The GI System
The gastro-intestinal system is essentially a long tube running right through the body, with specialised sections that are capable of digesting material put in at the top end and extracting any useful components from it, then expelling the waste products at the bottom end. The whole system is under hormonal control, with the presence of food in the mouth triggering off a cascade of hormonal actions; when there is food in the stomach, different hormones activate acid secretion, increased gut motility, enzyme release etc. etc.
Nutrients from the GI tract are not processed on-site; they are taken to the liver to be broken down further, stored, or distributed.
Once food has been chewed and mixed with saliva in the mouth, it is swallowed and passes down the oesophagus. The oesophagus has a stratified squamous epithelial lining (SE) which protects the oesophagus from trauma; the submucosa (SM) secretes mucus from mucous glands (MG) which aid the passage of food down the oesophagus. The lumen of the oesophagus is surrounded by layers of muscle (M)- voluntary in the top third, progressing to involuntary in the bottom third- and food is propelled into the stomach by
The stomach is a 'j'-shaped organ, with two openings- the oesophageal and the duodenal- and four regions- the cardia, fundus, body and pylorus. Each region performs different functions; the fundus collects digestive gases, the body secretes pepsinogen and hydrochloric acid, and the pylorus is responsible for mucus, gastrin and pepsinogen secretion.
The stomach has five major functions;
· Temporary food storage
· Control the rate at which food enters the duodenum
· Acid secretion and antibacterial action
· Fluidisation of stomach contents
· Preliminary digestion with pepsin, lipases etc.
The Stomach - Histology
· G- mucosa containing glandular tissue; different areas of the stomach contain different types of cells which secrete compounds to aid digestion. The main types involved are:
o parietal cells which secrete hydrochloric acid
o chief cells which secrete pepsin
o enteroendocrine cells which secrete regulatory hormones.
· MM- muscularis mucosae
· SM- submucosa
The stomach contains three layers of involuntary smooth muscle which aid digestion by physically breaking up the food particles;
o OM- inner oblique muscle
o CM- circular muscle
o LM- outer longditudional muscle
The Small Intestine (1)
The small intestine is the site where most of the chemical and mechanical digestion is carried out, and where virtually all of the absorption of useful materials is carried out. The whole of the small intestine is lined with an absorptive mucosal type, with certain modifications for each section. The intestine also has a smooth muscle wall with two layers of muscle; rhythmical contractions force products of digestion through the intestine (peristalisis). There are three main sections to the small intestine;
· The duodenum forms a 'C' shape around the head of the pancreas. Its main function is to neutralise the acidic gastric contents (called 'chyme') and to initiate further digestion; Brunner's glands in the submucosa secrete an alkaline mucus which neutralises the chyme and protects the surface of the duodenum.
· The jejunum
· The ileum. The jejunum and the ileum are the greatly coiled parts of the small intestine, and together are about 4-6 metres long; the junction between the two sections is not well-defined. The mucosa of these sections is highly folded (the folds are called plicae), increasing the surface area available for absorption dramatically.