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 food
Solubilizes dry food to be tasted
Oral hygiene
Initiates 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 factor
 Rennin 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 Liver
 Liver: 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 acid
5. Pancreatic Amylase - Digests starch
The Small Intestine
 This is where the final stages of chemical enzymatic
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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
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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-
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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 dependant
Vitamin C- Na dependant
Vitamin 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.