Digestive System
• The digestive system or the GI tract
starts with the mouth and proceeds to
the phyrynx, esophagus, stomach,
small intestine (duodenum, jejunum,
ileum), and then to the large intestine
(colon), rectum, and terminates at the
anus.
• Accessory Organs includes:
Salivary glands, Tongue, Teeth, Liver,
Gallbladder.
• The digestive System must accomplish
the following tasks:
• Ingestion
• Digestion: Physical or Chemical
• Absorption
• Egestion
 Oral cavity
• Teeth: grind and break down food to expose food to a
large surface area for enzyme action.
• Tongue: Rolls food into bolus (a ball of solid mass of
food) before swallowing.
• Salivary gland: secrets saliva in the buccal cavity.
• Saliva: A clear, tasteless, odorless, slightly acid (pH 6.8)
viscid fluid, consisting of the secretion from the parotid,
sublingual, and submandibular salivary glands and the
mucous glands of the oral cavity.
 Function of saliva
1- Preparation of food for swallowing
 Mucin acts as a good lubricant for swallowing.
 Mucin softens the mass of food.
 Saliva moistens the mouth and facilitates chewing
& mixing of food and thus in bolus formation &
deglutition.
2- Solvent actions
 Taste is a chemical sensation.
 Unless the substance be in solution the taste buds
cannot be stimulated.
 Saliva acts as a solvent.
3- Antibacterial or cleansing action
 Constant flow of saliva removes bacteria from any
area where bacteria can grow.
 Saliva contains several factors that can destroy
bacteria like several proteolytic enzymes like lysozyme,
thiocyanate ions.
 Saliva often contains significant amount of protein
antibodies that can destroy oral bacteria including
those that cause dental caries.
4- Speech
 Moistening of lips and mucous membranes by saliva
aids in speech.
 Decrease in salivary secretion cause impairment of
speech.
5- Digestive function :
 Saliva contain digestive enzymes such as salivary
amylase which aids a chemical breakdown of
polysaccharides to disaccharides.
ex. starch to maltose
 Also saliva contain mucous , a glycoprotein ,
which help in make a bolus.
 Presence of lingual lipase in very small amount.
6- Excretory function:
 Many substances both organic and inorganic are
secreted to saliva.
 In diabetes mellitus, glucose is secreted.
 In Nephritis, urea is secreted.
 Viruses of rabies and poliomyelitis.
 Alkaloids like morphine and antibiotics.
 Alcohol content of saliva has been used for
medicolegal purpose (alcohol test).
7- Regulation of water balance
 When liquid is lost from the body, salivary glands are
subjected to dehydration effect. This arouses a sense
of thirst.
8- Helps in heat loss
 Frothing found in animals (dog, sheep, etc.).
9- Removes irritating factor
 Saliva helps to remove the irritating factor by diluting
or neutralising it. It thus prevents injury to mucous
membranes.
 Applied
 Decrease or absence of salivary secretion 
Xerostomia
 Excessive salivary secretion  Sialorrhea (seen in
patients with cancer of esophagus).
Pharynx
Divided into three regions;
nasopharynx, the oropharynx,
and the laryngopharynx.
The pharynx plays a role in
both respiratory as well as
digestive system.
Function of the Pharynx:
• The pharynx is a common
passageway for air and food,
which is why pharynx has a dual
role.
•
The pharynx then opens into
two pathways, one that leads to
the esophagus or food passage
and the other trachea or air
passage.
Pharynx Function in Digestive System
• The moistened food bolus is moved to the back of the
mouth by the tongue and enters the pharynx.
• Here muscle contractions (circular muscle constrictions)
take place and the swallowing reflex is triggered.
• Part of the swallowing action takes place as a reflex
action, while part of it is under voluntary control.
• The food bolus is then pushed toward the esophagus,
which is a muscular tube extending from the esophagus.
This swallowing reflex prevents the food from entering in
the wind pipe or trachea.
• The flap like epiglottis covers the larynx which prevent
food from enters the trachea. The contraction of the
longitudinal muscles in the walls of the pharynx lifts the
walls of the pharynx during swallowing.
Pharynx Functions
•
•
Vocalization
The process by which humans are able to make vocal sounds and speak
is called vocalization. When air passes through the pharynx and then into
larynx, it causes the vocal cords in the larynx to vibrate, thereby helping
in production of sound, which is used for speech in humans.
