ANATOMY AND PHYSIOLOGY

THE GASTRO-INTESTINAL SYSTEM

HOW IT WORKS

THE MOUTH – Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant.

Oesophagus: A simple conduit between the mouth and stomach. Does not require gravity to conduct the food due to active peristalsis.

The Mouth

Saliva

It is composed of water, electrolytes, enzymes – salivary or alpha amylase, lysozyme, lingual lipase bacterial cells and mucous.

Lubrication and binding of foodSolubilizes dry food to be tastedOral hygieneInitiates starch digestion

The Stomach Stomach: Where the real action begins - enzymatic

digestion of proteins initiated and foodstuffs reduced to liquid form. The inner side is responsible for secretion and the longer outer side for peristalsis

Functions of stomach- reservoir as it allows for ingestion of food faster than

it can be digested and absorbed, kills some bacteria and parasites, begins process of digestion by exposing food to low

ph and pepsin- esp. on protein and mixing action- break up of food particles, delivers

the gastric contents now called chyme to the duodenum

Gastric juice

It contains pepsin, rennin, HCL, mucous and intrinsic factor

Partial protein digestion by pepsin, it is aided in this by hydrochloric acid

Absorption of vitamin B12 with intrinsic factorRennin aids the digestion of milk proteins.Mucus secreted by the gastric glands helps

protect the stomach lining from the action of gastric juice.

Bile

Bile is secreted by the gall bladder. After being secreted by the gall bladder it travels in the bile duct which opens into the duodenum.

Bile juice contains bile salt that helps to emulsify fats, that is they break them down into small globules which are easily digested by the enzyme lipase present in the pancreatic juice.

The LiverLiver: The centre of metabolic activity in the

body - its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats. The liver is the main organ of metabolism and energy production; its other main functions include:

Bile production Storage of iron, vitamins and trace elements detoxification conversion of waste products for excretion

by the kidneys

The Pancreas

It provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein.

both an endocrine and exocrine organ

The Pancreas

Pancreatic juice

1.Trypsinogen - trypsinogen is an inactive form of trypsin, it is activated by the enzyme enterokinase present in the intestinal juice. Trypsin has the same function like that of Pepsin, that is it breaks down protein to smaller peptide fragments.2. Chymotrypsin - same function as that of trypsin.3. Steapsin (Pancreatic Lipase) - Steapsin acts on fats converting them into fatty acids & glycerol.4. Carboxypolypeptidase - This enzyme converts peptides into amino acid5. Pancreatic Amylase - Digests starch

The Small Intestine

This is where the final stages of chemical enzymatic digestion occur and where almost all nutrients are absorbed into the portal system from the villi in the lining.

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;

The jejunum and the ileum are the greatly coiled parts of the small intestine, and together are about 4-6 metres long.

Blood containing digestive products from the small intestine is taken to the liver via the hepatic portal system.

Small Intestines

Intestinal juices

1. Entirokinase - entirokinase activates trypsinogen to trypsin.

2. Eripsin - Eripsin converts polypeptides to amino acids.

3. Maltase - Digests Maltose to glucose.4. Sucrase - Digests sucrose into glucose

and fructose.5. Lactase - Digests lactose into glucose and

galactose.

The Large Intestine

Main functions are:water is absorbed, bacterial fermentation takes place faeces are formed passing it into the rectum

to be expelled from the body through the anus.

The large intestine also contains areas of lymphoid tissue; these can be found in the ileum too (called Peyer's patches), and they provide local immunological protection of potential weak-spots in the body's defences.

Large Intestines

GI Hormones

The classical GI hormones are secreted by epithelial cells lining the lumen of the stomach and small intestine. GI hormones are secreted into blood, and hence circulate systemically, where they affect function of other parts of the digestive tube, liver, pancreas, brain and a variety of other targets.

