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How The Gut Works And Irritable Bowel Syndrome

How The Gut Works And Irritable Bowel Syndrome

How The Gut Works And Irritable Bowel Syndrome


The gastrointestinal tract is made up of the mouth, oesophagus, stomach, small bowel and large bowel. It digests and absorbs the food and holds what’s left until we are ready to expel it when we defecate. To understand the ‘how and why’ of medical treatment and investigation for irritable bowel syndrome (IBS), it’s important to have an idea of the basic structure and function of the gastrointestinal tract.


Is chewing important?

Chewing reduces the size of the pieces of food so that they can be swallowed. Eating quickly will probably have little effect on the digestion of most foods as long as you chew well enough, but it will result in more air being swallowed. Even when we just swallow a little saliva, a few millilitres of air will go down too. If we gulp down our food, we will inevitably swallow more air.

We now believe that the bloating experienced in irritable bowel syndrome is mostly due to excess air in the small bowel. Most of this air is swallowed. Hence if you suffer from bloating, eating slowly and chewing well may be important.

How much saliva do we make in a day?

About 1.5 litres (3 pints) of saliva are produced each day. The secretion of saliva is stimulated by food – so if no food is eaten, only 500 ml will be produced. Sleep, fatigue, fear and dehydration all reduce the secretion of saliva.

What does saliva do?

Saliva keeps the lining of the mouth moist and lubricates the food. It is alkaline to protect the teeth from acidic food and also contains an antibacterial enzyme. In addition, the digestion of carbohydrates begins in the mouth with the salivary enzyme amylase.



The oesophagus is a tube about 25 centimetres (10 inches) long that carries food from the mouth to the stomach. (Figure 1.1)




What can go wrong with the oesophagus?

Reflux of acid from the stomach up into the oesophagus seems to be getting more common, producing indigestion symptoms such as heartburn, chest pain, pain in the upper abdomen and difficulty swallowing. In severe cases of reflux, there can be ulceration and Gall bladder Transverse colon Ascending colon Small bowel: jejunum and Terminal ileum ileum Caecum Sigmoid colon Appendix Rectum bleeding. Cancer of the oesophagus is often first noticed because a person has difficulty swallowing (dysphagia). It is associated with smoking, alcohol consumption and acid reflux. Difficulty swallowing is also sometimes due to poor co-ordination of the muscles in the oesophagus.



The chest and the abdomen are separated by a dome-shaped sheet of muscle called the diaphragm. This is the chief muscle used in breathing in (inspiration). When it contracts, the contents of the abdomen are compressed down while the chest cavity is enlarged, drawing air into the lungs. Clearly, when the stomach is full, it becomes more difficult for the diaphragm to work, which is why it’s easier to exercise on an empty stomach.

The diaphragm has an opening in its centre through which the oesophagus empties into the stomach. The stomach is a J-shaped bag with an average capacity of 1 litre (2 pints; Figure 1.1). It is, however, capable of a lot of expansion (called dilatation), to hold food for the first processes of digestion and then to release it in a controlled fashion to the small bowel. Interestingly, the stomach can distinguish between fluids and solids, so that fluids taken on an empty stomach may pass straight through into the duodenum.

Functionally, the stomach works in two halves. The upper part expands to hold the food coming down from the oesophagus. It therefore acts as a reservoir of food for the lower part of the stomach. The lower part is controlled by a pacemaker that sends electrical signals to the muscle, causing it to contract rhythmically, which churns the food up to mix and digest it.

How long does food stay in the stomach?

About 1–3 hours. Fluids spend less time there, but fatty foods delay the emptying of the stomach so that whatever was eaten with them will get held back for longer as well.

How much digestive juice does the stomach produce?

The stomach produces 2–3 litres (4–6 pints) of gastric juice each day. This contains hydrochloric acid, which can make the con¬tents of the stomach very acid. You may hear healthcare professionals talking about the ‘pH’ of your stomach. A neutral pH is 7, with numbers bigger than this meaning alkaline, and smaller numbers meaning acid – the pH of your stomach can be as low as 1!

