The Digestive System
Tri-State Business Institute
Micheal H. McCabe
The digestive system is also called the
alimentary canal and the gastrointestinal
The organs of the digestive system have
four primary functions:
Ingestion involves all behaviors associated
with the acquisition and consumption of
food and beverage.
Ingestion involves not only the organs and
structures of the alimentary tract, but the
entire organism – body and mind.
Ingestion is often governed as much by
social convention as by hunger.
Food is broken down mechanically in the
mouth by chewing – this increases the
surface area of the food and speeds
Chemical action serves to break down food
into its component parts. Complex
materials are broken down into simpler
Solvents, including water and strong acids,
dissolve nutrients and allow for absorbtion.
Enzymes – are chemicals that speed-up
chemical reactions and help in the
breakdown of complex nutrients.
Stomach Acids – primarily hydrochloric acid
– dissolve minerals, and break down
complex materials (like cellulose).
Bile – emulsifies fat to allow absorbtion.
Digested food must by absorbed into
the bloodstream by passing through
the walls of the small intestine.
Carbohydrates (sugars) and amino
acids are distributed by the
bloodstream throughout the body
where they provide energy and raw
materials to the individual cells.
Amino acids are used as raw materials to
build new protein structures within the cells.
Excess carbohydrates are stored within the
liver as starch (glycogen.)
Fats are broken down into fatty acids and
glycerol. Fatty acids are then stored in
adipose tissue as an energy reserve.
Many materials that are ingested
cannot be absorbed.
These materials are considered solid
The large intestine (colon) collects and
concentrates this waste (called feces).
Wastes ultimately pass from the body
via the anus (defecation.)
Anatomy of the
The Oral Cavity:
The oral cavity (mouth) is the first part
of the digestive system.
Food is ingested (eaten) and the
digestive process begins within the
Mastication (chewing) is the first step
in the mechanical breakdown of
Anatomy of the Oral
Cheeks – form the lateral walls of the mouth.
Lips – surround the opening of the mouth.
Hard Palate – forms the anterior portion of the
roof of the mouth.
Soft Palate – muscular structure forming the
posterior portion of the roof of the mouth.
Rugae – are irregular ridges in the mucous
membrane covering the anterior portion of the hard
Uvula – small appendage of the soft palate.
Serves as an accessory organ for speech and acts
as a sense organ in swallowing.
Anatomy of the Oral
Tongue – large muscular organ located on the
floor of the oral cavity and attached to the
mandible by muscles. Moves food around during
mastication and swallowing.
Mastication – is the act of chewing.
Deglutition – is the act of swallowing.
Papilla – are small raised areas on the tongue that
contain taste buds; specialized sense organs that
respond to the chemical composition of food.
Tonsils – are masses of lymphatic tissue located
on both sides of the oropharynx.
Anatomy of the Oral
Gums – are made of fleshy tissue and surround
the sockets of the teeth.
Teeth – are specialized structures of several types
that are used to cut, pierce, and grind food during
A complete set of adult dentition includes 32
permanent teeth. “Milk Teeth” – sometimes
present in newborns are not true teeth – these are
specialized structures of cartilage. “Baby Teeth”
are called deciduous teeth and are replaced by
larger adult teeth between the ages of 5-10 years.
Diagram of the Oral
The Dental Arch (Upper):
Central Incisors – are designed to cut food – the
have a sharp chisel-shaped edge that allows you to
sever a portion of food from a larger mass.
Lateral Incisors – also serve to cut food, the
arch-shaped arrangement of the central and lateral
incisors allows a discrete “bite” of food to be taken
from a larger mass.
Canines – are pointed and provide the ability to
pierce through tough membranes present in food.
Sometimes called “Fangs” or “cuspids”– they serve
as a killing instrument in carnivorous animals.
Bicuspids – Also called pre-molars – these teeth
serve to crush food and break fibers up.
Molars – serve to pulverize and grind the broken-
up food particles into a fine mash.