•
Equalizing Air Pressure in Middle Ear
Auditory tubes connect the middle ear to the pharynx at an opening in
the nasopharynx region. This opening opens and closes, thereby
equalizing air pressure in the middle ear to that of the outside
atmosphere, so that sound conduction takes place properly.
Esophagus
• The esophagus is a narrow
muscular tube about 20-30
cm long which starts from the
end of pharynx, passes
through the thorax, and ends
at the beginning of stomach.
• There are a number of glands
lining the inner wall of the
esophagus that help in
keeping the passage moist
and also facilitate in
swallowing.
• The esophagus is playing a major
role in transporting food, saliva and
liquids to the stomach. This
function is carried out by a muscle
layers, lining the wall of the
esophagus, called sphincters.
• There are two parts of the
sphincters, the upper esophageal
sphincter and lower esophageal
sphincter. The upper sphincter is
usually closed, but opens when
food or liquid is swallowed.
• Food in the esophagus is carried to
the stomach by peristalsis, a
process where muscles contract to
push food through the esophagus
to the stomach.
• Sphincters also release certain
enzymes that help in the semi
digestion of food in the
esophagus.
• Even if the person is lying down
or is upside down, sphincters
enable the food to be pushed into
the stomach.
Abnormalities of Esophagus Function
 over consumption of carbonated drinks, spicy food, etc.
can lead to esophagus pain.
 Symptoms that appear during abnormal esophagus
function are chest pain, dysphagia or difficulty in
swallowing and heartburn.
 Another disorder that affects the esophagus function is
acid reflux disease. The stomach releases certain acids
during digestion. When the acid content increases, due to
over consumption of spicy or hot food stuff, it irritates
the lining of the esophagus. This occurs when the lower
esophageal sphincter that separates the stomach from
the lower esophagus does not function properly.
Stomach
• The human stomach is
located on the upper left
region of the abdominal
cavity and is a vital part
of the digestive system.
• It is a muscular and
hollow structure. It is
preceded by the
esophagus and followed
by duodenum.
• Two valves control the entrances of food into and out
of the stomach
1- Esophageal Sphincter - opening between esophagus
and stomach. It stops the acid in the stomach from
flowing back up into the esophagus.
2- Pyloric Sphincter – when relaxes, it opens the entrance
to the small intestine.
• A layer of mucus prevents the stomach from digesting
itself.
• Food in the stomach is converted into a thick acidic
liquid called chyme, which then moves into the small
intestine.
Stomach Structure
• Parts of the Stomach:
The stomach has five parts, which are named as cardia, fundus,
corpus (body), antrum and pylorus.
• Stomach Secretions:
• The inner surface of the stomach has different types of epithelial
cells that produce various secretions which aid the process of
digestion.
• This includes mucus cells that produce mucus, which is alkaline in
nature. This mucus prevents harm to the inner layer of the
stomach from acids.
• There are chief cells that produce an enzyme called pepsin (aids
in the stomach function of breaking down proteins)
• parietal cells that produce hydrochloric acid (fights
microorganisms and digests food).
• G cells also produce a hormone called gastrin.
What is the Function of the Stomach?
• The main function of the stomach is to breakdown the
large molecules into small parts to be easily absorbed.
• The chewed food reaches the stomach and gets mixed with the
acids and enzymes produced in the organ. This mixture is called
chyme, which is stored in the stomach and is released to the small
intestine in smaller amounts.
• The stomach is the organ that is an important part of the digestive
system. This is the organ that is responsible for digesting the food
that is carried from the mouth to the stomach through the
esophagus.
• The enzymes that are present in the stomach which digest the
food require an acidic pH to function properly. This is where the
hydrochloric acid in stomach comes into play.
• The main function of hydrochloric acid in stomach is to
provide an optimum pH for normal functioning of the
enzymes present there.
• For example, hydrochloric acid helps convert
pepsinogen to pepsin, (which is responsible for
breaking down proteins in the stomach).
• How Does the Stomach Prevent Itself from Digestion?
• There is a thick mucus protective layer which prevents
HCl from harming the cells of the stomach.
Furthermore, sodium bicarbonate buffering aids in
preventing damage to the cells of the stomach.
Movements in Stomach
Small Intestine
• Has a tiny projections called villi which absorb the
digested food into the capillaries.