The function of the acid is to help digest protein and to protect against infection by killing any bacteria ingested with the food. Surprisingly, reducing acid production with medicines, surgery or disease seems to make remarkably little difference to a person’s health. Gastric juice also contains enzymes; the main function of these is to digest protein.

What can go wrong with the stomach?

Ulcers in the stomach usually produce pain in the upper abdomen that is related to food. They are mostly caused by an infection acquired in childhood with a bacterium called Helicobacter pylori (H. pylori) or from taking non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, naproxen and many others. Ulcers associated with NSAIDs are painless in at least half the people who have them.

Cancer of the stomach is rare under the age of 50 years. It pres¬ents with upper abdominal pain, vomiting, weight loss or anaemia (a low level of iron in the blood). It is caused by H. pyloriinfection and environmental factors. Stomach cancer is very common in Japan where the diet is high in salt- and nitrate-containing foods such as dried fish, pickles and processed meats. Interestingly, Japanese people who adopt a Western diet reduce their risk of stomach cancer.

Problems with the movements of the stomach (dysmotility) are caused by poor co-ordination of the stomach muscles. This is one of the most common causes of indigestion. Typical symptoms include feeling full early on in a meal, a feeling of fullness and bloating in the upper abdomen, reflux, and upper abdominal pain that responds poorly to acid-reducing medicines. Dysmotility is fairly common in patients with irritable bowel syndrome.

What is a hiatus hernia? My friend suggested it could explain my problems.

The chest and the abdomen are separated by the diaphragm. It has an opening in its centre through which the oesophagus passes to empty into the stomach. In a hiatus hernia, part of the upper part of the stomach is shifted up through this opening and into the chest. A hiatus hernia is not visible from the outside. Its significance is that it allows more acid to reflux up into the oesophagus. The usual symptom is ‘heartburn’, although acid in the oesophagus can also cause pain in the upper abdomen. The medical treatment is medication to reduce acid production by the stomach. Self-treatment involves weight loss, stopping smoking, and drinking less alcohol and coffee.



The duodenum is the first 25 centimetres (10 inches) of the small bowel (Figure 1.1) and has a diameter of 4–5 centimetres (2 inches). It curves in a C shape around the head of the pancreas. The pancreatic duct and the bile duct (from the gall bladder) join to empty into the duodenum. The pancreas and gall bladder produce substances that help to break down the food (sugars, proteins and fats, respectively) so that it can be digested.

What can go wrong with the duodenum?

The acidic contents of the stomach empty into the duodenum, so it may become inflamed and ulcerated. Duodenal ulcers cause pain in the upper abdomen, and sometimes back pain that is related to food, or even occasionally relieved by food. As with stomach ulcers, infection with the bacterium H. pyloriand taking NSAIDs are the predominant causes. Cancer of the duodenum is very rare.



The small intestine, or small bowel, is made up of the jejunum (the upper half) and the ileum (the lower half; Figure 1.1). There is no sharp distinction between them, and it’s just convention to call the parts this. The length of the small bowel varies from 3 to 10 metres (10–33 feet), the average length being 6 metres (24 feet). It tapers from about 3 cen¬timetres down to about 2 centimetres (1 inch) in width.

The time taken for food to pass through the small bowel varies from about 2 to 6 hours, with an average of about 3.5 hours. Air can be propelled though the small bowel much more quickly, and swallowed air may reach the anus within half an hour.

The small intestine is one of the most important organs in the body. Most of the digestion and absorption of the food occurs here. Life without a small intestine is difficult and needs food solutions to be infused directly into the bloodstream. This is called total parenteral nutrition; in the long term, it is invariably complicated by serious infections. Transplantation of the small bowel is only in its early days and can’t yet be used to solve this problem.

What can go wrong with the small intestine?

The small intestine needs to absorb about 9 litres (18 pints) of fluid every day.This is made up of about 1.5 litres that has been taken in with the food and about 7.5 litres secreted as digestive juices. If the small intestine doesn’t do this, the consequence is severe diarrhoea with rapid dehydration. Small bowel dysfunction occurs with infection, coeliac disease and Crohn’s disease).

