How Your Digestion Works

Understanding the Gastrointestinal Tract

(Information reproduced from "Understanding Your Irritable Bowel" published by the IBS Research Appeal)

In my experience one of the most helpful things we can offer IBS patients is a proper understanding of why their symptoms occur. For example, patients will often be reassured if it is explained to them that episodes of crampy pains in their lower abdomen are likely to be occurring because the muscle surrounding the colon, which is responsible normally for achieving movement, is going into spasm. There are at least 13 symptoms arising from the intestines alone which can occur in IBS and a wide range of additional symptoms arising from sites outside the gastrointestinal tract. Because gastrointestinal symptoms feature so prominently in IBS, we present here a brief reminder of what the digestive system is and how it works.

               irritable bowel syndrome - gastrointestinal tract  

The Alimentary Canal

The digestive process takes place in tubes and sacs that extend from the mouth to the anus, this system being known as the alimentary canal. Its organs are the mouth, the pharynx, the oesophagus, the stomach, small and large intestines, the rectum and the anal canal. The stomach and the intestines are referred to as the gastrointestinal tract. Other organs that support the process are the teeth, tongue, salivary glands, pancreas, liver and gall bladder. In short, the job of the digestive system is to ingest solids and liquids, to masticate (chew) them with the teeth, to alter some foods chemically, to absorb useful products and to excrete the useless residue.

As we know, the process begins in the mouth where the teeth tear and crush the food. The tongue tastes it and the salivary glands lubricate the food and begin to digest some of the starches. The act of swallowing pushes the food through the throat and passes it to the oesophagus where the action of the oesophageal muscle propels it to the stomach. The stomach is a digestive bag that holds the food during the initial digestive action. Muscles in the wall of the stomach churn the food as gastric juices, secreted by glands in the gastric wall of the stomach, act on the food and digest it. These glands secrete about a quart (1 litre) of gastric juices every 24 hours. Certain cells secrete a mucus that forms a layer in the stomach, preventing the enzyme pepsin from digesting the stomach itself since some of the stomach's secretions are extremely powerful - hydrochloric acid being one of them. The gastric juices begin to digest or break down the food and liquid that we eat and drink.

The average meal remains in the stomach for about 1 - 3 hours while this process takes place. This food, by now partially digested and at this stage known as chyme, is then discharged into the upper intestine. Here the food mixes with bile and the digestive enzymes produced in the pancreas. The small intestine, which is about 4 - 6 metres long, is the power house of food handling. We eat about 80 grammes of protein a day, 80 grammes of fat a day and 250 grammes of carbohydrate a day. These are all complex food stuffs which are broken down to their basic constituents before absorption can take place. The digestive juices produced by the pancreas and important chemical enzymes in the lining of the intestine are responsible for the complex break-down process. Thus the fat we eat is absorbed in the form of the simple compounds fatty acids and glycerol. All the different proteins we eat are absorbed in the form of single molecules called amino acids and small peptides composed of two or three of these. The complex sugars that we eat are all absorbed in the form of single molecules called monosaccharides. The processes of digestion and absorption are enormously efficient. Most of the food we eat is absorbed in the first 80 cm or so of the small intestine.

Fluid

On average we drink about 1.5 litres, or 2.5 pints, of fluid a day. This is not the end, however, as far as the gastrointestinal tract is concerned, as the digestive juices add another 7.5 litres to the volume of fluid the gut has to deal with every day. Normally we pass around 200 ml or 200 grammes of stool a day, so it follows that the gastrointestinal tract has to absorb a lot of fluid every day.

Figure 2.2

Approximate daily volumes of fluid (ml)
handled by the human gastrointestinal tract.

