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The Digestive System
& Body Metabolism
Chapter 15
Biology Ch. 38-2
I. Overview of the Digestive
System
A. Functions of the Digestive
System
• 1. Ingestion – consuming food
• 2. Digestion – breakdown of ingested food into
smaller molecules
• 3. Absorption – passage of nutrients into blood
• 4. Metabolism – converting nutrients into raw
materials for production of cellular energy
(ATP)
• 5. Elimination – removing indigestible wastes
B. Types of Digestion
• 1. Mechanical Digestion:
– Chewing, mixing,
churning, cutting, tearing,
or mashing of food.
2. Chemical Digestion
• Enzymes break down food molecules
into their building blocks
– Carbohydrates are broken down to
simple sugars
– Proteins are broken down to amino
acids
– Fats are broken down to fatty acids and
alcohols
II. Organs of the Digestive
System
• A. Two main groups of digestive
organs:
• 1. Organs along the Alimentary canal
(Gastrointestinal tract): continuous
coiled hollow tube over 30 ft. long
– Mouth, pharynx, esophagus, stomach,
small and large intestine, rectum
• 2. Accessory digestive organs: organs
that contribute secretions (fluids) &
enzymes to aid in digestion
– Salivary glands, liver, gall bladder,
pancreas
B. Alimentary Canal Organs
1. ORAL CAVITY (Mouth, Teeth, Salivary
Glands)
a. Mouth: Food enters the mouth where
digestion begins
–
–
•
Mechanical digestion: Mastication
(chewing) of food by teeth
Chemical digestion: Saliva contains the
enzyme amylase that breaks down
carbohydrates & starches in the mouth
Bolus – masticated food mixed with
saliva
Frontal View of Mouth
b. Teeth
•Function: chew food
•Humans have 2 sets:
1. Deciduous (baby or
milk) teeth
•20 teeth are fully
formed by age two
2. Permanent teeth
•Replace deciduous
teeth between the
ages of 6 to 12
•A full set is 32 teeth,
but some people do
not have wisdom
teeth
Classification of teeth
•Incisors – used for
biting food
•Canines – used for
tearing food
•Premolars (bicuspids)
– used for grinding
food
•Molars – Used for
crushing food
•Parts of a tooth
•Crown – exposed
part of tooth made of
enamel
•Neck – connects
crown to root
•Root – Periodontal
membrane attached
to the bone, contains
blood vessels and
nerves
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c. Salivary Glands



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Produce saliva mixture of mucus and
serous fluids
Helps to form a food
bolus
Contains salivary
amylase to begin starch
digestion
Dissolves chemicals so
they can be tasted
2. PHARYNX
•The passageway for air and food
•Has 3 parts:
•(highlight parts in diagram in your notes)
• Swallowing:
– Reflex that causes the epiglottis to move
blocking the trachea (windpipe)
– The esophagus opens to receive the bolus
3. Esophagus
• “Food tube” links to the
stomach through the
diaphragm
• Muscle layers contract
to squeeze food through
to the stomach
– Longitudinal inner layer
– Circular outer layer
Peristalsis – alternating waves of
muscle contraction
4. Stomach
• A. Structure & Path of
Food
– Thick-walled, J-shape,
muscular organ on left
side of abdominal cavity
– Bolus (food) enters
through the
cardioesophageal
sphincter
• Prevents backup of
stomach acid into
esophagus (acid reflux)
• Bolus goes through the
four regions of stomach:
– Cardiac region – near
heart
– Fundic region
– Body region – main
part of stomach
– Pyloric region – funnel
shaped end
• Chyme (mix of bolus and
digestive juices) exits
through pyloric sphincter
to small intestine
• Mechanical Digestion: Muscles of stomach
contract to churn and mix food with gastric
juices
• Deep folds called rugae allow stomach to
expand
• B. Coverings of the
Stomach
– Peritoneum –
membrane that covers
the abdominal wall and
organs
• Visceral peritoneum –
covers organ
• Parietal peritoneum –
covers cavity walls
– Mesentery holds the
stomach and intestines
in place in the
abdominopelvic cavity
• C. Gastric Juices
– Chemical Digestion:
Lining of stomach
secretes gastric juices
that break down food.
• Pepsin breaks down
proteins into amino acids
• Hydrochloric acid (HCl) is
a very strong acid (pH 1 or
2) activates enzymes and
kills bacteria
• Mucous protects the lining
of the stomach from the
acid environment
– Replaced every few days
• If the mucus layer is broken down by increased
acid, an ulcer may form
• Peptic Ulcer: open sores on the lining of the
digestive tract that may bleed and be very
painful
– Caused by bacteria (Heliobacter pylori), regular use
of pain relievers or other medications
Gastric Bypass
Lap-band Surgery
5. Small Intestine
• A. Overview
– Muscular tube
extending from
the pyloric
sphincter to the
ileocelcal valve
– Up to 22 feet long!
