The Gastrointestinal System

 

The gastrointestinal system is essentially a long tube running through the body with specialized sections that are capable of digestive material put in the mouth and extracting any useful components from it, then expelling the waste products from the anus.

Food after ingestion undergoes three types of processes in the body:

  • Digestion
  • Absorption
  • Excretion

The entire GI system is under hormonal control with the presence of food in the mouth triggering a cascade of hormonal actions. When food reaches the stomach, different hormones activate acid secretion, increased gut motility, enzyme release etc.

Nutrients from the GI tract are not processed on- site but instead will be absorbed and taken to the liver through the blood circulation to be broken down further, stored, or distributed.

Esophagus

Once food is chewed and mixed with saliva in the mouth, it is swallowed and passes down the esophagus.  The esophagus has a stratified squamous epithelial lining (SE) which protects the esophagus from trauma.  The submucosa lining (SM) secretes mucus from mucous glands (MG) which aid the passage of food down the esophagus.  The esophageal wall muscle layer helps to push the food into the stomach by waves of motion called peristalsis.

Stomach

The stomach is a ‘J’-shaped organ with two openings- the esophageal and the duodenal- and four regions- the cardia, fundus, body and pylorus. Each region performs different functions including mixing of the food with digestive enzymes and strong acid. The layer of mucus produced prevents the stomach from digesting itself.

The stomach’s major functions are:Temporary food storageControl the rate at which food enters the smallintestine

  • Acid secretion and antibacterial action
  • Fluidization of stomach contents
  • Preliminary digestion with pepsin, lipases etc.

Small intestine

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 occurs.The whole of the small intestine is lined with an absorptive mucosal layer, 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 (peristalsis). 

There are three main setions to the small intestine:

The duodenum forms a ‘C’ shape around the head of the pancreas. Its main function is to neutralize the acidic gastric contents (called ‘chyme’) and to initiate further digestion; Brunner’s glands in the submucosa secrete alkaline mucus which neutralizes the acidic chyme of the stomach and protects the surface of the duodenum.

The jejunum and the ileum are greatly coiled parts of the small intestine, and together are about 4-6 meters long; the junction between the two sections is not wll-defined. The mucosa of these sections is highly folded (the folds are called plica), increasing the surface area available for absorption dramatically.

Large intestine

The large intestine is the last part of the digestive tube and the location of the terminal phases of digestion. It is the part of the digestive tube between the terminal small intestine and anus. Within the large intestine, three major segments are recognized:

The cecum is a blind-ended pouch that in humans carries a worm-like extension called the vermiform appendix.

The colon constitutes the majority of the length of the large intestine and is sub-classified into ascending, transverse, and descending segments.

The rectum is the short, terminal segment of the digestive tube, continuous with the anal canal.

Functions of the Large Intestine

Recovery of water and electrolytes from digested food:

A considerable amount of water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large intestine. This is what goes wrong when you have diarrhea and constipation

Formation and storage of feces:

As digested food passes through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces.

Microbial fermentation:

Fermentation is the enzymatic decomposition and utilization of foodstuffs, particularly carbohydrates, by microbes. The large intestine does not produce its own digestive enzymes, but contains huge numbers of bacteria which have the enzymes to digest and utilize many substrates.

 

 

 

The duodenum forms a ‘C’ shape around the head of the pancreas. Its main function is to neutralize the acidic gastric contents (called ‘chyme’) and to initiate further digestion; Brunner’s glands in the submucosa secrete alkaline mucus which neutralizes the acidic chyme of the stomach and protects the surface of the duodenum.
The jejunum and the ileum are greatly coiled parts of the small intestine, and together are about 4-6 meters long; the junction between the two sections is not wll-defined. The mucosa of these sections is highly folded (the folds are called plica), increasing the surface area available for absorption dramatically.