The Digestive System I
Anatomy
The digestive system
• The organs of the digestive system can be
separated into 2 main groups:
1. Alimentary canal: digests food and absorbs
the digested fragments through its lining into
the blood
2. The accessary digestive organs: assist the
process of digestive breakdown in various
ways
Organs of the Alimentary Canal
• It is also called the gastrointestinal (GI) tract
• It is a continuous, coiled, hollow, muscular tube
that pass through the ventral body cavity and is
open at both ends
• Its organs are:
- The mouth
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
Organs of the Alimentary Canal
• In a cadaver, the alimentary canal is
approximately 9 meters long, but in a living
person, it is considerably shorter because of
its relatively constant muscle tone
• Food material within this tube is technically
outside the body, because it has contact only
with cells lining the tract and is open to the
external environment at both ends
Mouth
• Mouth: oral cavity, a mucous membrane-lined
cavity
• Lips: Labia, protect the mouth’s anterior opening
• Cheeks: Form the mouth’s lateral wall
• Hard palate: Forms the mouth’s anterior roof
• Soft palate: Forms the mouth’s posterior roof
• Uvula: A fleshy fingerlike projection of the soft
palate, which extends downward from its
posterior edge
Mouth
• Vestibule: The
space between
the lips and
cheeks externally
and the teeth and
gums internally
Tongue
• The muscular tongue occupies
the floor of the mouth
• The tongue has several bony
attachments:
1. 2 hyoid bones
2. The styloid process of the
skull
• The frenulum, a fold of
mucous membrane, secures
the tongue to the floor of the
mouth and limits its posterior
movements
• The Papillae on the tongue
containing taste buds, or taste
receptors
Pharynx
• From the mouth, food
passes posteriorly into the
oropharynx and
laryngopharynx: Both of
Which are common
passageways for food,
fluids and air
• Oropharynx: posterior to
the oral cavity
• Laryngopharynx:
continuous with the
esophagus below
Pharynx
• The walls of the pharynx contain two skeletal
muscle layers:
- The cells of the inner layer run longitudinally
- The cells of the outer layer run around the
wall in a circular fashion
- Alternating contractions of these two muscle
layers propel food through the pharynx into
the esophagus
Esophagus
• The esophagus, or gullet, runs
from the pharynx through the
diaphragm to the stomach
• The mechanism that esophagus
propel food down is called
peristalsis
Walls of the alimentary canal organs
• The walls of the esophagus to the large intestine
are made up of the same four basic tissue layers
or tunics:
1. The mucosa: The inner most layer, a moist
membrane that lines the cavity or lumen of the
organ
Walls of the alimentary canal organs
2. The submucosa: Is found just beneath the
mucosa. It is a soft connective tissue layer
containing blood vessels, nerve endings, lymph
nodes and lymphatic vessels
Walls of the alimentary canal organs
3. The muscularis externa: A muscle layer
typically made up of an inner circular layer and
an outer longitudinal layer of smooth muscle
cells
Walls of the alimentary canal organs
4. The serosa: the outermost layer of the wall. It
consists of a single layer of flat serous fluidproducing cells, the visceral peritoneum
Nerve Plexus
• The alimentary canal wall contains 2
important nerve plexus (a branching network
of axons outside of the central nervous
system): Submucosal nerve plexus and the
mysenteric nerve plexus
• They help regulate the mobility and secretory
activity of GI tract organs
Stomach
• The C shaped
stomach is on the
left side of the
abdominal cavity
• The stomach has 2
sphincters:
- Esophageal
sphincter
(Superior)
- Pyloric sphincter
(Inferior)
Stomach
• The fundus is the
expanded part of the
stomach
• The body is the midportion
• The funnel shaped
pylorus is the terminal
part
Stomach
• The stomach is approximately 25 cm long, but
its diameter depends on how much food it
contains:
- When its full, it holds about 4 liters of food
- When its empty, it collapses inward on itself
and its mucosa is thrown into large folds
called rugae (roo’ge)