• Functions
– Main digestive organ – most digestion
occurs in the small intestine
• Carbohydrate and protein digestion is
completed
• Fat digestion begins
– Main site of nutrient absorption into the
blood
b. Structure
• Small intestine has 3 parts:
– Duodenoum
• First 25 cm, attached to the stomach
• Receives secretions from the liver
and pancreas to aid in digestion
– Jejunum
• Next 3 feet
• Contains folds and villi for absorption
– Ileum
• Last 6-7 feet
• Extends from jejunum to large
intestine
• Walls of the small
intestine
• Has 3 features that
increase the surface
area for maximum
absorption of nutrients
• 1. Circular foldspermanent transverse
folds in the mucosa
and submucosa
• 2. Villi – fingerlike
projections of the mucosa
– Goblet cells in villi produce
mucus to lubricate chyme
– Villi contain absorptive cells
& blood capillaries to
absorb nutrients
• 3. Microvilli – microscopic
extensions from the
plasma membrane
c. Chemical Digestion in the
Small Intestine
• Digestion is completed in the
small intestine
• Accessory organs aid in
digestion by secreting
enzymes into the small
intestine
– Bile from the gall bladder
• Breaks down lipids (fat)
– Pancreatic juices from the
pancreas
• Sodium bicarbonate
neutralizes the acid
d. Absorption in the Small
Intestine
• When food has been broken down to its nutrients &
waste products, the nutrients must be absorbed by
the body
• Most nutrient absorption occurs in the small
intestine
– Most substances are absorbed by active
transport through cell membranes
– Lipids are absorbed by diffusion
• After absorption, nutrients are transported to
the cells through the bloodstream
6. Large Intestine
• Once digestion & absorption of nutrients
is complete, the chyme moves into the
large intestine by peristalsis
• Also known as the Colon
• Large in diameter, but shorter than the
small intestine
a. Functions of the Large Intestine
• Absorption of water
• Does not participate in digestion of food
• Eliminates indigestible food from the
body as Feces
• Goblet cells produce mucus to act as a
lubricant
b. Structure
• Ascending Colon – includes the cecum
(saclike structure) with a small
projection called the appendix
• Transverse Colon
• Descending Colon
• Sigmoid Colon
c. Food Breakdown & Absorption
in the Large Intestine
• Bacteria breaks down indigestible
materials
– Source of odor
– Produce some vitamin K & B
– Release gases
• Water, vitamins K and vitamin B are
absorbed
• Remaining materials are eliminated as
feces
7. Rectum
• Feces moves from the sigmoid colon of
the large intestine to the rectum by
peristalsis
• Feces is stored in the RECTUM until the
involuntary internal anal sphincter is
triggered to open
• Defecation or expulsion of feces occurs
when the voluntary external anal sphincter
relaxes and the feces passes out of the
body
– Happens daily
– Triggered by presence of feces in rectum
8. Accessory Digestive Organs
• 1. Pancreas
– Produces a wide spectrum of digestive enzymes
that break down all types of food
– Enzymes are secreted into the duodenum of the
small intestine
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
– a) Secretions of the Pancreas
• 1) Pancreatic Amylase: helps complete
digestion of starch
• 2) Trypsin: carry out about half of all
protein digestion
• 3) Lipase: responsible for fat digestion
• 4) Nucleases: digest nucleic acids
• 5) Sodium Bicarbonate: neutralize acidic
chyme
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
– Enzymes are transported to the small
intestine through the Common Bile
Duct (shared with the liver)
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
2. Liver
• Largest organ in the
body
• Has 2 main lobes
– Larger right lobe
– Smaller left lobe
• The liver also
contributes to the
digestion by producing
bile
a. Bile
• Bile is a greenish substance that
emulsifies fat
• Stored in Gall Bladder
• Composed of bile salts & pigments
(biliruben from hemoglobin breakdown),
cholesterol, phospholipids, &
electrolytes
• Helps to digest fatty foods
b. Role of Liver in Metabolism
• Removes drugs and alcohol from blood
• Degrades hormones
• Produces cholesterol, blood proteins
(albumin and clotting proteins)
• Plays a central role in metabolism
• Stores products of digestion as
glycogen to be released as needed
3. Gall Bladder
• Sac found in hollow fossa of liver
• Stores bile produced by the liver
• Bile is introduced into the duodenum in
the presence of fatty food
• Gallstones can cause blockages
III. Control of Digestive Activity
• Mostly controlled by reflexes via the
parasympathetic division of nervous
system
• Chemical and mechanical receptors are
located in organ walls that trigger
reflexes
III. Control of Digestive Activity
• Stimuli include:
– Stretch of the organ
– pH of the contents
– Presence of breakdown products
• Reflex include:
– Activation or inhibition of glandular
secretions
– Smooth muscle activity
Digestion Animation
Processes of the Digestive System