Dentists use special terms to describe the surfaces
Labial – is used to describe the surface of incisors
and canine teeth adjacent to the lips.
Buccal – describes the surface of bicuspid and
molar teeth nearest the cheek.
Some dentists use the term facial to describe both
the labial and buccal surfaces.
Opposite the facial side, all teeth have a lingual
surface near the tongue.
The mesial surface of a tooth is the face
nearest the midline of the body.
The distal surface of a tooth is the face
farthest from the medial line.
Bicuspids and Molars have an additional
surface called the occlusal surface. This is
where the teeth come together when
Incisors and Cuspids both have a sharp
incisal edge. That serves as the cutting
Inner Anatomy of a
Inner Anatomy of a
Crown – the portion of the tooth visible
above the gum line.
Root – the portion of the tooth below the
Enamel – is the dense, hard, white
substance that forms the outermost
protective layer of the crown. Enamel is the
hardest substance in the human body.
Inner Anatomy of a
Dentin – forms the main substance of the
tooth. Dentin is a yellow substance, similar
to bone, that lies beneath the enamel and
extends throughout the crown.
Pulp – lies beneath the dentin. Pulp is a
soft and delicate tissue that forms the
center of the tooth. Blood vessels, nerve
endings, connective tissue, and lymph tissue
are all found in the pulp canal (also called
the root canal.)
Three pairs of salivary glands surround
the oral cavity.
These glands produce Saliva that
contains important digestive enzymes
such as salivary amylase that begin
chemical digestion of food while still in
The pharynx (throat) is a muscular tube
appx. 5 inches long, lined with mucous
The pharynx serves as a common
passageway for both air traveling from the
nose to the trachea, and food traveling from
the oral cavity to the esophagus.
When swallowing (deglutition) occurs, a flap
of tissue, the epiglottis, covers the trachea
so that food can’t enter.
The esophagus is a muscular tube
extending from the pharynx to the stomach.
Rhythmic contractions of muscles in the wall
of the esophagus propel food towards the
Peristalsis is the name given to this
progressive, involuntary, rhythmic
contraction of smooth muscle observed in
most of the organs in the digestive system.
Food passes from the esophagus into the
stomach through the cardiac (esophageal)
The cardiac sphincter normally closes after
passing a bolus of food into the stomach –
this prevents gastric reflux (heartburn.)
Abnormalities with the cardiac sphincter
may result in “GERD” (gastroesophageal
Within the stomach, gastric acid
(primary hydrochloric acid) and
enzymes breakdown food particles into
simpler substances that can later be
absorbed by the intestines.
The stomach “churns” – rhythmically
contracts to thoroughly mix food with
stomach acids and enzymes.
Anatomy of the Stomach:
Yet More About the
The stomach is a hollow, muscular organ
that serves as a “staging area” where
digesting food is held prior to passage into
the small intestine.
Rugae is a specialized tissue present in the
stomach, urinary bladder, and similar hollow
organs that allows the stomach to expand in
size without causing injury.
Specialized cells in the stomach wall
produce hydrochloric acid and digestive
After food has been thoroughly mixed with
stomach acid, the mixture (now called
chyme) passes through the pyloric sphincter
into the duodenum.
Within the duodenum, bile and pancreatic
juice are added to the chyme to emulsify
fats and break proteins up into the 29 basic
The duodenum is the shortest segment of
the small intestine; normally about a foot in
Accessory organs of
The liver is a multi-purpose organ that
produces bile as one of its principal
Bile is a “salt” made from acid and alkali
compounds that serves to emulsify fats to
The pancreas is also a multi-function organ
that produces digestive enzymes needed to
break down complex proteins into
component amino acids.
The Gallbladder and Bile
Bile is manufactured by specialized cells in
It moves through the hepetic duct to the
cystic duct and is stored in the gallbladder
When fatty foods are ingested, the
gallbladder contracts, forcing bile into the
duodenum via the cystic duct and common
The Pancreas and
Digestive enzymes are made by specialized cells in
the pancreas and secreted via the pancreatic duct
into the duodenum.
