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					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
    –   Digestion
    –   Absorbtion
    –   Elimination

   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.
Chemical Digestion:

   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.
Absorbtion, Continued:

   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
Digestive System
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
Cavity, Continued:
   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
Cavity, Continued:
   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):
Tooth Classification:
   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.
Tooth Classification:
   Molars – serve to pulverize and grind the broken-
    up food particles into a fine mash.
   Dentists use special terms to describe the surfaces
    of teeth:
   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.
Tooth Classification:
   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
    chewing food.
   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
    gum line.
   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.)
Salivary Glands:

   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 mouth.
Salivary Glands:
The Gastrointestinal
   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
    reflux disease.)
Stomach, continued:

   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
The Duodenum:
   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
    amino acids.
   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
    allow absorbtion.
   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
    the liver.
   It moves through the hepetic duct to the
    cystic duct and is stored in the gallbladder
    until needed.
   When fatty foods are ingested, the
    gallbladder contracts, forcing bile into the
    duodenum via the cystic duct and common
    bile duct.
The Pancreas and
Pancreatic Duct:
   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
    intestinal wall.
   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 Jenunum:

   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:
   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:
   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:

   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.
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
    descending colon.
Descending Colon:

   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.
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:
   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.
Elimination, Continued:

   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
Gastrointestinal Tract:

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