Family Doctor Books


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Family Doctor Books
Preview of Understanding Irritable Bowel Syndrome

The gastrointestinal tract
The gastrointestinal tract is a long passageway that connects the mouth and anus. Digestion starts in the mouth, where food mixes with salivary enzymes. When you swallow, food is propelled down the oesophagus (gullet) into the stomach, where it is broken down by the powerful digestive enzymes and the hydrochloric acid found in gastric juices.

From the stomach, food passes into the small intestine (duodenum, jejunum and ileum), where juices from the pancreas and gallbladder continue the digestive process. It is here that most nutrients are absorbed from the food as peristalsis (movement caused by alternating muscle contraction and relaxation) moves the intestinal contents along.

Undigested waste (faeces) moves into the first part of the large intestine (the colon), where muscle contractions slowly move it along while excess water is removed.

Just before defecation (bowel action), the waste is moved into the rectum and is then eliminated through the anus.

The gastrointestinal tract and the digestive process. Food must be broken down so that the body can absorb the nutrients. Undigested material and waste are expelled.

Daily fluid intake and loss

The intestines are capable of both absorbing and secreting fluid. Overall, it is estimated that nine litres of fluid pass through the intestines each day, of which only around two litres come from food and drink. The other seven litres are secreted by the body itself, in the form of saliva, bile and the juices of the stomach, pancreas and intestine. These secretions provide the necessary conditions for rapid digestion of nutrients and for optimal absorption of nutrients and minerals. Of the nine litres, approximately 8.8 litres or more are reabsorbed back into the circulation, so that less than 200 grams of water are excreted in the stools each day. The intestines are therefore efficient, reabsorbing as much as 98 per cent or more of the water and minerals that pass through them. If anything prevents this from happening, so that less than 98 per cent of water is reabsorbed, then stool output will increase and you will have diarrhoea.

The intestines both absorb and secrete fluid. Overall, it is estimated that nine litres of fluid pass through the intestines each day, of which only around two litres come from food and drink. The other seven litres are secreted by the body itself, in the form of saliva, bile and juices of the stomach, pancreas and intestine. The intestines are very efficient, reabsorbing as much as 98 per cent or more of the water and minerals that pass through them.


The daily intake of water and the secretions within the body are efficiently absorbed by the gastrointestinal tract.
Classification Quantity of water
Oral intake 2,000 ml
Salivary glands 1,500 ml
Stomach 2,500 ml
Bile 500 ml
Pancreas 1,500 ml
Intestine 1,000 ml
Oral intake 2,000 ml
Total water presented to the intestines 9,000 ml
Expelled in faeces 200 ml
Absorbed by the intestines
8,800 millilitres (ml)

The large intestine

Normally, in the colon, the liquid material entering from the small intestine solidifies as water is absorbed from it, and the remaining waste matter is stored until it is convenient for you to open your bowels. If you are an adult consuming a typical Western diet, about 90 per cent of the 1.5 litres or so of liquid reaching your colon in a 24-hour period is absorbed, leaving less than 200 millilitres of semi-solid material to be excreted.

Liquid material enters the colon from the small intestine; it progressively solidifies as water is absorbed from it.

Food spends around one to three hours in the stomach, two to six hours in the small intestine and 12 to 48 hours in the colon. Normally, it passes through the colon relatively slowly so as to allow fluid to be absorbed – a process that occurs mainly in the right ascending and mid-(transverse) colon. Powerful muscle contractions propel solidified stool into the lower (sigmoid) colon and rectum several times a day. Defecation ultimately occurs as a result of complex interactions between sensory and motor nerves within the gut wall and the central nervous system. This interaction stimulates the muscles in the pelvis and rectum and the ring of muscle that controls the anus (the anal sphincter) to relax and contract in a coordinated way, and so to empty the rectum.

Approximate passage times through the gut.

Transit times through the colon are usually shorter in men than in women, and men’s stools are heavier.
KEY POINTS
  • The gastrointestinal tract extends from the mouth to the anus; it digests and absorbs food and liquids and propels them along by muscle contraction
  • The small and large intestine are very efficient at absorbing fluid, so that most people excrete no more than 200 millilitres of stools per day. If absorption is impaired by disease, the result is diarrhoea
  • The large intestine consists of the colon, rectum and anus. Transit through the colon is slow, which helps absorption of fluid
  • Muscle contractions move solid stool into the lower colon and rectum several times a day
  • Defaecation occurs as a result of the interaction of several different parts of the nervous system, which cause pelvic, rectal and anal sphincter muscles to relax and contract