Title: The Digestive System

Key words: gastrointestinal tract, salivary glands, liver, gall bladder, pancreas, small intestine, large intestine, ileum, caecum, ingestion, absorption, digestion, assimilation, defaecation, mucosa, serosa

Date: Aug 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

The Digestive System

The digestive system consists of the gastrointestinal tract (also known as the GI tract, gut, digestive tract or alimentary canal) and its associated organs (e.g. salivary glands, liver, gall bladder, pancreas).

The function of the digestive system is to take in food (ingestion), break it down to its component parts (digestion), absorb the components (nutrients) needed by the body for growth and energy (metabolism), then remove the remainder, along with any waste products from the process (excretion).

The gastrointestinal tract consists of the mouth, pharynx (throat), oesophagus (gullet), stomach, small intestine (gut), colon, rectum and anus. Structurally, the entire length of the tract is a tube composed of four layers, each with its own specialised function. From the inner to the outer surfaces of the tube, these layers are:

 

The mucosa consists of a mucous membrane containing mucus secreting cells and, in some places, glandular cells or drainage tubes from glands which produce digestive juices. The mucus acts as a lubricant for food passing through the lumen of the tract and also helps to protect the lining of the tract against the effects of its own digestive juices.

The digestive tract is lined by a mucous membrane

Around the mucosa is a thin layer of smooth muscle and the submucosa, a layer of connective tissue. This contains blood and lymph vessels, lymphatic tissue and nerves. The larger glands that drain into the lumen of the tract are located in the submucosa. Around the mucosa are a layer of circular and a layer of longitudinal smooth muscle, referred to as the muscularis. Reducing friction between this layer and the surrounding tissue is the serosa, a thin, smooth layer of epithelial-type covering cells or mesothelium, on a base membrane.

 

 

The walls of the digestive tract contain smooth muscle

The mouth, or oral cavity, has several functions, as it is the entry point to both the digestive and respiratory systems. The digestive process begins when food is chewed and mixed with saliva. Saliva is secreted by a large number of minor glands and three pairs of large glands, the parotid, submandibular and sublingual. Saliva contains salivary amylase or ptyalin, the first digestive enzyme, which starts to break carbohydrates down to sugars. The jaws, jaw muscles, teeth, salivary glands and tongue are all involved in preparing food for digestion further down the GI tract through chewing or mastication. This forms a paste which the tongue and cheeks mould into a small mass, called a bolus. The bolus is swallowed and passes through the pharynx into the oesophagus.

Food is prepared for digestion in the mouth by chewing, which breaks the food up and mixes it with saliva

 

Food is moved down the oesophagus into the stomach by swallowing and peristalsis. Swallowing involves a complex set of reflex actions, which prevent food entering the windpipe or air entering the oesophagus. Peristalsis is a less complex, rhythmic cycle of contraction and relaxation of the smooth muscle layers, which moves food along throughout the GI tract.

A major difference between the stomach wall and the rest of the GI tract is an extra (third) set of smooth muscle fibres, which help the stomach to mix and break down the food. Other differences include the many folds or rugae of the stomach wall, which increase the surface area for secretion and absorption and allow the stomach to expand more easily, to store food.

The stomach has an extra layer of smooth muscle to aid the mixing of food.

The cardiac sphincter or lower oesophageal sphincter (LOS) is a valve at the entrance to the stomach, which stops the stomach contents passing back into the oesophagus. Around this sphincter, the cardiac glands in the stomach wall secrete mucus. Mucus lubricates food and helps to protect the stomach against its own secretions. Gastric mucus contains a high proportion of bicarbonate ions which help to neutralise acid. The pyloric glands also secrete mucus over the gastric lining and secrete the hormone gastrin into the bloodstream when food enters the stomach.

Gastrin acts on cells in the main body of the stomach, to stimulate acid and pepsin production, muscle activity and regeneration of mucosal cells. The pyloric glands are located around the pyloric sphincter, a valve that keeps the stomach contents separate from the intestine until they have been suitably prepared.

 

The stomach is protected by mucus against the acid it contains

 

When food enters the stomach, it also triggers the release of gastric juice from the gastric glands. These glands (see figure X) contain 3 types of cell:

    • mucous cells
    • parietal cells
    • peptic cells.

