thwarts the absorption of foreign chemicals into the body. Besides protecting the intestinal mucosa from physical damage and injuries caused by chemicals, the gastrointestinal mucus also serves as a lubricant. In fact, all the cells present in the esophagus, stomach as well as the small and the large intestines make mucus. About 95 per cent of the mucus is water, while the remaining portion composes of proteins, salts, mucins and nucleic acids. Mucins, on the other hand, are made up of carbohydrates, proteins and lipids and are responsible for the gluey, jelly-like texture of mucus. As mucus is sticky, it has the aptitude to lock in large molecules like those of metallic chemicals. In addition, it also possesses the ability to trap bacteria and parasites and can even truss viruses, which are eventually eliminated from the body as feces.
The acid microclimate layer of the small intestine is yet another barrier that puts off the absorption of harmful chemicals into the body. This layer is mainly made up of protons and its pH level is 5.9. Compared to the mildly alkaline lumen, which has a pH level of 7.3, the small intestine's acid microclimate layer is acidic. The acid microclimate layer is effective in preventing the entry of weak alkalis and acids into the small intestine as it keeps away acids and counteracts alkalis.
The strong presence of bacteria in the colon or the large intestine forms the fourth barrier. In fact, over 400 different bacteria species thrive inside the colon and they work to metabolize various chemicals as well as drugs. Nevertheless, the manner in which bacteria metabolize these chemicals is totally different from the way the body metabolizes various substances. Bacterial metabolism re-establishes the original form of a xenobiotic (a chemical or substance that is alien to an organism) and permits it to enter the circulatory system again, thereby allowing it to be included in the toxic load of the body again.
The gastrointestinal track has yet another defence mechanism against harmful microbes and chemicals. When the cells of the intestines begin to shed rapidly they prevent the absorption of such detrimental substances into the body. In fact, the intestinal cells divide very actively and are among the fastest in our body. It may seem incredible, but as many as 100 new intestinal cells are formed every hour, while several billion of them are discarded daily. The intestines may excrete the metals as well as chemicals soluble in lipids with the old cells regularly.
The gastrointestinal tract is the second most important detoxification site in the body where both detoxification systems - Phase I and Phase II, occur here. The harmful chemicals in the body are changed during the Phase I detoxification enabling the addition of a small molecule during the Phase II system. The biotransformation enzymes found in the gastrointestinal tract are same as those present in the liver. However, compared to the liver, the pace of metabolism in the gastrointestinal tract is sluggish. The gastrointestinal tract does not always transform toxic chemicals into harmless substances. On the contrary, it possesses the aptitude to convert a xenobiotic into a weak toxin or a potent toxic chemical, subject to the chemical's composition.
Hydrochloric acid and pepsin are secreted in the stomach and they can facilitate the breaking down of toxins and other chemical substances. Similarly, the bile acids and a variety of enzymes, including lipases, proteases and glucuronidases, are present in the intestines. They also possess the aptitude to break down toxins and chemicals. On the other hand, the amine oxidase system has a mixed function and it is highly vigorous in the duodenum, which is considered to be the opening to the small intestine.
The amount of cytochrome P-450 activity is the utmost in the enterocytes, the cells that form the intestines' lining, when they become mature. However, the action of the enzymes that are active during the Phase-I detoxification system diminishes in the colon compared to that in the duodenum.
It is important to note that the competency of the intestines in metabolizing xenobiotics is influenced by several factors. For instance, the action of several metabolic enzymes diminishes when people take diets that lack nourishments or those that are partially malnourished. It also decreases when people are fasting or taking semi-fasting diets. The action of cytochrome P-450 can diminish greatly owing to deficiency of essential mineral iron or the vital trace element selenium in one's diet. On the contrary, consumption of plants belonging to the Cruciferous family, such as broccoli, cauliflower and cabbage, helps to augment the action of amine oxidase system, which has a mixed function.
Similarly, chemicals can also diminish or even augment the action of the enzymes, subject to the nature of the chemical. It has been found that the activity of the enzymes in a particular part of the small intestine known as jejunum is lower in women compared to that in men. In addition, it has been found that the action of the detoxification enzymes is also low in aged people as well as those who are very young.
In addition to the organs mentioned above, even the stomach or the gut have the ability to work as a detoxifying agent and excrete toxic substances. The cells that form the intestinal wall lining also possess the aptitude to discharge xenobiotics into the small and large intestines. The bloodstream also secretes many potent acids as well as digitalis chemicals into the lumen of the small intestine and subsequently they are eliminated from the body in the form of feces.