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Gastrointestinal Tract - Part 2: the mouth and oesophagus

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VOL: 102, ISSUE: 07, PAGE NO: 26

Marion Richardson, BD, CertEd, DipN, RGN, RNT, is senior lecturer/programme leader, emergency nursing, Department of Nursing and Midwifery, University of Hertfordshire

This article, the second in this series on the GI tract, describes the form and function of the mouth and oesophagus.

 

This article, the second in this series on the GI tract, describes the form and function of the mouth and oesophagus.

 

 

Function
We eat approximately 1kg of solid food each day and have a fluid intake of approximately 1.2kg. Some of the food intake is in response to hunger, which is sometimes accompanied by stomach contraction. Of course, eating and drinking are also under voluntary control and there is a strong social element.

 

 

Food intake is regulated by the hypothalamus and there appear to be two centres - one that tells us we are hungry, the other that we have had enough. Appetite is also affected by emotional state.

 

 

The mouth
The mouth is a cavity and, like the rest of the gastrointestinal tract, it is lined with a mucous membrane (Fig 1). Its functions are to begin the mechanical and chemical digestion of food and to start it on its journey through the gastrointestinal tract.

 

 

The entrance to the mouth is protected by the lips and the teeth. The cheeks form the two sides and the palate (the bony hard palate at the front and the muscular soft palate at the back) make up the roof of the mouth. The lymphatic tonsils and the uvula, which hangs between the tonsils, guard the back of the mouth.

 

 

The floor of the mouth is formed by the tongue, which is made of strong skeletal muscle covered in mucous membrane. The tongue is held down towards the front of the mouth by a fold of skin - the frenulum.

 

 

Some individuals are born with a short frenulum, which makes it difficult for them to pronounce words properly. All over the surface of the tongue are papillae, which contain specific taste receptors. These allow us to enjoy food as we eat it and can discriminate five basic tastes: sweet; sour; bitter; salty; and umami.

 

 

The tongue is often used as an indicator of health and nutrition - it should be pink and moist. A smooth or cracked tongue, or one that is furred or brown, indicates a problem in the gastrointestinal tract. A dry tongue is a sign of dehydration.

 

 

Chewing
Chunks of food are bitten or torn off by the incisor teeth at the front of the mouth and drawn by the tongue into the oral cavity where they are mixed with saliva, which moistens the food and makes it easier to chew.

 

 

Saliva is a mixture of mucous and serous fluids and contains an enzyme, salivary amylase, which begins to digest starches. The enzyme allows some chemical breakdown of food to begin as soon as it enters the digestive tract.

 

 

Saliva is secreted by the salivary glands, in particular the parotid, submandibular and sublingual glands via the salivary ducts into the mouth. It is secreted continuously (about 1.5L per day) and keeps the mouth moist. Much larger amounts are secreted from the glands when we think about food or eat. It has a disinfectant role, containing substances that inhibit bacteria, and also dissolves the chemicals in food allowing us to taste it. In mumps, the salivary glands become inflamed and eating and swallowing become painful.

 

 

The mixture of food and saliva is chewed or masticated by the teeth and rolled around and mixed with saliva in the mouth by the tongue. This begins the mechanical breakdown of food.

 

 

Adults have up to 32 permanent teeth, which have a number of functions. At the front of the mouth, the incisors have a sharp cutting edge to bite through food, the canines (or cuspids) tear food, while towards the back of the mouth the premolars (or bicuspids) and molars (or tricuspids) have large flat surfaces for grinding food. Although no food is absorbed from the mouth, some drugs (for example nitroglycerine) are easily absorbed through the oral mucosa.

 

 

Swallowing
When food has been sufficiently chewed, we initiate the swallowing process or deglutition. The food bolus is pushed to the back of the mouth by the tongue into the pharynx (Fig 2a).

 

 

Once it enters the pharynx, we lose voluntary control and the parasympathetic nerves (especially the vagus) take over. Re-entry to the mouth is blocked by the tongue (Fig 2b) while the soft palate closes over the nasal passages and the flap-like epiglottis covers the entrance to the trachea (Marieb, 2005) (Fig 2c). All of this allows food only one exit route from the mouth - into the pharynx.

 

 

The two muscle layers in the walls of the pharynx (see NT Clinical, 7 February, p26, for a description of the layers of the lumen of the gastrointestinal tract) contract alternately and the resultant peristalsis (wave-like contractions) propels food through the pharynx and into the oesophagus (Fig 2d).

 

 

The oesophagus
The oesophagus is about 25cm (10 inches) long and runs from the pharynx, through the diaphragm to the stomach. Its function is simply to conduct food from the mouth to the next processing area in the gastrointestinal tract - the stomach.

 

 

Like the rest of the gastrointestinal tract, the oesophagus is lined with a mucus-producing membrane so that food slides easily over it. Also like the rest of the tract, it contains muscle layers, which are responsible for peristalsis, which move the food forward. Gravity ceases to play a significant part in the movement of food through the gastrointestinal tract once it has left the mouth so food will be moved to the stomach even if we eat standing on our heads.

 

 

Sphincters guard the oesophagus at either end. These valve-like rings of muscle help to keep the food moving in one direction and separate areas of the tract. This is particularly important at the far end of the oesophagus where the cardiac or gastro-oesophageal sphincter prevents the hydrochloric acid secreted by the stomach from flowing back.

 

 

If acid breaches this sphincter, it will damage the oesophageal mucosa and gastro-oesophageal reflux disease may result. This can range from mild heartburn to the more serious symptoms of regurgitation and difficulty in swallowing, or of hiatus hernia. In the latter condition, the stomach bulges through a gap in the diaphragm beside the oesophagus. Once food reaches the cardiac sphincter, the sphincter opens and the food enters the stomach.

 

 

- This article has been double-blind peer-reviewed.

 

 

For related articles on this subject and links to relevant websites see www.nursingtimes.net

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