Functions of Alimentary Tract (With Diagram)

In this article we will discuss about the functions of alimentary tract, explained with the help of suitable diagrams.

In most of the cases of foodborne illness we consider, the pathogenic (disease producing) effect occurs in the alimentary tract giving rise to symptoms such as diarrhoea and vomiting. Since these are essentially a dysfunction of the gut, a useful starting point would be to outline its normal operation and the role micro organisms play in this process.

The alimentary or gastrointestinal tract is not an internal organ of the body but a tube passing through it from the mouth to the anus (Figure 6.6). Its principal functions are the digestion and absorption of food and the excretion of waste.

Unlike most of the body’s other external surfaces, it is not lined with a dry protective skin and so, although it possesses some protective features, it offers a more congenial environment for micro-organisms and an easier route by which they can penetrate the body. 

In the mouth, food is mixed with saliva and broken down mechanically to increase the surface area available for attack by digestive enzymes. Saliva is an alkaline fluid containing starch-degrading (amylase) enzyme and the antimicrobial factors immunoglobulin (IgA), lysozyme, lactoferrin and lacto peroxidase.

It provides lubrication to assist chewing and swallowing and performs a cleansing function, rinsing the teeth and mouth to remove debris. On average, an adult secretes and swallows about 1.5 1 of saliva each day.

The variety of foods consumed and the range of micro-environments in the mouth result in a diverse and continually changing microflora. On the teeth, bacteria are associated with the formation of dental plaque – an organic film in which bacteria are embedded in a matrix derived from salivary glycoproteins and microbial polysaccharides.

The microbial composition of plaque varies with its age but filamentous Fusobacterium species and streptococci are common components. Plaque offers a protective environment for bacteria and its development is often a prelude to conditions such as dental caries and periodontal disease.

Swallowed food descends via the oesophagus into the stomach; a bulge in the alimentary tract which serves as a balance tank from which food is gradually released into the small intestine for further digestion.

In the stomach, food is blended with gastric juice, an acidic fluid containing hydrochloric acid. This has a marked effect on ingested micro-organisms, killing most. Normally only acid-tolerant vegetative cells and spores survive and the microbial count in the stomach is low, although lactobacilli are frequently found in association with the stomach wall.

Gastric acidity generally provides very effective protection for subsequent sections of the intestine but is not, as we shall see, an invulnerable defence.

Bacteria can evade prolonged exposure to the acid by being sheltered in food particles or as a result of accelerated passage through the stomach as occurs, for instance, when the stomach is full. Alternatively, acidity may be neutralized by the food or absent as a result of illness.

The digestive functions of the stomach are not confined to those of a mechanical churn with antimicrobial features. Proteases, such as pepsin, and lipase which can operate at low pH partially digest the stomach contents. The gastric mucosa also secretes a protein responsible for efficient absorption of vitamin B₁₂.

Little absorption of nutrients occurs in the stomach, with the notable exception of ethanol, but some material transfer is often necessary to adjust the osmotic pressure of the stomach contents to ensure they are isotonic with body fluids.

From the stomach, small quantities of the partially digested mixture of food and gastric juice, known as chyme, are released periodically into the small intestine. In this muscular tube over 6 metres long most of the digestion and absorption of food occur.

Its internal lining is extensively folded and the folds covered with finger-like projections or villi which are themselves covered in microvilli. This gives the inner surface the appearance and texture of velvet and maximizes the area available for absorption (Figure 6.6).

In the first section of the small intestine, the duodenum, large-scale digestion is initiated by mixing the chyme with digestive juice from the pancreas and bile from the gallbladder which neutralize the chyme’s acidity. The pancreatic juice also supplies a battery of digestive enzymes, and surfactant bile salts emulsify fats to facilitate their degradation and the absorption of fat soluble vitamins.

Further digestive enzymes that break down disaccharides and peptides are secreted by glands in the mucous lining of the duodenum called, with evocations of a Gothic horror, the crypts of Lieberkuhn.