It was once thought that the small bowel was not involved in irritable bowel syndrome. However, recent evidence convincingly shows that abdominal bloating is predominantly due to air that has been retained in the small bowel.

The small bowel is almost sterile, normally containing few if any bacteria. In some people, bacteria from the large bowel manage to pass back up into the small bowel. These usually get flushed out, but if they manage to take hold, we get a condition called ‘small bowel bacterial overgrowth’. These bacteria then interfere with the diges¬tion and absorption of the food. The symptoms that result can be similar to those seen in irritable bowel syndrome. In fact, small bowel bacterial overgrowth has been suggested as a cause of irritable bowel syndrome. Cancer of the small bowel is very rare.



The large intestine varies in length, but is usually about 1–1.5 metres (5 feet) long and about 5 centimetres (2 inches) wide. There is a valve between the small bowel and the large bowel called the ileocaecal valve, which to some extent prevents backflow.The different parts of the large bowel are given their own names (caecum, ascending colon, transverse colon, descending colon, sigmoid colon; see Figure 1.1), but there are really no distinct divisions between them.

The large bowel acts to absorb about 1 litre (2 pints) from the mixture that pours in from the small bowel. This is because a more solid formed stool is easier to hold until we are ready to defecate. It normally takes 24–48 hours for food residue to pass through the large bowel, but gas is conducted much faster and can reach the anus within half an hour.

The large bowel is full of bacteria; indeed, half of the stool that we pass is made up of bacterial cell bodies. It is therefore not surprising that antibiotics disturb bowel function, usually leading to diarrhoea.

What can go wrong with the large bowel?

Compared with our other organs, the large bowel is relatively unimportant. You can easily live without part or all of it, and many people indeed do just that because large bowel problems are common.

Infective diarrhoea caused by viruses or bacteria is common in the large bowel. When the rectum becomes inflamed, from whatever cause, we feel an urgent call to stool whenever anything enters it – liquid, solid or gas. But because the large bowel is required to absorb only about a litre of fluid a day, fluid loss is minimal compared with small bowel dysfunction and rarely causes severe dehydration. Large bowel dysfunction is therefore usually associated with frequent, small-volume, loose stools.

Cancer of the large bowel affects about 1 in 25 people, presenting as a change in bowel habit to loose stools, bleeding or anaemia. It is rare under the age of 50 and is often cured by surgical resection.

Diverticular disease is increasingly common as we get older, affecting up to 60% of 60-year-olds. In many people, diverticular disease causes no problems at all. In others, it is associated with left-sided spasmodic pains and a change in bowel habit that cannot be distinguished from that of irritable bowel syndrome. In a few people, diverticular disease produces more serious complications such as bleeding and infections.

Inflammatory bowel disease is a consequence of an overactive immune system in the bowel. It causes inflammation not related to infection, which damages the bowel. This shows up as diarrhoea, usually with blood. There are several ‘inflammatory bowel diseases’, as they are called, but ulcerative colitis and Crohn’s disease are the most common. In their milder forms, they can be mistaken for irritable bowel syndrome, but irritable bowel syndrome does not develop into Crohn’s disease or ulcerative colitis. Conversely, however, many people with inflammatory bowel disease will go on to also develop irritable bowel syndrome.

Although bloating is now recognised to be mainly a consequence of problems in the small bowel, most of the other symptoms of irritable bowel syndrome are thought to arise largely from the large bowel.



The lowermost part of the large bowel is designed to keep a person continent, that is, stop the faeces leaking out when we don’t want them to. The rectum (see Figure 1.1) is 12–15 centimetres (about 5–6 inches) long. It is named from the Latin word regeremeaning ‘to straighten, correct or rule’, but although the rectum is straight in some mammals, in humans it is actually curved along the sacrum and attaches to the anal canal at an angle of 80–90 degrees.

