Introduction to the
Digestive System
For student copy
Digestive system & homeostasis
• Digestive system contributes to
homeostasis of body by breaking down
food into forms that can be absorbed &
used by body cells.
• also:
– absorbs :
• water, vitamins, minerals
– eliminates:
• wastes
2 groups of organs:
• aka Alimentary Canal
• continuous tube
mouth  anus
• mouth
• most of pharynx
• esophagus
• stomach
• small & large
intestine
•
•
•
•
•
•
GI Tract
Accessory Organs
teeth
tongue
salivary glands
liver
gallbladder
pancreas
vocabulary
• Ingestion: eating
• Secretion: ~7 liters water, acids, buffers, &
enzymes into lumen of GI tract
• Motility: contraction/relaxation of smooth
muscle in wall of GI tract mix & propel
food & secretions  anus
• Mechanical Digestion: physically breaking
down food
• Chemical Digestion:
• Absorption
• Defecation: wastes leave body/ material
defecated called feces/ gas called flatus
Layers of the gi tract
(inner  outer)
1. mucosa
2. submucosa
3. muscularis
4. serosa
Mucosa: 3 layers
1. epithelium
–
–
–
–
mouth  esophagus: stratified sq. epith for
protection
stomach  intestines: simple columnar
cells slough off q 5-7 d
exocrine mucus glands (mucus & water) & several
types endocrine glands called enteroendocrine
cells interspersed
Mucosa: 2nd layer:
lamina propia
• areolar CT
• rich in blood & lymph
vessels
• contains most of
MALT (mucosaassociated lymphatic
tissue)
Mucosa: 3rd layer:
muscularis mucosa
• thin layer of smooth
muscle
• creates small folds in
epithelium 
increases surface area
for digestion &
absorption
submucosa
• areolar CT that binds mucosa to
muscularis
• rich blood & lymphatics, glands
• submucosal plexus: extensive network
of neurons
Muscularis
• skeletal muscle in mouth, pharynx,
superior & middle parts of esophagus,
external anal sphincter
– voluntary swallowing & defecation
• 2 sheets smooth muscle in rest of GI tract
with myenteric plexus between them
1. outer longitudinal
2. inner circular
Serosa
• parts of GI tract suspended in
abdominopelvic cavity have this
superficial layer = visceral peritoneum
• esophagus lacks a serosa; has adventitia
a single layer of areolar CT
Innervation of the
gi tract
Enteric Nervous System
(ENS)
• 100 million neurons that extends from
esophagus  anus
• 2 plexuses:
1. myenteric plexus (plexus of Auerbach)
–
motor neurons of longitudinal & circular
muscle
2. submucosal plexus (plexus of Meissner)
–
w/in submucosa supplying the secretory cells
ANS
• parasympathetic fibers via X to most of GI tract
(not to 2nd ½ large intestine: gets it from sacral
spinal cord)
– increase secretions & motility
• sympathetic fibers from thoracic & upper lumbar
spinal nerves
– decrease secretions & motility
Peritoneum
• largest serous membrane of body
• simple squamous epithelium with
underlying areolar CT
• divided:
1. parietal peritoneum
2. visceral peritoneum
Peritoneal Cavity
• space between parietal & visceral peritoneums
– contains serous fluid: decreases friction
– Ascites: excess serous fluid ass’c w/ some diseases
Retroperitoneal
• “behind peritoneum”
• kidneys & most of pancreas, end of sigmoid
colon under parietal peritoneum
5 Major Peritoneal Folds
1.
2.
3.
4.
5.
greater omentum
falciform ligament
lesser omentum
mesentery
mesocolon
Greater Omentum
• largest peritoneal fold
• “fatty apron”
• drapes over transverse colon & coils of sm
intestine
• extends downward anterior to sm intestine
• contains adipose cells, macrophages, plasma
cells;
• function to combat & contain infections
Greater omentum
Falciform Ligament
• attaches liver to
anterior
abdominal wall
& diaphragm
– liver only organ
in
abdominopelvic
cavity attached
to anterior abd
wall
Lesser Omentum
• arises as 2 folds in serosa of stomach &
duodenum
Mesentery
• fold of peritoneum
that attaches small
intestine to
posterior
abdominal wall
– starts @ posterior
wall  wraps
around sm
intestine 
reflects back to
posterior abd wall
– route of bld
supply to sm
intestine
Mesocolon
• binds large intestine
to posterior abd wall
• route of blood supply,
lymphatic drainage
Peritonitis
• acute inflammation of the peritoneum:
contamination by infectious microbes
– from punctures of abdominal organs
• trauma
• surgical wounds