   

Fluid volume (ml)
Input Diet
1500
  Digestive juices
7500
  Total
9000
Absorption Small intestine
7500
  Colon
1300
Output Stool
200












As shown in figure 2.2, most of this fluid is absorbed in the small intestine. This is achieved in conjunction with the absorption of digested food. Some 1.5 - 2 litres of fluid is absorbed during the passage of contents through the colon or large intestine

Fibre

Part of the food we eat is derived from plants, ie. cereals, pulses, vegetables and fruit. The cells wall of all plants contains a carbohydrate different in structure from the starch in our diet, called non-starch polysaccharide. This is generally known as fibre. Unlike the starch in our diet, fibre is not broken down and absorbed in the small intestine. Thus, following the digestion and absorption of food, fibre, together with the digestive juices and a few other chemicals, travels down the small intestine in liquid form, and a volume of 1.5 - 2 litres a day enters the colon. Here the fluid and electrolytes are absorbed, the stool is manufactured and proportions of the fibre are broken down to the chemicals (short chain fatty acids) which are responsible for maintaining the integrity of the lining of the colon. Some 150 - 200 grammes of solidified contents pass out of the colon everyday in the form of stools.

Motility

It follows from this discussion of the events that take place to achieve the digestion and absorption of food that a very important function of the gastrointestinal tract is a propulsive one - that is, the moving of ingested food and digestive juices from the stomach to the colon and anus. It is the contractile or motile function of the gut that is responsible for inducing this movement. In IBS it is the abnormalities of this motility (contractility) of the intestines that lead to many of the symptoms from which patients suffer. Gastrointestinal physiologists have over the years developed many different techniques for assessing the rates of movement of contents through the gastrointestinal tract and have usually expressed their results in terms of transit time. There are broadly three components to gastrointestinal transit - emptying of the stomach (gastric emptying), transit through the small intestine and transit through the colon.

            irritable bowel syndrome - transit through the human gastrointestinal tract

            

Gastric Emptying

Many different factors, including importantly the nutrient content of the meal, influence emptying times, but in general the greater part of a solid meal has emptied from the stomach within three hours. Small intestinal tract Bearing in mind that the small intestine is 4 - 6 metres long, its motility or contractility is a very efficient process, as all its contents are moved right the way through in about 4 hours. In the immediate response to eating, the characteristics of motility (contractility) are geared up to preventing movement of contents from the upper regions, so as to allow digestion and absorption to take place here. The rate of movement of the residual contents after these events is then very rapid. Movement is achieved by the powerful contractile forces that move down the intestine in an orderly way, called the migrating motor complex, or MMC.

Colonic Transit

Remember that 1 - 2 litres of fluid enter the colon each day. Unlike the emptying of the stomach and the movement through the small intestine, transit through the colon is slow and in normal subjects it takes up to 48 hours for the contents to move through the ascending, transverse and descending segments of the colon to the bottom portion or rectum. Again, it is the disturbances of motility or contractility of the colon that are responsible for abnormal propulsion, giving rise to symptoms in IBS sufferers. It follows that in IBS, movement of contents is abnormally slow in those with constipation and abnormally rapid in those with diarrhoea. Absorption and evacuation After the various foods have been digested to their soluble and easily usable products, they are absorbed through the intestinal lining cells, mostly in the small intestine. The amino acids, glucose, vitamins and other substances diffuse into the blood system to drain to the liver before passing into general circulation. The products of fat digestion (glycerol and fatty acids) are absorbed and reformed into neutral fats and carried away from the intestine by the lymphatic system. Alcohol and some drugs are absorbed in the stomach. Water is absorbed in the small and large intestines. At the junction of the stomach and the small intestine, and at the junction of the small and large intestine, are circular muscles called sphincters. When they relax, food passes from one structure to another. After passing through the sphincter between the small and large intestines, the intestinal contents pass successively through the ascending colon, then into the transverse colon and then the descending colon, the sigmoid colon, the rectum and the anus. Water continues to be absorbed as contents pass along the large intestine and solidified material is formed. This solidified material is stored as faeces in the lower end of the colon, or rectum, and finally passed through the anal canal by means of a coordination of muscles in this area of the body, a process that is called defaecation.

In the revised edition of the acclaimed book, 'Understanding Your Irritable Bowel', Dr David Silk gives a detailed and easy to follow explanation of this complex condition.

Our online catalogue shows extracts from the book.  Click here to visit the catalogue and read about the book, or access the catalogue from our 'Publications' page linked in the sidebar at the left.

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