It can be helpful to think of the digestive system as a tube running through the body with an opening at each end, the mouth at the top and the anus at the bottom.

The tube isn’t a neat, constant shape throughout its course. It swells into a muscular storage organ at the stomach where the food you swallow is thoroughly mixed with digestive enzymes that start breaking it up into a form that the body can use.

At the exit from the stomach more essential digestive juices are added from the gallbladder and pancreas. Food particles get smaller and smaller until, in the exquisitely adapted small intestine (small only in diameter, not length) the tiniest fragments of nutrients can pass easily though the intestine wall straight into the bloodstream.


Digestive System

The basic materials we need to function are carbohydrates, fats, proteins, minerals and vitamins. Carbohydrates (starchy foods) are large molecules that can be turned into sugar (glucose) to provide the main energy source of the body. Glucose can also be transformed into fat and protein.

Fat is a very high energy source used throughout the body. Proteins are found in, for example, meat, pulses and grains. They are complex molecules formed from amino acids. Amino acids are like building bricks that, once digested, become available to make all the other body structures, such as muscles, nerves, organs, blood and skin.

Minerals include calcium, potassium, sodium, iron and many more. Each mineral has an important role to play in the body’s metabolism, so there are specialized digestive processes that harvest them from food.

Vitamins are biochemicals of a particular type which the body cannot, on the whole, make for itself, but which are essential to cell metabolism. Even though tiny amounts are required, they are vital for making blood, bones, skin, the nervous system, energy production and much more.

The Digestive Process

Enzymes break the raw food molecules into smaller fragments, which are absorbed and transported all around the body. This begins in the mouth, where saliva starts the breakdown of starch. Digestion begins in earnest in the stomach, where food plunges into a warm highly acidic bath of hydrochloric acid mixed with enzymes.

The digestive tract can be thought of as a single, long, irregular tube and consists of the mouth and throat (pharynx), oesophagus, stomach, small and large intestines (bowels) and the anus Other organs that arc not part of the digestive tract but are important to digestion are the salivary glands, the liver, the pancreas and the gallbladder.

Food enters the digestive tract at the mouth, where it is mashed by the teeth and mixed with saliva. It passes down the oesophagus to the stomach, where it is mixed with digestive enzymes and broken down further into a semiliquid mixture. This mixture passes into the small intestine, where it is broken down into molecules that are absorbed into the blood via the liver. Indigestible material then passes into the large intestine, where some water is reabsorbed before it is excreted as faeces.

The liver produces the digestive juice bile, which is stored in the gallbladder until it is released into the small intestine.

The pancreas also secretes a digestive juice and produces the hormone insulin.
As food passes through the digestive tract it is broken down into nutrients, which are absorbed by the small intestine, and water, which is absorbed by the large intestine. Nutrients pass to the liver in the bloodstream. Faeces form in the large Intestine and collect in the rectum before excretion.

The chemical resources of the digestive system break food down within hours into usable components.The liquid mass now passes into the small intestine, the site of the bulk of both digestion and absorption. Yet more

enzymes from the pancreas attack the food, breaking down protein. Bile from the liver dissolves fat molecules.

The lining of the small intestine is composed of billions of frond-like outgrowths, which vastly increase the surface area. The outer layer of each frond contains specialized cells that actively transport molecules from the food slurry into the blood stream. Within the blood stream yet more specialized proteins pick up the newly digested food and cany it mainly to the liver for further processing.

The digestive process is by and large complete by the time that material reaches the large intestine – called the colon. Huge quantities of bacteria live in the large intestine and complete the digestion of tough carbohydrate fibres. Water and minerals are also reabsorbed there.


The process of digestion is under an array of biochemical and nerve controls, which are far from fully understood. These cause the right enzymes to appear at the right times and regulate acid and bile production. They influence the complicated muscle layers in the walls of the bowels, which sweep food through the system and which expel it at the end.

Digestive Disorders

Many bowel problems revolve around the production of excess acid (ulcers, indigestion and heartburn), the upward escape of acid (hiatus hernia) and disordered movements of the intestine (constipation, diarrhoea and irritable bowel syndrome). The rapid turnover of cells within the digestive tract predisposes to cancer, especially of the stomach and large intestine.

Large Intestine

We hear a lot. about the large intestine (large in diameter) or colon, or more familiar still, the bowel, because it seems to be the focus of many common complaints. Its function in the body is really very simple, it’s there to ensure that precious nutrients and water aren’t lost. Its main job is to absorb water from its fluid contents. When the stream of digested food leaves the small intestine it ’s liquid, very runny. Wren we pass it as a stool it’s solid. Its passage through the large intestine has brought about this change, purely through the absorption of water.


The rectum too can absorb water from faeces, in fact, it will wring faeces dry if they stay in the rectum too long. So if the “call to stool” is ignored for any length of time faeces can become stony hard and difficult, even painful, to pass.

This form of constipation is often thought of as a fault of the bowel or diet, but it isn’t. It’s simply due to not emptying the rectum soon enough. And it can be cured by heeding the call to stool as soon as you’re aware of it, even though the pressure to ignore it and do something else is strong. Laxatives aren’t necessary to cure this type of constipation, only retraining yourself to heed what your bowel is telling you to do. If you don’t, your bowel won’t bother to send you the signal and your “constipation” will worsen.

Oesophageal Cancer and Bowel Cancer

If I were to try to pull out the headlines of what we’ve discovered over the last decade two topics would immediately spring to mind, both of them cancer, oesophageal cancer and bowel cancer. The first is referable to heartburn. For decades heartburn has been thought of as nothing more than a mild irritation, a troublesome form of indigestion. In the last live or so years, however, we’ve come to realize that recurrent heartburn, especially if it’s left untreated, can eventually result in malignant change of the lower end of the gullet where it enters the stomach.

It’s caused by the chronic irritation from acid gastric contents, which regurgitate into the lower part of the oesophagus. Here, the lining has no protection from the burning effect of stomach acid. By contrast the stomach lining is covered with a thick protective layer of mucus, so acid never reaches the delicate walls of the stomach lining.

Rigorous treatment of heartburn can prevent cancerous change, so you must seek advice from your doctor if you have a long-term problem.

We know more and more about bowel cancer. In the UK it kills approximately 18,000 people every year and it’s almost certainly due to die fact that our Western diet now contains too little fibre (mainly fruit and vegetables). We can help the next generation by making sure our children eat the magic number of five fruits a day.

Signs of Bowel Cancer

There are two cardinal signs of bowel cancer. The first is seeing blood in your stool, though that’s a rather late sign. Before that you’ll probably have noticed a change in your bowel habits, going more or less often than you used to, a touch of constipation or diarrhoea where previously you had none. Or a sudden change in the shape of the stool narrow where previously it had been round. This change in the bowel habit, in middle age or later, is a sign you must heed and discuss with your doctor.

Peptic ulcer now has a new cause. We used to think it was due to too much stomach acid. We now know it’s due to an infection, a new infection, with Helicobacter pylori. Treatment has been revolutionised in the last 10 years.

Now antibiotics are the Ivnchpin and, as the bacterium is difficult to eradicate, the course lasts a minimum of two weeks, along with other medicines to increase the effectiveness of the antibiotic. One course of treatment and nearly everyone is cured! Now, that’s an advance on the old days.