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Digestive System
Ch. 14
Digestive System
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The digestive system has a simple job, gather
nutrients.
There are two divisions:
Alimentary – Ingests, digests, absorbs, defecates
(stomach, intestine, etc) aka gastrointestinal tract
 Accessory – helps the alimentary organs complete
their goal. (teeth, pancreas, etc)
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Gastrointestinal Tract
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A twisty, hollow tube that runs continuously
from the mouth to the anus.
It is divided into sections, each with a specific
purpose.
Includes: Mouth, pharynx, esophagus, stomach,
sm. Intestine, and lg. Intestine.
The terminal opening is the anus.
Mouth
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Lips protect anterior
Cheeks form lateral walls
Hard and soft palate
form roof
Muscular tongue
occupies the floor of the
oral cavity.
Tongue attached to the
bottom of the mouth by
the lingual frenulum.
Mouth
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The area between your
cheeks/lips and your
teeth is called the
vestibule. The area
behind your teeth is the
oral cavity proper.
Two sets of tonsils –
palatine in the back,
lingual cover the back of
the tongue just further
along. When inflamed, it
hurts to swallow.
Mouth
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Mastication – food in the
mouth is chewed using
teeth to break it down
(give it more surface
area)
Salivary glands produce
saliva with amylase in it.
The tongue mixes the
food around like a
washing machine.
Teeth
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Teeth are used to rip and
grind food into small
bits.
From birth – 2yr our
milk teeth grow in, 20
total.
From 7yr – 25yr our
second set (permanent)
of teeth grow in.
Full set is 32. This
number changes based
on impaction and
wisdom teeth.
Teeth
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Teeth are named based
on their shape.
Incisors for cutting
canines for piercing and
ripping
premolars and molars for
grinding.
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Tooth
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Teeth have two major
areas, the crown and the
root.
The crown is covered in
enamel – hardest
substance in the body
and heavily mineralized
with calcium.
Gums or gingiva bridge
the gap between teeth.
Tooth
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Below the gum line is the
root. Held in place by
the periodontal
membrane and the
cementum.
Internally, the tooth is
made up of dentin, a
bone like material and
the pulp.
Tooth
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The pulp is a central
cavity filled with
connective tissue, blood
vessels and nerve fibers.
Cavities eat away enamel,
making the tooth weaker.
If the decomposition
gets too deep near the
pulp, a root canal is
performed.
Pharynx
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After the mouth, food enters the “throat” or
pharynx.
The walls of the pharynx contain two skeletal
muscle layers which begin peristalsis (more
later).
Esophagus
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10” tube that connects the throat to the stomach
Made of four layers (tunica)
Mucosa – innermost, lines the lumen, surface
epithelium and smooth muscle. Friction-resisting.
 Submucosa – soft connective tissue with blood
vessesls, nerves, lymph vessels and nodes.
 Muscularis Externa – Muscles for Peristalsis
 Serosa – serous fluid producing. Part of the
continuous visceral peritoneum that lines the
abdominopelvic cavity, including mesentary.
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Stomach
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On the left side of the abdominal cavity.
Approximately 10” long, diameter changes as
food fills it.
Can hold approximately 1 gallon of food
Separated from the esophagus by the
cardioesophageal sphincter and separated from
the Sm. Intestine by the Pyloric Sphincter.
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Stomach
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Stomach has three layers of muscles, running
longitudinally, transversely, and obliquely.
These three layers contract in rhythm to
constantly churn and pummel the food.
Proteins begin to be broken down here by
gastric juice.
Food stays here until it is the consistency of a
heavy cream. Called chyme.
Activity of the Stomach
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Produces Pepsin – enzyme that breaks down
protein.
Pepsin requires an acidic environment, stomach
also produces hydrochloric acid.
Stomach tissue is protected from this acid by
mucus. If no mucus, the stomach could eat
itself called an ulcer.
Stomach muscles constantly mix food with acid.
Once liquefied, Chyme, food enters the sm.
Intestine via the pyloric sphincter.
Small Intestine
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The major digestive
organ.
7 meters long
Can only process a small
amount at a time, kept in
stomach.
Three subdivisions
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Duodenum
Jejunum
Ileum
Duodenum
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The first and smallest section. Starts at the end
of the stomach, turns down and then transverse.
Makes a C Shape.
Pancreatic duct empties here, releasing
pancreatic digestive enzymes.
Bile duct from liver also empties into the
duodenum.
Activities of the Small Intestine
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The pancreas and gall bladder empty their
contents into the small intestine. This releases
enzymes and neutralizes the chyme back to
about pH 8
Enzymes
Pancreatic amylase, lactase, maltase and sucrase
finish the breakdown of carbs that was started in the
mouth
 Trypsin continues the breakdown of proteins
 Bile and pancreatic lipase begin to breakdown fats
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Sm. Intestine
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Nearly all food absorption occurs in the sm.
Intestine.
A large amount of surface area is produced
because the inside layer of the sm. Intestine is
highly folded (circular folds). These folds have
villi on them which are made up of cells with
microvilli. All of which make more surface area
for absorption.
Activities of the Small Intestine
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As chyme is constantly digested by these various
enzymes, nutrients are absorbed by microvilli of
the intestinal walls.
These nutrients are routed into the hepatatic
portal vein.
They are taken to the liver for detoxification
before being emptied into the blood stream.
Chyme left in the small intestine is pushed
through by peristalsis.
Large Intestine
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Its name refers to its diameter, not its length
(5ft)
It extends from the ileocecal valve of the sm.
Intestine to the anus.
Major function is to dry out the indigestible
food residue by absorbing water and to eliminate
the residue from the body as feces.
Sections: cecum, appendix, colon, rectum, and
anal canal.
Activities of the Large Intestine
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Chyme in the Lg. Intestine has very few
nutrients.
Bacteria living in the lumen can breakdown the
chyme releasing a few vitamins, but not much.
The liquid is absorbed by the lg. intestine, this
makes the feces more solid.
Accessory Digestive Organs
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Salivary Glands – make saliva – mucus and
serous fluids. Mucus moistens and binds the
food into a mass called the bolus.
Serous fluid contains enzymes amylase to break
down carbs. Also contains lysozyme and
antibodies to inhibit bacteria.
Accessory Digestive Organs
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Teeth
Pancreas – produces enzymes and alkaline fluid
to break down food and neutralize acidic chyme.
Liver – produces many enzymes, digestively,
bile. Emulsifies fats
Gallbladder – stores bile when digestion is not
occuring.
Six Steps of Digestion
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Ingestion
Propulsion
Mechanical Breakdown
Chemical Breakdown
Absorption
Defecation