Food triggers the release of gastric juice when it enters the stomach

Mucous cells help maintain the protective mucus layer. The parietal cells secrete hydrochloric acid, which kills harmful organisms and breaks down proteins and essential minerals to an absorbable form. They also secrete intrinsic factor, which enables the body to absorb Vitamin B12. Peptic cells in the lower third or body of the glands produce pepsinogen, for conversion to pepsin by hydrochloric acid. Pepsin breaks proteins down into amino acids for absorption.

The gastric glands have another important function, replacing the epithelial lining of the stomach. Because the gastric contents are so corrosive (acid + enzymes), this lining is continuously being replaced to ensure it remains intact. Cell turnover is high, with around half a million cells shed every minute. New cells are formed in the neck of the gastric glands and migrate to the mucosal surface over 2–3 days.

 

The stomach has a very good blood supply, receiving up to 25% of the heart's output during digestion. About half of this flows through the gastric mucosa, to fuel cell production, aid gastric secretion and remove both harmful substances and nutrients that penetrate the mucosal layer. As a result of this rich blood supply, any damage to the gastric mucosa causes serious bleeding, which can become life threatening.

Bleeding from the gastric mucosa can be serious

To minimise this risk, the stomach has several protective mechanisms, such as the mucus layer described earlier. The mucous layer also contains bicarbonate ions (HCO3-)which are secreted in response to prostaglandins. These ions neutralise any acid ions (H+) reaching the cells beneath the layer of mucus. Drugs that affect prostaglandin production, such as aspirin and NSAIDs used for pain relief, can reduce the effectiveness of this protective mechanism.

The tightly packed epithelial cells are also protective, as they prevent harmful substances reaching the deeper tissues that they cover. The cells themselves can be replaced within minutes if damaged, to maintain the overall integrity of the gastric mucosa. When the stomach is empty, other mechanisms reduce total gastric secretion, to avoid unnecessary exposure of the lining to acid and enzymes.

Some drugs can reduce the protection of the stomach lining

 

Table X: Maintaining mucosal integrity in the stomach

Positive (defensive) factors

Harmful (aggressive) factors

mucus layer

gastric acid and pepsin

prostaglandin-induced mucus + bicarbonate secretion

Exogenous (outside) factors - alcohol, drugs eg aspirin and NSAIDs

high cell turnover - rapid epithelial cell replacement

duodenal juice (from small intestine)

rich blood supply - removes harmful acid (H+)ions

prolonged gastric emptying - damage to mucosa increases if stomach contents stay longer

tightly packed epithelial cells

Once food has been mixed and partly digested in the stomach, it is passed into the small intestine in a paste-like form known as chyme. Here the remaining acid is neutralised by sodium bicarbonate secreted by the pancreas. Digestive enzymes from the pancreas, gall bladder and the intestinal mucosa complete the breakdown of fats, carbohydrates and proteins in the chyme to forms that can be absorbed.

Food leaves the stomach in the form of chyme

 

The small intestine consists of the duodenum, jejunum and ileum. The duodenum is 20-25cm long, the jejunum is around 3.5m long and the ileum 4.5m long. The jejunum and ileum are arranged in coils and loops. Inside the small intestine, the surface area is increased by circular folds and villi, small (1mm high and 0.1mm wide) finger-like projections. The villi, in turn, consist of epithelial cells with many microvilli, to further increase the surface area for digestion and absorption. The total surface area is calculated to be around 200m2. Almost all of our nutritional needs are absorbed in the small intestine.

The small intestine is where almost all of our food is absorbed

The large intestine is the last section of the GI tract. It consists of five parts, the caecum (which has an appendix), the ascending colon, transverse colon, descending colon, sigmoid colon and the rectum which ends at the anus, the valve or sphincter through which faeces are released during defaecation. The large intestine does not contain villi or microvilli and only water, vitamins and some minerals are absorbed here.

Only water, vitamins and some minerals are absorbed from the large intestine

The contents of the large intestine are mainly roughage and indigestible materials left after the digestive process. This builds up inside the colon and rectum until stretch receptors in the rectum trigger the urge for defaecation. At a suitable time and place, the anal sphincter relaxes and faeces is squeezed out by peristaltic contractions in the colon.

Defaecation removes the waste products of digestion from the body