Deglutition

Swallowing, known scientifically as deglutition, is the reflex in the human body that makes something pass from the mouth, to the pharynx, into the esophagus, with the shutting of the epiglottis.

oral preparatory – MASTICATION or chewing, oral- use of tongue and in co-ordination with cranial

nerves pharyngeal- using the cartilages of the larynx to force the

bolus via peristalsis into the oesophagus, Esophageal-The upper oesophageal sphincter relaxes to

let food past, after which various striated constrictor muscles of the pharynx as well as peristalsis and relaxation of the lower esophageal sphincter sequentially push the bolus of food through the esophagus into the stomach.

Peristalsis

It s the rhythmic contraction of smooth muscles to propel contents through the digestive tract.

Peristaltic movement is initiated by circular smooth muscles contracting behind the chewed material to prevent backflow, followed by a contraction of longitudinal smooth muscles which pushes the digested food forward.

This process works in one direction only and its sole purpose is to move food from the mouth into the stomach, and then from stomach into the intestines.

Reverse peristalsis or vomitting

Defecation

It is the act or process by which organisms eliminate solid or semisolid waste material (feces) from the digestive tract via the anus.

The rectum ampulla (anatomically also: ampulla recti) acts as a temporary storage facility for the unneeded material. As the rectal walls expand due to the material filling it, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate.

The rectum shortens as material is forced into the anal canal and peristaltic waves push the feces out of the rectum. The internal and external anal sphincters allow the feces to be passed by muscles pulling the anus up over the exiting feces.

PROTIENS

Proteins are polymers of amino acids linked together by peptide bonds. Chain length varies tremendously and many dietary proteins have been modified after translation by addition of carbohydrate (glycoproteins) or lipid (lipoprotein) moieties.

Very short proteins, typically 3 to 10 amino acids in length, are called peptides.

Enzymes that hydrolyze peptide bonds and reduce proteins or peptides to amino acids are called proteases or peptidases.

LIPIDS

True fatty acids possess a long hydrocarbon chain terminating in a carboxyl group.

The most abundant storage form of fat in animals and plants, and hence the most important dietary lipid, is neutral fat or triglyceride

Triglycerides cannot be efficiently absorbed, and are enzymatically digested by pancreatic lipase into a 2-monoglyceride and two free fatty acids, all of which can be absorbed.

CARBOHYDRATES

Monosaccharides or simple sugars are either hexoses (6-carbon) like glucose, galactose and fructose, or pentoses (5-carbon) like ribose.

Disaccharides are simply two monosaccharides linked together by a glycosidic bond. E.g.- lactose, sucrose & maltose

Polysaccharides are the most abundant dietary carbohydrate

Starch is a major plant storage form of glucose. It occurs in two forms: alpha-amylose, in which the glucoses are linked together in straight chains, and amylopectin, in which the glucose chains are highly branched.

Cellulose is the major constituent of plant cell walls, and more than half of the organic carbon on earth is found in cellulose and which no vertebrate has the capacity to enzymatically digest.

Digestion of electrolytes and fluid

Sodium diffuses into enterocytes down it's concentration gradient, but is also actively taken up in co-transport with both glucose and amino acids. It is also co-transported with other ions such as chloride, or exchanged such as with hydrogen ions.

Sodium ion absorption drags other ions such as chloride with it, electrical effect.

Water also passes through the tight junctions by bulk flow, following the movement of ions.

Digestion of Vitamins

The vitamins can be separated into the fat soluble and water soluble classes. All the vitamins are absorbed by passive diffusion

Special cases-Vitamin B1- Na dependantVitamin C- Na dependantVitamin B12 – intrinsic factor from

stomach to form complex that is absorbed in the ileum.

Calcium & Iron

The absorption of calcium from the gut is dependent upon parathyroid hormone.

Iron uptake in the gut is mediated by an binding protein, transferrin. This protein binds two molecules of iron and is then taken up into the cell by receptor mediated pinocytosis.

Things to do

Separate from your food the carb, protein, fat, vitamin, mineral and fluid contents.

Observe time taken by your body to digest each food item by varying your diet.

Observe process of mastication and deglutition and try to visualize the process.

Observe amount of saliva secreted in mouth and all that causes its secretion.