Ch25: Digestive System
Objectives:
Discuss the general functions and anatomy
of the digestive tract
Describe the individual organs of the
system, including a discussion of the gross
and microscopic anatomy.
Digestive
System
consists of:
Muscular, hollow tube (= “digestive tract”)
+
Various accessory organs
Function
Individual parts
function in:
The function of the system as a
whole is processing food in such a
way that high energy molecules can
be absorbed and residues
eliminated.

ingestion

mechanical digestion

chemical and enzymatic digestion

secretion

absorption

compaction

excretion and elimination
Histological Organization
Tube made up of
four layers.
Modifications
along its
length as
needed.
1
2
Muscularis
3 externa
4
The 4 Layers of the Gut
Fig 25.2
1) Mucosa
Epithelium – usually simple columnar with goblets; may be stratified
squamous if protection needed
Lamina propria - connective tissue deep to epithelium
Muscularis mucosae -produces folds - plicae (small intestine) or
rugae (stomach)
2) Submucosa – made up of loose connective tissue contains
submucosal plexus and blood vessels
3) Muscularis externa – smooth muscle, usually two layers
(controlled by the myenteric plexus ) outer layer: longitudinal
inner layer: circular
4) Serosa
visceral layer of mesentery or adventitia depending on location
Membranes
Peritoneum - generic serous membrane in
abdominal cavity
Mesenteries - double sheets of peritoneum,
surrounding and suspending portions of
the digestive organs

Greater omentum - "fatty apron", hangs anteriorly from
stomach, double layer encloses fat

Lesser omentum - between stomach and liver

Mesentery proper - suspends and wraps the small
intestine

Mesocolon - suspends and wraps the colon, parts are
i. transverse mesocolon
Fig. 25.4
ii. sigmoid mesocolon
Oral Cavity

Also called buccal cavity - lined
with oral mucosa (type of
epithelium ?)

Hard and soft palates - form roof
of mouth

Tongue - skeletal muscle

Salivary glands - three pairs

Teeth
Three pairs of Salivary
Glands
1-1.5 l / day for
digestion (?)
lubrication (swallowing)
moistening (tasting)

Parotid – lateral side of face, anterior to ear, drain by parotid
duct to vestibule near 2nd upper molar
– mumps

Submandibular – medial surface of mandible – drain near
lingual frenulum drain posterior to lower molars

Sublingual – in floor of mouth - drain near frenulum
Structure of Teeth
Crown - exposed surface of tooth
Neck - boundary between root and crown
Fig 25.7
Enamel - outer surface
Dentin – bone-like, but noncellular
Pulp cavity - hollow with blood vessels and
nerves
Root canal - canal length of root
gingival sulcus - where gum and tooth
meet
Types and Numbers of Teeth
Dental succession
Deciduous (baby, milk) teeth - 20, replaced by
Permanent teeth - 32 teeth
Gross Anatomy of the Stomach
Lesser curvature
Greater curvature
Cardia - end under the heart
Fundus - bulge above the esophageal opening
Body - largest region
Pylorus - J curve, inferior end, terminates in
Cardiac and Pyloric sphincters (importance?)
Rugae – highly extendable interior folds
Figs 25-10/11
Histology of Stomach
Fig 25.13
Type of epithelium lining stomach?
Gastric pits – shallow pits, external half rapidly reproduces
for replacement
Gastric glands – deep in lamina propria, 3 types of cells
1.
Parietal cells (produce HCl and intrinsic factor)
2.
Chief cells (produce pepsinogen)
3.
Enteroendocrine cells – G cells (several hormones
including gastrin which stimulates both parietal and chief cells)
Regions of Small Intestine
SI is longest part of dig. tube

Duodenum (short, 12 inches)
– fixed shape & position
– Mixing bowl for chyme & ?

Jejunum (2.5 m long)
– Most of digestion

Ileum (longest at 3.5 m)
– Most of absorption, ends in

Ileocecal valve – slit valve into large intestine (colon)
Structure of Small Intestinal Wall
Fig 25.15
Plicae circulares – circular pleats around the interior of the
small intestine
Villi – minute finger-like projections, contain capillaries &
lacteals
Microvilli – sub-microscopic size, projections on single cells
Function of all three?
Intestinal glands (crypts)
– intestinal juice production
– Cell regeneration
Histology in lab
Regions of Large Intestine
Cecum – pocket at proximal end with
Appendix
Colon
Ascending colon - on right, between
cecum and right colic flexure
Transverse colon - horizontal portion
Descending colon - left side,
between left colic flexure and
Sigmoid colon - S bend near
terminal end
Fig 25-17
Rectum – terminal end is anal canal - ending at the anus which has internal involuntary sphincter and external voluntary
sphincter
Histology of Large Intestine
1. Mucosa - abundant goblet cells,
stratified squamous epithelium near anal
canal
2. No villi
3. Longitudinal muscle layer incomplete,
forms three bands or taenia coli
4. Circular muscle - forms pockets or
haustra between bands
Fig 25.19
Liver
On right under diaphragm, largest
organ made up of 4 lobes (left and
right, caudate, and quadrate)
Hilus (porta hepatis) – underside
"entry" point
Extremely versatile: Know a few
functions?
Gall bladder
Blood supply to liver
Microscopic anatomy: Liver lobules and triads
Fig 25.20
Fig 25.21
Pancreas

Retroperitoneal

Endocrine or
exocrine gland?

Common bile duct
and pancreatic duct
lead to duodenal
ampulla and papilla
Compare to Figs 25-22 / 23