• Participate in digestion of proteins, lipids and
carbohydrates partially.
• Divided into three parts:
1. Duodenum – upper part; connected to the stomach.
The duodenum is also where the bile and pancreatic
juices enter the intestine.
2. Jejunum.
3. Ileum.
Movement in small intestine:
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•
•
•
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Mixing: Segmental contraction that occurs in small intestine
Secretion: Lubricate, liquefy, digest
Digestion: Mechanical and chemical
Absorption: Movement from tract into circulation or lymph
Elimination: Waste products removed from body
Large Intestine
• The large intestine (colon) extends from the end of
the ileum to the anus.
• divided into the cecum, colon, rectum, and anal
canal.
• There are different kinds of bacteria in the colon,
most of them are helpful, where they can synthesis
vitamins as B12 and K. also they can breakdown
toxins and prevent the proliferation of harmfull
organisms. They also stimulate the immune system
to produce short chain fatty acids which can
prevent the incidence of colon cancer.
• Water is absorbed from the undigested food
making the waste harder until it becomes solid.
Accessory Organs
Pancreas
• It has a dual function:
1. Exocrine: where it produce enzymes that break
down all categories of digestible foods
(trypsinogen, chymotrypsinogen, pancreatic lipase,
and amylase ).
2. Endocrine: where it secrete hormones by islets of
Langerhans as follow:
• Beta cells--- producing insulin.
• Alpha cells--- producing glucagon.
• Delta cells--- producing somatostatin.
• PP cells--- producing pancreatic polypeptide.
• Gall bladder
• It is connected to the liver and the duodenum by
biliary tract.
• Stores bile, which is released when food containing
fat enters the digestive tract, stimulating the
secretion of cholecystokinin (CCK).
• Bile is an emulsifier NOT an enzyme. Emulsifier –
dissolves fat into the watery contents of the
intestine.
Functions of liver
1. The liver produces and excretes bile requires for
dissolving fats. Some of the bile drains directly into the
duodenum, and some is stored in the gallbladder.
2. The liver performs several roles in carbohydrate, protein
and lipid metabolism.
3. The liver produces coagulation factors I (fibrinogen), II
(prothrombin), V, VII, IX, X and XI, as well as protein C,
Protein S and antithrombin.
4. The liver breaks down hemoglobin, creating
metabolites that are added to bile as pigment.
5. The liver breaks down toxic substances and most
medicinal products in a process called drug
metabolism.
6. The liver converts ammonia to urea.
7. The liver stores a multitude of substances, including
glucose in the form of glycogen, vitamin B12, iron, and
copper.
8. In the first trimester fetus, the liver is the main site of
red blood cell production.
9. The liver is responsible for immunological effects the
reticuloendothelial system if the liver contains many
immunologically active cells, acting as a 'sieve' for
antigens carried to it via the portal system.
Path of Digestion
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•
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•
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•
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Mouth
Pharynx
Esophagus
Stomach
Small Intestine
Large Intestine
Anus
ABSORPTION AND TRANSPORT OF NUTRIENTS
• Digested molecules of carbohydrates, proteins,
lipids and vitamins as well as water and minerals
from the diet, are absorbed from the cavity of the
upper small intestine. Most absorbed materials
cross the mucosa into the blood and are carried off
in the bloodstream to other parts of the body for
storage or further chemical change.
How is the digestive process controlled?
The hormones that control digestion are:
•Gastrin causes the stomach to produce an acid for dissolving and
digesting some foods. It is also necessary for the normal growth of
the lining of the stomach, small intestine, and colon.
•Secretin causes the pancreas to send out a digestive juice that is
rich in bicarbonate. It stimulates the stomach to produce pepsin, an
enzyme that digests protein, and it also stimulates the liver to
produce bile.
•Cholecystokinin (CCK) causes the pancreas to grow and to produce
the enzymes of pancreatic juice, and it causes the gallbladder to
empty.
Nerve regulations
• Two types of nerves help to control the action of the digestive
system:
1. Extrinsic nerves cause muscle contractions that push food along
the digestive tract. They also cause the stomach and pancreas to
produce more digestive juice.
2. Intrinsic nerves within the walls of the esophagus, stomach, small
intestine, and large intestine react when their walls are stretched by
food. They cause the release of substances that speed up or slow
down the movement of food, and the production of juices, by the
digestive organs.