Stomach
• The lateral convex surface of the stomach is
called the greater curvature
• The medial surface is the lesser curvature
Stomach
• The lesser omentum is a
double layer of peritoneum
extends from the liver to the
lesser curvature
• The greater omentum
drapes downward and
covers the abdominal organs
like a lacy apron
• The infection of peritoneum
is called peritonitis : the
peritoneal membranes tend
to stick together around the
infection site
Stomach
• The stomach acts as a temporary “storage
tank” for food as well as a site for food
breakdown:
1. Physical break down:
- Besides the usual longitudinal and circular
layers, its wall contains a third obliquely
arranged layer of muscle in the muscularis
externa
- This muscle allows the stomach to churn, mix
and physically breaking the food down
Stomach
2. Chemical breakdown:
- The mucosa of the stomach is dotted with millions
of deep gastric pits, which lead into gastric glands
that secrete the solution called gastric juice
- The chief cells produce protein-digesting enzymes,
mostly pepsinogens
- The parietal cells produce corrosive hydrochloric
acid
- The mucous neck cells produce a sticky alkaline
mucus, which clings to the stomach mucosa and
protects the stomach wall itself from being
damaged
Stomach
• Most digestive activity occurs in the pyloric
region of the stomach
• After food has been processed in the stomach,
it resembles heavy cream and is called chyme
• The chyme enters the small intestine through
the pyloric sphincter
Small Intestine
• It is the body’s major
digestive organ
• It is the longest
section of the
alimentary tube (avg.
2m)
• The small intestine is
a convoluted tube
extending from the
pyloric sphincter to
the ileocecal valve
Small intestine
• The small intestine has 3 subdivisions:
- duodenum: beginning
- Jejunum: middle
- Ileum: terminal
Small intestine
• The small intestine is able to process only a
small amount of food at one time
• The pyloric sphincter controls food movement
into the small intestine from the stomach
Small intestine
• Enzymes produced by the
intestinal cells and by the
pancreas (more important)
are ducted into the
duodenum through the
pancreatic duct
• Bile (produced by the liver,
stored in the gallbladder)
also enters the duodenum
through the bile duct
• The pancreatic and bile
duct join at the duodenum,
to form the flask-like
hepatopancratic ampulla
Small intestine
• The wall of the small intestine has 3 structures
that increase the absorptive surface:
- Microvilli: tiny projections of the plasma
membrane of the mucosa cell, sometimes it is
referred to as the brush border
- Villi: fingerlike projection of the mucosa
- Circular folds: deep folds of both mucosa and
submucosa layers
Villi and microvilli
Villi
• Within each villus is a
rich capillary bed and
a modified lymphatic
capillary called a
lacteal
• The digested food are
absorbed through the
mucosa cells into both
the capillary and the
lacteal
Circular folds
• Unlike the rugae folds of the stomach, the
circular folds do not disappear when food fills
the small intestine
• The number of folds decrease toward the end
of the small intestine
• Payer’s patch (lymphatic tissue) increase
toward the end of the small intestine
Large Intestine
• The large intestine is
about 1.5 m long
• Its major functions
are to dry out the
indigestible food
residue by absorbing
water and eliminate
these residues from
the body as feces
Large intestine
• It frames the small intestine on three sides
and has the following subdivisions:
- Cecum
- Appendix
- Colon
- Rectum
- Anal canal
Colon
• The colon is divided into several distinct
regions:
- The ascending colon
- The transverse colon
- The descending colon
- Sigmoid colon
Anal Canal
• The anal canal has an external voluntary
sphincter formed by skeletal muscle and internal
involuntary sphincter formed by smooth muscle
Large intestine
• Because most nutrient absorption has
occurred before the large intestine, no villi are
seen in the large intestine
• There are tremendous numbers of goblet cells
in its mucosa that produce mucus
• The pocket like sacs on the large intestine are
called haustra