The pancreatic duct communicates with the
common bile duct and duodenum – infection or
inflammation within any of these organs is readily
transmitted to the other two.
Example: pancreatitis resulting from infection can
present clinically with jaundice as the gallbladder
and liver become involved.
Likewise, gallstones or inflammation of the
gallbladder can result in inflammation of the
Absorbtion of Nutrients in
the Small Intestine.
Absorbtion of nutrients begins in the duodenum as
chyme moves past millions of microscopic villi.
The villi resemble small fingers extending from the
Each villi contains a rich bed of capillaries that
serve to absorb nutrients and carry it into the
Each villi also contains a specialized lymph vessel
called a lacteal that serves to absorb emulsified fat
and conduct it into the lymphatic circulation.
The second segment of the small
intestine is called the jejunum.
The jejunum is appx. 8 feet long and
continues the process of digestion and
Chyme is passed from the jenunum
into the third section of the small
The ileum is the third section of the small intestine
and is the last section of the gastrointestinal tract
to be concerned with the absorbtion of nutrients.
The ileum is also the longest section of the small
intestine and is approximately 11 feet long.
By the time that chyme leaves the small intestine
via the ileocecal valve, most water-soluble and
emulsified nutrients including carbohydrates,
protein, and fat have been absorbed.
The remaining matter consists largely of waste and
is called feces, or stool, once it enters the large
The Large Intestine:
The large intestine is divided anatomically
into seven parts:
– The cecum
– The appendix
– The ascending colon
– The transverse colon
– The descending colon
– The sigmoid colon
– The rectum
The cecum is a pouch on the lower end of
the ascending colon.
Fecal matter enters the cecum through the
ileocecal valve where the cecum
communicates with the ileum.
The principal function of the large intestine
is recovery of water, minerals, bile, and
enzymes from the waste stream.
“Recover, Recycle, and Reuse.”
The appendix has no clear function in
If clogged or blocked by fecal matter,
the appendix can become infected and
inflamed – “appendicitis.”
Rupture of an inflamed appendix can
result in peritonitis, sepsis, and death.
The Ascending Colon:
The ascending colon extends upward
from the cecum (lower right side) to
the undersurface of the liver.
Below the liver, a 90 degree bend in
the colon (called the hepatic fixture)
occurs and fecal matter passes into
the transverse colon.
The transverse colon passes across
the abdominal cavity towards the
A second 90 degree bend in the colon
occurs near the spleen (splenic fixture)
and fecal matter progresses into the
From the splenic fixture, fecal matter
moves downward through the
descending colon toward the pelvic
At the pelvic crest, the descending
colon makes an “S” turn and becomes
known as the sigmoid colon.
The “S” shaped section of the colon
serves to carry fecal matter from the
abdominal cavity into the pelvic cavity.
The lower (distal) end of the sigmoid
colon connects with the rectum.
The rectum is an expansive, muscular organ
that serves to store fecal matter until it can
be expelled from the body.
The opening of the rectum is called the
Anus and is equipped with two sphincters –
the inner sphincter is involuntary and
controlled by the autonomic nervous
The outer sphincter is under voluntary
control – to a point.
The elimination reflex is triggered by a
sense of pressure in the rectum.
When the sensation of pressure in the
rectum becomes apparent, the inner
(involuntary) sphincter relaxes and the need
to pass stool becomes “urgent.”
Voluntary control of the external sphincter
allows reasonable selectivity regarding the
time and place of elimination.
The extent of voluntary control is
limited in scope and duration.
If voluntary defecation isn’t
undertaken, stool continues to collect
in the rectum and pressure increases.
Beyond a certain point, voluntary
control of the external sphincter is lost
and defecation occurs automatically.
Diagram of the