The Gut

What is the gut?

The gut (gastrointestinal tract) is the long tube that starts at the mouth and ends at the anus.

Where is the gut found?

The mouth is the first part of the gastrointestinal tract. When we eat, food passes down the oesophagus (gullet), into the stomach, and then into the small intestine. The small intestine has three sections - the duodenum, jejunum and ileum. The duodenum is the first part of the small intestine and follows on from the stomach. The duodenum curls around the pancreas creating a c-shaped tube. The jejunum and ileum make up the rest of the small intestine and are found coiled in the centre of the abdomen. The small intestine is where food is digested and absorbed into the bloodstream.

Following on from the ileum is the first part of the large intestine, called the caecum. Attached to the caecum is the appendix. The large intestine continues upwards from here and is known as the ascending colon. The next part of the gut is called the transverse colon because it crosses the body. It then becomes the descending colon as it heads downwards. The sigmoid colon is the S shaped final part of the colon which leads on to the rectum. Faeces is stored in the rectum and pushed out through the anus when you go to the toilet. The anus is a muscular opening that is usually closed unless you are passing stool.
The large intestine absorbs water, and contains food that has not been digested, such as fibre.

What does the gut do?

The gut processes food - from the time it is first eaten until it is either absorbed by the body or passed out as faeces. The process of digestion begins in the mouth. Here your teeth and enzymes (chemicals made by the body) begin to break down food. Muscular contractions help to move food into the oesophagus and on to the stomach. Chemicals produced by cells in the stomach begin the major work of digestion.

While some foods and liquids are absorbed through the lining of the stomach, the majority are absorbed in the small intestine. Muscles in the wall of the gut mix your food with the enzymes produced by the body. They also move food along towards the end of the gut.

Food that can't be digested, waste substances, bacteria and undigested food all get passed out as faeces.

How does it work?

The mouth contains salivary glands which release saliva. When food enters your mouth the amount of saliva increases. Saliva helps to lubricate food and contains enzymes that start chemically digesting your meal. Teeth break down large chunks into smaller bites. This gives a greater surface area for the body's chemicals to work on. Saliva also contains special chemicals that help stop bacteria from causing infections.

The amount of saliva released is controlled by your nervous system. A certain amount of saliva is normally continuously released. The sight, smell or thought of food can also stimulate your salivary glands.

To pass food from your mouth to the oesophagus you must be able to swallow. Your tongue helps to push food to the back of the mouth. Then the passages to your lungs close and you stop breathing for a short time. The food passes into your oesophagus. The oesophagus releases mucous to lubricate food. Muscles push your meal downwards towards the stomach.

The stomach is a j-shaped organ found between the oesophagus and duodenum. When empty it is about the same size as a large sausage. Its main function is to help digest the food you eat. The other main function of the stomach is to store food until the gut is ready to receive it. You can eat a meal faster than your intestines can digest it.

Digestion involves breaking food down into its most basic parts. It can then be absorbed through the wall of the gut into the bloodstream and transported around the body. Just chewing food doesn't release the essential nutrients, so enzymes are needed.

The wall of the stomach has several different layers. The inner layers contain special glands. These glands release enzymes, hormones, acid and other substances. These secretions form gastric juice, the liquid found in the stomach.

Muscle and other tissue form the outer layers. A few minutes after food enters the stomach the muscles within the stomach wall start to contract (tighten). This creates gentle waves in the stomach contents. This helps to mix the food with gastric juice.

Using its muscles the stomach then pushes small amounts of food (now known as chyme) into the duodenum. The stomach has two sphincters, one at the bottom and one at the top. Sphincters are bands of muscles that form a ring. When they contract the opening the control closes. This stops chyme going into the duodenum before it is ready.

Digestion of food is controlled by your brain, nervous system and various hormones released in the gut. Even before you begin eating, signals from your brain travel via nerves to your stomach. This causes gastric juice to be released in preparation for food arriving. Once food reaches the stomach, receptors (special cells which detect changes in the body) send their own signals. These signals cause the release of more gastric juice and more muscular contractions.

When food starts to enter the duodenum this sets off different receptors. These receptors send signals that slow down the muscular movements and reduce the amount of gastric juice made by the stomach. This helps to stop the duodenum being overloaded with chyme.

Some disorders of the gut

Details of the following disorders can be found at www.patient.co.uk/display/16777234/

• Acid Reflux & Oesophagitis
• Anal Fissure
• Appendicitis
• Barrett's Oesophagus
• Cancer of the Bowel
• Cancer of the Liver
• Cancer of the Oesophagus
• Cancer of the Pancreas
• Cancer of the Stomach
• Cholecystitis
• Coeliac Disease
• Constipation
• Crohn's Disease
• Cystic Fibrosis
• Diarrhoea
• Diverticula
• Duodenal Ulcer
• Dyspepsia
• Gallstones
• Gastroenteritis
• Haemorrhoids (Piles)
• Helicobacter Pylori & Stomach Pain
• Hernia
• Hiatus Hernia
• Irritable Bowel Syndrome
• Mesenteric Adenitis
• Pancreatitis
• Pruritus Ani (Itchy Bottom)
• Pyloric Stenosis
• Rectal Bleeding (Blood in Faeces)
• Stomach (Gastric) Ulcer
• Threadworms
• Toddler's Diarrhoea
• Ulcerative Colitis

Further more detailed information available free online

The Digestive Apparatus

From Gray's Anatomy Online
Web: www.bartleby.com/107/241.html

Gastrointestinal Function And Disorders

From Textbook in Medical Physiology And Pathophysiology
Web: www.mfi.ku.dk/ppaulev/chapter22/Chapter%2022.htm

Human Physiology/The Gastrointestinal System

From Wikibooks, the open-content textbooks collection
Web: http://en.wikibooks.org/wiki/Human_Physiology/The_gastrointestinal_system