The Gastrointestinal Tract

What is the gastrointestinal tract?

The gastrointestinal tract is a long passage way or tube that connects the mouth with the anus. Digestion begins in the mouth, where food mixes with salivary enzymes. When you swallow, food is then propelled down the oesophagus (gullet) into the stomach. In the stomach the food is broken down by the powerful digestive enzymes produced by the lining of the stomach and the hydrochloric acid which is present in the gastric juices.

From the stomach, the food passes into the small intestines. The small intestine is composed of the duodenum, the jejunum and the much longer ileum. In the small intestines juices from the pancreas and gallbladder continue the digestive process. It is here that most nutrients are absorbed from the food. This absorption happens as the intestine or contents are moved along the small intestines by peristalsis. Peristalsis is the movement caused by alternating muscle contraction and relaxation of muscles of the bowel.

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

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

Daily fluid intake and loss

The intestines are capable of both absorbing and secreting fluid. Overall it is estimated that about 9 litres of fluid pass through the intestines each day, of which only about 2 litres come from food and drink. The other 7 litres of fluid is 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 digestive and of nutrients and for optimal absorption of nutrients and minerals. Of the 9 litres, approximately 8.8 litres or more are on absorbed back into the bloodstream, so that less than 200 mls of water are excreted in the stools each day.

The intestines are therefore very efficient, reabsorbing as much as 98% or more of the water and minerals that pass through them. If anything prevents this from happening, so that less than 98% of water is reabsorbed, then stool output will be more watery and you will have diarrhoea.

The large intestine

Normally, in the colon, the liquid material entering from the small intestines becomes solid as water is absorbed from it. This solid waste is then stored until it is convenient for you to open your bowels.

If you are an adult consuming a typical Western diet, about 90% of the 1.5 litres or so of liquid reaching your colon in a 24 hour period are absorbed. This leaves 200 mL of semisolid material to be excreted.

Food spends around 1 to 3 hours in the stomach, 2 to 6 hours in the small intestines and 12 to 48 hours in the colon. Normally, it passes through the colon relatively slowly so as to allow fluid to be reabsorbed. This absorption occurs mainly in the ascending and 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 which empty the rectum. The muscles in the pelvis and rectum contract and the ring of muscle that controls the anus (the anal sphincter) relaxes in a coordinated way.

Transit times through the colon are usually shorter in men than in women, and men’s stools are heavier.