Digestive system

It is extremely rare that a day goes by without our eating. An extensive system of organs is responsible for converting the raw materials we ingest into substances that are suitable for our body. A large number of different parts of the body, from the teeth and cheeks to the liver, the pancreas and the intestines, co-operate to break down all the food we consume into its smallest components.

In the course of its journey through the body, food is ground and mixed with various digestive juices by powerful muscular contractions.

In general we assume that man is an omnivore – a creature who eats everything. Our body seems to be built for digesting both meat and vegetables. Even the diets of our ancestors included both types of foods, although one can question whether the composition of the modern diet is precisely what nature intended. Humans have succeeded in making eating a social activity. Over the centuries an enormous cult has grown up around the way in which we prepare food. Opinions on the manner in which food should be eaten differ from time to time and from country to country. Even so, the process of ridding ourselves of the superfluous remnants in the form of stools is as essential to the digestive process as eating and drinking. This is the only part performed in a place where no one can see us.

The digestive system is the means by which we convert our food into the materials necessary for the continuation of life. Eating, an essential pleasure, is the start of the food’s long journey along the digestive tract. This is an intricate anatomical structure containing a complex chemical processing plant capable of reducing the most elaborate of meals to the basic chemicals that we must absorb to build, repair, fuel and maintain our bodies.

Food transport through the body

The process of digestion begins as soon as” we start to eat. We bite off chunks of food and grind them into smaller pieces by chewing. After swallowing, the food enters the stomach where it is churned about by muscular contractions and thoroughly mixed with the gastric juices. This process reduces it to a soft sludge known as chyme, which enables it to pass through the pylorus, the exit valve of the stomach, into the duodenum, the beginning of the small intestine. Once in the small and, later, in the large intestine, this mixture is propelled along by a process called peristalsis. The reflex contraction of the muscle immediately behind the food causes the muscle in front to relax. This causes the food to be pushed down in the intestine and starts the process in the next section. Eventually the non-absorbable part of the food is expelled in the act of defecation.

Nutrition and digestion

The type and quantity of food we eat is of vital importance to our health and nutrition-based disorders can have serious consequences not only for the delicate organs of the digestive system but also for the rest of our bodies. The three principal constituents of food are protein, fat and carbohydrate, which are all needed in varying degrees in a healthy diet. A diet that contains too much fat, for example, produces higher levels of cholesterol in the blood which increases the risk of a heart disease. Similarly problems may arise from diets that lack vital elements. Scurvy, the often fatal disease that once was the blight of sailors, was eventually found to be caused by a lack of vitamin C in the diet, which is found in raw fruit. One of the most important breakthroughs in recent years is the realization of the importance of dietry fibre, which consists of the indigestible elements of plant cells, especially the cellulose-containing plant cell walls and is present in, among other things, brown rice, wholemeal bread and peas and beans of all sorts. The colon needs fibre for normal function and its absence has been blamed for many of our commonest disorders including constipation, cancer of the bowel and irritable bowel syndrome.

Digestion and secretions

The chemical breakdown and absorption of nutrients depends upon the secretion of different gastric juices.

Many litres of these various fluids are secreted every day and once again, the process begins in the mouth and finishes only at the anus. Secretions perform many separate tasks. First, certain secretions act as lubricants to prevent food from either sticking in the bowel or causing damage to the delicate mucous membranes that line the alimentary canal from mouth to anus. Second, the secretions form the digestive juices which not only chemically break down the constituents of the diet, but also help with their absorption. Third, the mucous membranes are protected against the ravages of their own digestive juices by providing a mucus barrier. Where this protection fails, the damage inflicted can lead to the formation of ulcers.

The digestive juices are comprised of two main sets of components. The first is the group that controls the acidity or alkalinity of the bowel and the second is the enzyme component responsible for food breakdown and digestion. The stomach secretes hydrochloric acid, so the first part of the digestive process is carried out in a very acidic environment. The pancreas secretes sodium bicarbonate which neutralizes the acidic stomach contents passing into the intestine, to provide the best possible mixture suitable for the action of the enzymes in the small intestine. The digestive enzymes are essentially catalysts which, by vastly accelerating chemical reactions, break down the large organic molecules that comprise our food. Each is specific in its actions and only small amounts are needed. Some enzymes, untypically, work only in an acid environment: pepsin, for instance, functions only in the stomach. Most enzymes, however, work best in the slightly alkaline conditions in the small intestine.

The enzymes in the small intestine come from two sources: glands in the wall of the small intestine, and the pancreas, which produces the powerful protein-digesting enzymes trypsin and chymotrypsin, the fat-digesting enzyme lipase and the starch-digesting amylase.

Yet another secretion is bile from the liver. This contains cholesterol derivatives, the bile acids, which help in fat absorption, and the bile pigments such as bilirubin, which are produced as a waste product of the breakdown of red blood cells that have reached the end of their useful life. These pigments give faeces the characteristic brown colour.

Control of digestive processes

Movements of food along the alimentary canal and the production of secretions are controlled at three distinct phases. The first occurs as a response to the sight, smell and taste of food and is called the cephalic phase. The second phase occurs when food is in the stomach and is called the gastric phase. The third occurs when the acidic chyme enters the small intestine, to pass on to the large intestine, and is known as the intestinal phase. At any of these three phases, the precise means whereby movement or secretion is controlled depends on the nervous system and/or the hormonal (endocrine) system. Bowel motility, for example, alters with food intake and with the phase of the digestive process. Bowel motility, together with the three phases mentioned above, are regulated by the dual control system of the autonomic nervous system (ANS) and the gastrointestinal hormonal secretions.

A nerve called the vagus nerve, originating in the brain, is responsible for varying the muscle tone of the wall of the stomach and the pylorus, which is the valve between the stomach and duodenum. It also stimulates the secretion of acid, mucus and pepsin (the digestive enzyme) from glands in the stomach wall. The vagus is stimulated by the sight and smell of food in the cephalic phase, and by presence of food in both the stomach, the gastric phase, and in the small intestine, the intestinal phase. The presence of food in the stomach and bowel accelerates peristalsis by way of the vagal nerve, a process known as the vagal reflex. Peristalsis is also affected by the bowel’s inherent nerve network. There is a degree of peristaltic control innervated by nerves of the autonomic nervous system that emanate from the spinal cord. In addition to controlling bladder and sexual functions, they also control the anal sphincter and act of defecation.

The other regulatory system, which is hormonally controlled, brings about many of the changes in both gastrointestinal motility and secretion. Hormones, which are chemical messengers secreted by cells within specialized gland tissues, are manufactured in places such as the pancreas and the bowel wall. Gastrin is a hormone secreted by cells within the stomach wall. Its main effect is to increase the output of hydrochloric acid in the stomach, although it also stimulates the secretion of pepsin (one of the gastric enzymes) and brings about an increase in both motility and secretion of other chemicals in the intestines. Another hormone, secretin, is released by the duodenum when acid and partly-digested chyme enter from the stomach. This stimulates the pancreas to release sodium bicarbonate and acts on the biliary tract to produce bile. Pancreozymin is also released from the duodenum by the effect of fat, partly-digested protein, and to a lesser extent acid on the mucous lining of the duodenum. Pancreozymin stimulates the external secretory activity of the pancreas, but it affects the release of digestive enzymes rather than bicarbonate. It also acts on the gall bladder causing it to contract and expel bile.

Many other important hormones are secreted from the pancreas and the walls of the bowels, to co-ordinate secretion and motility of other secretions of the bowel, achieving the optimum conditions for digestion with a fine degree of control.