Contrary to popular belief, the rectum is not a storage area for the stool but is usually empty. It is more sensitive to distension than other parts of the large bowel and can detect a volume as low as 80–120 ml (a small cupful). At larger volumes, for example 200–300 ml, the sensation of pressure in the rectum is greater, leading to an increasingly strong urge to defecate. In most people, the urge to defecate is usually irresistible at a rectal volume of over 400 ml. These volumes vary between individuals, however, and fall with age.In people with irritable bowel syndrome, the rectum is more sensitive to distension. This partly explains why they have to rush to the toilet more frequently just to pass small amounts of stool or fluid.

My doctor mentioned the pubo-rectalis muscle. What is this?

The pubo-rectalis is one of the muscles in the pelvis. It runs from the pubic bones at the front, towards the back, around the lower end of the rectum, and back to the pubic bone (Figure 1.2). So it acts as a sling, pulling the rectum forward to create an 80–90 degree angle between the rectum and the anal canal. When we defecate, the pubo-rectalis muscle relaxes, allowing this angle to straighten out so that the stool can pass out more easily.

Pubo Rectalis Muscle

Pubo Rectalis Muscle


I’ve always been a bit embarrassed to ask, but what happens when we defecate?

The anal canal is normally held tightly shut by the muscle bands that encircle it. These are called the anal sphincters. When the rectum is distended by stool or gas passing down from the colon, the internal sphincter (the upper one) relaxes slightly, allowing some of the rectal contents to pass into the anal canal.

The lining of the anal canal is very sensitive and can detect the difference between solid, liquid and gas. If it detects gas, the anal sphincters usually relax even more, allowing the gas to escape as flatulence. Liquid in the anal canal usually leads to a desperate desire to defecate, which has to be countered by a conscious effort to tighten the anal sphincter. When the anal canal detects solid matter, it gives us a choice: settle down to defecate, or contract the anal sphincters strongly, pushing the faeces back into the rectum until it is more convenient for us to go to the toilet.

To defecate, we sit or squat in order to straighten the angle between the anal canal and the rectum. An earlier answer described how this angle is partly maintained by a band of muscle called the pubo-rectalis, which relaxes so that the angle can be straightened out. The rectum now lines up with the anal canal. The muscles in the abdominal wall then contract, which increases the pressure within the abdomen and forces the stool to pass out of the large bowel.

Does it do any harm to put off going to the toilet?

Most of us resist the urge to defecate, the ‘call to stool’, every so often for social convenience. This is perfectly normal and will do no harm. But doing this regularly will lessen the message to defecate. The rectum can then become more tolerant of large volumes, which is a common cause of constipation.

Sometimes I need to rush to the toilet to pass a stool soon after eating or even during the meal. Is the food passing right through me that quickly?

You are actually describing a prominent gastrocolic reflex. A reflex is an unconscious automatic response in which stimulation of one part of the body results in activity in another. For example, tapping the tendon under the knee leads to reflex contraction of the thigh muscle. In the gut, filling and distension of the stomach leads to nervous impulses that stimulate activity in the bowel. This is a normal reflex. It is stronger in some people than others, and it is often more prominent in irritable bowel syndrome and inflammatory bowel disease.

The simplest way of controlling it is to eat smaller meals, or to eat more slowly so as to reduce the distension of the stomach. Eating less fat may help too. Fatty foods tend to slow emptying of the stomach, so a fatty meal will tend to distend the stomach more. You may also wish to avoid taking caffeine with your meal as it will increase the stimulation of your bowel.



The gastrointestinal tract is basically a long tube from the mouth to the anus.

Food normally spends about 1–3 hours in the stomach, 2–6hours in the small bowel and 24–48 hours in the large bowel.

Gas, such as swallowed air, will pass through much more quickly and may reach the anus within half an hour.

The small bowel is the most important organ of digestion but, compared with other organs, it is rarely involved in disease.

The small bowel is virtually sterile; any bacteria that enter it are flushed out into the large bowel.

The large bowel is full of bacteria.

The large bowel collects undigested food, solidifies it by absorbing excess water and holds it until we defecate.

Although the large bowel is, compared with the small bowel, relatively unimportant, it is a common cause of disease.

The most common large bowel problem is irritable bowel syndrome. The most serious problem is large bowel cancer.