The duodenum is a relatively short section of the small intestine, accounting for only about 2% of its overall length. Food is swept along by waves of muscle contraction, known as peristalsis, from the duodenum into the jejunum and thence into the ileum.

During this passage, nutrients such as amino acids, sugars, fats, vitamins, minerals and water are absorbed into capillaries in the villi from where they are transported around the body. Absorption is sometimes a result of passive diffusion, but more often involves the movement of nutrients against a concentra­tion gradient; an active process entailing the expenditure of energy.

The microbial population increases down the length of the small intestine: counts of 10²-10³ ml^-1 in the duodenum increase to around 10³-10⁴ in the jejunum, 10⁵ in the upper ileum and 10⁶ in the lower ileum. This corresponds with a decreasing flux of material through the small intestine as water is absorbed along its length.

In the higher reaches of the duodenum, the flow rate is such that its flushing effect frequently exceeds the rate at which micro-organisms can multiply so that only those with the ability to adhere to the intestinal epithelium can persist for any length of time.

As the flow rate decreases further along the small intestine, so the microbial population increases, despite the presence of antimicrobial factors such as lysozyme, secretory immunoglobulin, IgA, and bile.

In the healthy individual, the microflora of the small intestine is mainly comprised of lactobacilli and streptococci, although, as we shall see, other bacteria have the ability to colonize the epithelium and cause illness as a consequence.

Extensive microbial growth takes place in the colon or large intestine where material can remain for long periods before expulsion as faeces.

During this time active absorption of water and salts helps to maintain the body’s fluid balance and to dry faecal matter. Bacterial cells account for 25-30% of faeces, amounting to 10¹⁰-10¹¹ cfu g^-1 the remainder is composed of indigestible components of food, epithelial cells shed from the gut, minerals, and bile.

Obligate anaerobes such as Bacteroides and Bifidobacterium make up 99% of the flora of the large intestine and faeces. Members of the Enterobacteriaceae, most commonly Escherichia coli, are normally present at around 10⁶ g^-1, enterococci around 10⁵ g^-1, Lactobacillus, Clostridium and Fusobacterium, 10³-10⁵ g^-1, plus numerous other organisms, such as yeasts, staphylococci and pseudomonads, at lower levels.

The interaction between the gut microflora and its host appears to have both positive and negative aspects and is the subject of much current research and conjecture. Addition of antibiotics to feed has been shown to stimulate the growth of certain animals, suggesting that some gut organisms have a deleterious effect on growth.

A normal gut microflora confers some protection against infection. One example of this effect is the inflammatory disease pseudomembranous colitis caused by Clostridium difficile.

Normally the organism is present in the gut in very low numbers, but if the balance of the flora is altered by antibiotic therapy, it can colonize the colon releasing toxins. Similarly, the infective doses of some other enteric pathogens have been shown to be lower in the absence of the normal gut flora.

It appears that protection is not simply a result of the normal flora occupying all available niches, since entero-toxigenic E. coli adheres to sites that are normally vacant. Some direct antagonism through the production of organic acids and bacteriocins probably plays a part, but stimulation of the host immune system and its capacity to resist infection also appear to be factors.

In monogastric animals such as humans, gut micro-organisms do not play the same central role in host nutrition as they do in ruminants.

Some facultative anaerobes found in the gut, such as E. coli and Klebsiella aero-genes are known to produce a variety of vitamins in vitro and studies using animals reared in a germ-free environment and lacking any indigenous microflora have shown that in vivo vitamin production by micro-organisms can be important on certain diets.

In humans, however, the evidence is less convincing. Some have questioned the efficiency of absorption of vitamins produced in the large intestine pointing to the fact that vegans have developed vitamin B₁₂ deficiency despite its production in the gut and excretion in the faeces.

It appears that an adequate balanced diet will probably meet all the body’s requirements in this respect and that, short of coprophagy, access to vitamins produced in situ is limited.