Title: The gastro-intestinal tract

Key words: structure, function, digestion, defence mechanisms

Date: May 1999

Category: 8. The Gut

Type: Article

Author: DJE Candlish

The gastro-intestinal (GI) tract

Structure of the GI tract

The GI (or digestive) tract consists of the mouth, pharynx (throat), oesophagus (gullet), stomach, small intestine, large intestine, rectum and anus together with associated glands and organs (e.g. salivary glands, liver, gall bladder, pancreas).

Structurally, the tract can be thought of as one continuous tube. It has four key functions:

The entire length of the GI tract has a similar overall structure, consisting of four layers, although variations occur at different locations:

The network of nerves that controls the activities of the GI tract, Auerbach’s complex, is located between the two muscle layers.

 

The mouth and pharynx

The mouth is involved in tasting, chewing and swallowing the food we ingest. Food is chewed or broken up and mixed with saliva by the teeth and tongue, to form a small mass or bolus of soft paste. Saliva is released when the taste buds, tiny structures on the tongue, are stimulated by the presence of food. It softens and lubricates the food and also contains an enzyme which begins the digestive process.

After it passes from the mouth into the pharynx, the bolus is swallowed i.e. sent down the oesophagus into the stomach by a co-ordinated series of muscular contractions, peristalsis. The pharynx is the space at the back of the mouth which is also connected to the nasal passages and the trachea, which takes air into the lungs. Food is prevented from entering the trachea by the epiglottis,a thin flap of tissue that acts as a valve, opening when we breathe and closing when we swallow.

The oesophagus is a muscular tube, averaging 30cms in adults. It contains two layers of muscle, circular and longitudinal. Rings of strong circular muscle, called sphincters, control the passage of food into and out of the oesophagus. The upper ring is known as the pharyngeal sphincter and the lower as the cardiac sphincter or lower oesophageal sphincter (LOS).

The stomach

The stomach is one of the most important parts of the digestive system. It is essentially a muscular pouch, kept sealed at the top by the LOS and at the bottom by another "valve", the pyloric sphincter. It is distensible (can expand) with a typical capacity of around 1 – 2 litres.

The stomach consists of four regions:

the cardia – just below the LOS

the fundus – the rest of the upper stomach

the corpus or body of the stomach – the major region

the pyloric region – ending in the pyloric sphincter

The stomach wall contains an additional muscle fibre layer (oblique fibres) which helps it to "churn" its contents. The wall has rugae or folds which enable it to expand as food or liquid enters. (dia.)

The inner lining or mucosa of the stomach contains many glands which secrete the acid and gastric juices that break food down for absorption. Ingested food and liquid is mixed with these secretions in the stomach and converted into chyme, a semi-digested liquid which is passed into the small intestine in pulses, over a period of several hours.

The small intestine

The small intestine is where most of the digestion and absorption of nutrients takes place. It is around 4 – 8 metres long and consists of three regions:

The inner layer of the small intestine is formed into villi, small projections that increase the surface area for absorption. There are 10-40 villi/mm2 of mucosa. Each villus is richly supplied with fine blood vessels or capillaries and a small branch of the lymphatic system, which is involved in the transport of fats and in body defence systems. The surface of the villi is covered by a layer of epithelial cells, topped with microvilli projecting into the central area or lumen of the intestine. The microvilli form a brush border which further increases the area available for absorption. (dia.)

The large intestine

Towards the end of the digestive process, the undigested components of food are passed from the small intestine into the large intestine. This is around 1.5m in length and consists of three regions:

The last section of the descending colon is known as the sigmoid colon (Greek for ‘S’ shaped). The main functions of the large intestine are the formation, storage and expulsion of faeces, the solid waste products of digestion. However, water and some electrolytes (chemical ions like Na+ and K+) are reabsorbed in the colon.

The inner lining of the large intestine differs from the small intestine, as there are no villi here. Instead, there are deep clefts or crypts, lined with goblet cells that secrete mucus, which lubricates the faeces ready for expulsion.

The rectum and anal canal are the last sections of the GI tract. Their function is to expel faeces from the body. The anus is a muscular tube around 5cm long with a muscular valve at each end – the inner and outer sphincter. Only when these sphincters relax can defecation occur.

The hepatic portal system

The serosa or outer layer of the GI tract provides it with a rich blood supply. This is needed for two reasons:

Oxygenated blood is supplied from the heart to the stomach and intestines via the arteries, while deoxygenated blood containing the absorbed nutrients is removed by the veins. These veins lead back, not to the heart but to the liver. This venous system is known as the hepatic portal system. All the nutrients absorbed in the stomach and small intestine pass via the hepatic portal system to the liver, before being sent into the systemic circulation to the rest of the body.

