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Gastrointestinal Tract - Part 1: form and function

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VOL: 102, ISSUE: 06, 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

The human gastrointestinal (GI) tract, or alimentary system, is a single tube approximately nine metres long when relaxed, though shorter in life because of the tone of the muscles within it (Marieb, 2005).

 

The human gastrointestinal (GI) tract, or alimentary system, is a single tube approximately nine metres long when relaxed, though shorter in life because of the tone of the muscles within it (Marieb, 2005).

It is open to the outside world at each end - the mouth and the anus - and includes several regions: the oral cavity; pharynx; oesophagus; stomach; small and large intestines; rectum; and anal canal (Fig 1).

Various accessory digestive organs open into the tract and there are a number of sphincters and valves. Its healthy functioning is vital because it breaks down food and converts it into components to build and provide energy for all the cells of the body.

Food that is in the GI tract is not really inside the body. To enter the body, food must be broken down and enter the blood or lymphatic system.

General structure

The same basic four-layered structure (Fig 2) is found throughout the GI tract, though different parts are adapted for different functions. Sphincters and valves ensure that food usually moves in one direction only and help to separate the different parts of the canal.

The mucosal layer


This is the innermost layer of the GI tube and is subject to a good deal of wear and tear as ingested food is in direct contact with it. It is particularly thick, with several layers of cells, in the mouth, oesophagus and rectum. In other areas the mucosa is thinner with a single layer of columnar cells.

Throughout the mucosa are scattered specialised goblet cells, which secrete mucus to lubricate and protect the gut lining.

In much of the GI tract, the mucosal layer is folded to provide a larger surface area for digestion and absorption. A thin layer of muscle beneath the mucosa is responsible for movements in these folds (Godfrey, 2005).

The submucosal layer

This layer of loose connective tissue supports the structures needed to supply and drain the cells of the tract. It contains:

- Blood vessels, which bring oxygen and nutrients to the cells, and remove waste and digestion products;

- The lymphatic system, which helps to drain excess and unwanted substances and defends the body against disease;

- Nerve fibres from the autonomic nervous system to control reflexes such as smooth muscle contraction.

The muscularis layer

This consists of muscle fibres arranged in two distinct sheets: a thicker, inner sheet made of two layers of circular fibres and an outer sheet of longitudinal fibres. The muscle fibres are smooth and are not under voluntary control except at the two ends of the tract.

The two layers contract together but, because their fibres lie in different directions, they produce slow, wave-like contractions that mix the contents of the tube - ‘segmentation’ - and propel it along - ‘peristalsis’. Peristalsis is continuous and rhythmic but can be influenced by hormones and by the many nerves that enter the muscularis layer.

At intervals throughout the GI tract, the circular muscle layer is modified to form rings of tissue called sphincters. These help to separate one section of the tract from another and control the speed of contents through the tract.

The adventitia or serosa

This outermost, protective layer is made of loose connective tissue and squamous (flat, scale-like) cells and carries nerves and blood vessels to supply the inner layers of the tract. This layer forms the inner, or visceral, part of a much larger organ - the peritoneum.

Blood and nerve supply

The gut requires a rich blood supply to support its digestive activities.

Arterial blood is supplied mainly by the coeliac artery to the stomach, pancreas, spleen and liver and by the mesenteric arteries to the intestines.

Venous blood drains from the stomach, pancreas and spleen via the hepatic portal vein into the liver, where the products of digestion undergo further processing and detoxification.

Blood from the oesophagus and rectum does not go through the liver but drains directly into the venous system.

There are two types of nerve supply to the GI tract. The enteric system, found within the walls of the GI tract, is sometimes known as the ‘gut brain’ and controls movement and secretion within the gut.

Nerves from the autonomic nervous system also supply the GI tract. The sympathetic nerves generally reduce blood flow to the gut and decrease secretions, motility and contractions, while stimulation of the parasympathetic nerves leads to an increase in motility and secretion within the tract and relaxation of the gut sphincters.

The vagus nerve (Xth cranial) supplies the oesophagus, stomach, pancreas, bile duct, small intestine and upper colon.

General functions

Marieb (2005) describes the digestive tract as a ‘disassembly line’ carrying food from one stage to the next so that it can be broken down and absorbed into the body. The stages involved in this breakdown process are ingestion, mastication, mechanical digestion (chewing, churning and pulverising), chemical digestion (by digestive enzymes), absorption into the blood and lymph, assimilation of useful components into the body by the liver and, finally, elimination in the faeces of non-usable residues such as animal and plant fibre (Godfrey, 2005; Smith, 2005).

Conditions in the lumen of the GI tract are controlled to maximise its efficiency. Mechanical digestion is mainly controlled by reflexes of the parasympathetic nerves. The receptors that stimulate this activity are found in the walls of the alimentary canal and respond to stretch, the acidity of the contents and the presence of certain breakdown products. When stimulated they set off reflexes that activate or inhibit the glands that secrete digestive juices and the smooth muscle of the muscularis layer that propels food along the tract (Marieb, 2005).

Chemical digestion - the process by which food molecules are broken down to their building blocks - is achieved by digestive enzymes secreted from the attached glands (salivary, liver, gall bladder, pancreas) into the lumen of the GI tract. The food needs to be soft and moist to enhance this process so water is needed in the diet. Carbohydrates are broken down to the simple sugars glucose, fructose and galactose, proteins are digested to amino acids while fats or lipids become fatty acids and an alcohol called glycerol (Godfrey, 2005; Marieb, 2005).

Forthcoming articles will discuss the more precise mechanisms of these functions.

- 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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