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The gut or alimentary canal can be considered as an extension or inversion of the epithelium or outer skin, as it is physiologically not part of the body. It runs from the mouth from the rectum, and varies in thickness. It has the following functions (comments in brackets for my Yorkshire-born counterparts):

• Ingestion (eetin’ an’ swallowin’ it)
• Peristalsis (movin’ it along)
• Digestion (smashin’ it up)
• Absorption (mekin’ it part of you)
• Egestion (defaecation or “shitting”)

The wall of the alimentary canal consists of:

• a lumen through which the food travels
• a mucosa, consisting of epithelium, connective tissue and muscles in the ileum which cause the movement of microvilli
• a submucosa
• circular muscle which contracts behind, and relaxes in front of the bolus. This widens in places to form sphincter muscles, like the cardiac and pyloric sphincter, which keep the food in the stomach for as long as is needed and enables the irregular feeding of carnivores. The anal sphincter keeps faeces in your body until a suitable time; otherwise life could be quite awkward!
• longitudinal muscle which relaxes behind, and contracts in front of the bolus.
• a serosa on the outside which protects the gut from friction. The epithelium surrounding it is called the mesentery and joins the various organs of the gut together.

The buccal cavity (gob) has a tongue (no…!), which rolls food into a bolus so that it passes easily down the oesophagus.
The stomach has cardiac and pyloric sphincters which keep food in and regulate it’s flow into the small intestine in small squirts. It is surrounded by muscle which helps to churn and mix the contents.
The small intestine (duodenum, jujenum, ileum) is long thin and folded, and therefore it has a large area over which to digest food. It has villi and microvilli, which further increase the available surface area. Its cells produce mucous, which lubricates the passage of food and reduce damage and friction.
The large intestine (colon) has the job of re-absorbing all the fluids secreted and ingested into the alimentary canal, and compacting the undigested remains for storing in the rectum. It is fairly wide. Bacteria exist here which use some of the undigested remains (fibre) and synthesise vitamins such as K, B₁₂ and riboflavin.

Digestion of protein, starch and lipids (walk-through) and control of secretions

 

Physical digestion begins in the mouth with mastication. This process breaks the food up and increases the surface area for enzymes to work on. Saliva from the salivary gland contains amylase, which begins the digestion of starch to maltose. Stimulation of salivary glands is by the nervous system in response to real or imagined food. Saliva also lubricates the movement of the food, which is then rolled into a bolus by the tongue and travels down the oesophagus to the stomach. Here it is mixed with gastric juice (pH 2), which is produced in gastric pits/glands in the stomach epithelium. Gastric juice is released in response to the presence of food in the buccal cavity. As the stimulation by the nervous system wears off, the presence of food stimulates cells in the stomach lining to produce gastrin. This passes into the bloodstream and stimulates the release of gastric juice for several hours. These consist of:
 

• Chief cells, which secrete pepsinogen, which is converted to its active enzyme by hydrochloric acid and pepsin itself. Producing the inactive form first prevents it from digesting the cell cytoplasm contents.
• Oxyntic cells, which produce hydrochloric acid. This converts pepsinogen to its active form as mentioned above, provides optimum pH for pepsin to work in, and kills most bacteria. The stomach epithelium is protected from ulceration by a thick layer of mucous produced by goblet cells.

 

Gastric juice therefore contains pepsin and pepsinogen, as well as HCl, and chemical digestion begins. Both pepsin and HCl catalyse hydrolysis. Pepsin is an endopeptidase: it is an enzyme which catalyses the hydrolysis of proteins to polypeptides. It does this by hydrolysing the inside of the protein molecule, thus increasing the number of ends for the exopeptidases to work on. The pyloric sphincter opens when the chyme (the mixture of food and gastric juice) is of the correct consistency, and releases the food into the small intestine. Lipid stimulates the production of entergasterone by the stomach mucosa, which has an antagonistic effect on gastrin. Therefore, the more fat the food contains, the longer it remains in the stomach. The first section of this is the duodenum, which runs nearly horizontally across your body below your stomach. When chyme enters the duodenum, its lining releases a number of hormones. One of these is secretin, which stimulates the production of the sodium hydrogencarbonate portion of the pancreatic juice. Cholecystokinin-pancreozymin (CCK-PZ) stimulates the gall bladder to contract, and the release of digestive enzymes from the pancreas. At the beginning of this, ducts running from the gall bladder (bile duct) and the pancreas (pancreatic duct) form the common duct and secrete bile and pancreatic juice into the duodenum. Pancreatic juice

• Is rich in sodium hydrogencarbonate, which neutralises the acidic chyme from the stomach, raising the pH to about 7.
• Is also rich in enzymes. They are summarised in the table below. The bits in bold must be learnt.

Type of enzyme	Name of enzyme	Substrate	Product(s)
Polysaccharidase	Amylase	Starch	Maltose/sucrose/lactose
Endopeptidase	Trypsin	Protein	Polypeptides
Endopeptidase	Chymotrypsin	Protein	Polypeptides
Exopeptidase	Carboxypeptidase	Polypeptides	Dipeptides/Dipeptides
Lipase	Lipase	Lipids	Fatty acids and glycerol
Nuclease	Nuclease	Nucleic acids	Nucleotides

Bile

• Is stored in the gall bladder
• Contains no active enzymes
• Contains bile salts which emulsify fats, increasing the surface area for the action of lipase  

The duodenum contains deep folds between its villi, which have Brunner’s glands below them. The latter secrete a viscous fluid containing water, hydrogencarbonate ions and mucoprotein, and serves to protect the wall of the duodenum from the pepsin and chyme. Both extracellular digestion and intracellular digestion occur in the small intestine. The cells of the epithelium contain enzymes in their cytoplasm and embedded within their cell surface membrane. Digestion therefore occurs outside, on the way into, and inside the cytoplasm of, these cells. Resulting from the digestion in the lumen of the small intestine, carbohydrates have been hydrolysed to disaccharides and proteins to dipeptides and tripeptides. Their further digestion takes place in the cell surface membrane of the microvilli of the intestinal mucosa. Disaccharidases break disaccharides down into their constituent monosaccharides. Most of these are released back into the lumen of the gut. A similar process occurs with dipeptides and tripeptides, which are taken up by the microvilli and broken down into their respective amino acids. Some also diffuse into the cytoplasm of the cells of the villi, where they are digested.