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
Muscular, hollow tube (= “digestive tract”)
+
Various accessory organs
consists of:
Function
ingestion
mechanical digestion
chemical and enzymatic digestion
secretion
absorption
compaction
excretion and elimination
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.
Individual parts
function in:
Muscularis
externa
Histological Organization
Tube made up of
four layers.
Modifications
along its
length as
needed.
1
2
3
4
The 4 Layers of the Gut
1) Mucosa
Epithelial cells – protection, absorption
2) Submucosa – made up of loose connective tissue contains submucosal plexus and blood vessels
3) Muscularis externa – smooth muscle, usually two layers -
outer layer: longitudinal
inner layer: circular
4) Serosa
outer layer, functions in protection
Ingestion
Ingestion is the first
step in the process
of digestion.
Ingestion means
that food is taken
into the mouth,
chewed, and
swallowed.
Oral Cavity
Also called the mouth, the oral cavity is the place where ingestion happens.
Hard and soft palates - form roof of mouth
Tongue - skeletal muscle
Salivary glands - three pairs
Teeth
Oral Cavity
Digestion starts here by two
means:
Mechanical – teeth tear,
shred, and grind food.
Chemical – enzymes in
saliva break down food
molecules such as
proteins and complex
sugars.
Structure of Teeth
Crown - exposed surface of tooth
Neck - boundary between root and crown
Enamel - outer surface
Dentin – bone-like, but noncellular
Pulp cavity - hollow with blood vessels and nerves
Root canal - canal length of root
Types and Numbers of Teeth
Dental succession:
Deciduous (baby, primary) teeth - 20, replaced by
Permanent teeth - 32 teeth
Types and Numbers of Teeth
All teeth are formed
before milk teeth or
primary teeth are lost.
It is the action of adult
teeth moving into
place that causes
primary teeth to fall
out.
Three pairs of Salivary Glands
Salivary glands help with:
• digestion
• lubrication (swallowing)
• moistening (tasting)
They secrete enzyme-filled
saliva to help break down
food
Ingestion
The tongue is a
muscle that pushes
your food toward
teeth to be
mechanically
processed and
shapes your food into
small rounded pieces
to be swallowed.
Swallowing
As you swallow, your
tongue pushes the food to
the back of the mouth.
Since both air and food go
through the pharynx, a flap
of tissue called the
epiglottis covers the larynx
to prevent food from
travelling into the lungs
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
Gross Anatomy of the Stomach
Histology of Stomach
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)
Fig 25.13
Regions of Small Intestine
SI is longest part of dig. tube
Duodenum (short, 12 inches)
– fixed shape & position
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)
Plicae circulares – circular pleats around the interior of the small intestine. Slows movement of food, increases surface area of intestine.
Villi – minute finger-like projections, contain capillaries & lacteals. Further increase surface area.
Microvilli – sub-microscopic size, projections on single cells. Aid in absorption of nutrients.
Intestinal glands (crypts)
- intestinal juice production
- Cell regeneration
Structure of Small Intestinal Wall Fig 25.15
Histology of Small Intestinal Wall
Histology of S.I. Wall (cont)
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
Regions of Large Intestine
Fig 25-17
Rectum – terminal end is anal canal - ending at the anus -
which has internal involuntary sphincter and external voluntary
sphincter
Structure of the Large Intestine
1. Mucosa - abundant goblet cells (produce mucous), 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
Histology of Large Intestine