The liver is one of the major organs with a number of functions including:

Function of the GI tract

The GI tract processes all the food and drinks we consume, to obtain from them the essential nutrients needed for health and growth. These include:

These nutrients are all obtained from food and drinks as they pass through the GI tract. The processes involved are briefly described below.

Digestion

Digestion is a combination of mechanical and chemical processes, aimed at breaking foodstuffs down into their component parts for ease of absorption and assimilation – the incorporation of these components into body tissue. The mechanical process involves muscles of the GI tract, mixing food with the digestive juices and moving it along for further processing. The chemical processes involve digestive enzymes, specialised proteins that break down specific nutrients.

Saliva provides the first digestive enzyme, salivary amylase or ptyalin. This begins the chemical breakdown of starches. Saliva also contains mucins, which act as lubricants for the food bolus as it moves towards the stomach.

When we see, smell or taste food or when a bolus of food enters the stomach, the stomach lining produces gastric juice, which consists of:

pepsin, a digestive enzyme which acts on proteins

hydrochloric acid

mucus – which protects the stomach against the acid

 

The hydrochloric acid is produced and secreted by specialised parietal cells present in the gastric glands located in the inner mucosal lining of the stomach.

Gastric acid has two important roles:

The stomach itself is protected against damage from the acidic gastric juice by a thick, jelly-like protective mucus layer.

The control mechanisms for acid production and secretion are complex and inter-related, involving nerve pathways, the hormone gastrin, and the body chemical histamine. Histamine can act directly on parietal cells to increase both the rate and volume of acid secretion. The release of pepsin is controlled by gastrin, itself released in response to the presence of food in the stomach.

The main digestive juices of the small intestine are intestinal juice, bile and pancreatic juice. Intestinal juice is secreted by specialised epithelial cells in the small intestine. Bile is formed in the liver and stored in the gall bladder until the presence of fatty food in the intestine stimulates its release. Bile is involved in the breakdown and absorption of fats and fat soluble vitamins.

The pancreas produces pancreatic juice, which contains amylase to break down carbohydrates, lipase to break down fats and trypsinogen. This is activated in the lumen of the small intestine by enterokinase, to form trypsin, which breaks down protein. The intestinal mucosa produces enterokinase, mucus and sodium bicarbonate ions to neutralise stomach acid.

The remaining chyme enters the large intestine at the caecum and is then converted into faeces, which consist of undigested food and plant materials, inorganic material, bacteria and bacterial debris and cell debris from the GI tract. Peristaltic movements propel faeces towards the rectum and anus, ready for expulsion by defecation.

 

Defence mechanisms of the GI tract

The GI tract provides a possible route of entry into the body for infectious micro-organisms. There are three defence mechanisms against these organisms:

The physical defences are the epithelial layer of the mucosa and the mucus layer over this which block the entry of micro-organisms into the body. Micro-organisms and their products are also physically removed from the body during defecation. The chemical defences are the various digestive enzymes, bile salts and gastric acid, which are toxic to micro-organisms. Both of these defence systems can be weakened by diseases, such as inflammatory bowel disease, or by physical injury.

The immune system defends the body against micro-organisms and other ‘foreign’ substances. It uses both cellular and non-cellular components to do this. The cellular components include:

The white blood cells or leucocytes present in circulating blood come from these cell types. Macrophages and neutrophils are also present in body tissues, especially after injury or during infection. They engulf micro-organisms, dead cell materials and other debris by the process of phagocytosis, then destroy them with enzymes from intracellular granules called lysosomes. (dia)

Mast cells and eosinophils also contain granules, which can release enzymes and other chemicals by degranulation. These substances attack invading organisms that are too large to be removed by phagocytosis.

Lymphocytes are mainly located in nodes in the lymphatic system, which is involved in the transport of fats as well as the immune system. The two most important types of lymphocyte are T cells and B cells. Some T cells attack and kill cells infected by viruses, while others control the activities of B cells. B cells produce special proteins called antibodies (Ab) or immunoglobulins (Ig), which are the major non-cellular component of the immune system. Substances that provoke the B cells to produce antibodies are called antigens. Special B cells ‘remember’ antigens, to speed up the immune response to them if they are encountered again.

There are five major classes of antibody: IgA, IgD, IgE, IgG and IgM. Each binds to one specific type of antigen, either to neutralise it directly or to attract other components of the immune system, such as complement proteins, to destroy or remove it.