Digestive System
B
Tunics of the Abdominal GI Tract
Figure 26.9
Mucosa
There are three components to this deepest layer:
• superficial epithelium
• an underlying areolar connective tissue,
called the lamina propria
• a thin layer of smooth muscle, called the
muscularis mucosae
Submucosa
Components include:
•
•
•
•
lymphatic ducts
mucin-secreting glands
blood vessels
Nerves
The nerves and their associated ganglia are referred to as the submucosal nerve
plexus (Meissner plexus)
Muscularis
•
Two layers of smooth muscle:
•
•
•
Two exceptions:
•
•
•
inner circular layer—constricts the lumen and forms
sphincters
outer longitudinal layer—shorten the tube
esophagus has a mix of both smooth and skeletal
muscle
stomach contains three layers of smooth muscle
Nerve fibers and associated ganglia between the
two layers of muscles is called the myenteric nerve
plexus (Auerbach plexus)
Tunics of the Abdominal GI Tract
Figure 26.9
Adventitia or Serosa
• Outermost layer
• Areolar connective tissue with collagen and elastic fibers
Blood Supply
Three unpaired arteries supply the abdominal GI tract:
• celiac trunk
• superior mesenteric artery
• inferior mesenteric artery
Lymphatic Vessels and Structures
• Lacteals are present in villi of the small intestines.
• Lymph ducts transport lymph to the cisterna chyli, which drains into
the thoracic duct.
• Lymphatic structures called MALT (mucosa-associated lymphatic
tissue) are found in the small intestine and appendix. These aggregate
nodules are called Peyer patches.
Nerves
Both autonomic and sensory; the three autonomic plexuses follow the
distribution of the unpaired artery of the same name:
• celiac plexus
• superior mesenteric plexus
• inferior mesenteric plexus
Esophagus
• A tubular passageway that conducts ingested materials from the
pharynx to the stomach.
• Passes through an opening in the diaphragm called the esophageal
hiatus as it connects to the stomach.
• Approximately 25 cm long coursing anterior to the vertebral bodies
• About 1.5 cm is in abdomen prior to changing into the stomach
Esophagus
Figure 26.1
Esophagus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mucosa
Stratified
squamous
epithelium
Muscularis
mucosae
Submucosa
Muscularis
Mucosa
Lamina
propria
Adventitia
LM 11x
(a) Esophagus, transverse section
Muscularis
mucosae
LM 65x
(b) Esophageal mucosa
a: © Alfred Pasieka/Peter Arnold; b: © The McGraw-Hill Companies, Inc./Photo by Dr. Alvin Telser
Figure 26.10
Esophageal Sphincters
The esophagus has two sphincters:
• Superior esophageal sphincter—at the junction of the pharynx
and the esophagus; closes during inhalation preventing air from
entering the GI tract
• Inferior esophageal sphincter—at the junction of the esophagus
and the stomach; along with the esophageal opening of the
diaphragm, prevents materials from regurgitating from the
stomach into the esophagus
Phases of Swallowing
There are three phases of swallowing:
• voluntary phase
• pharyngeal phase
• esophageal phase
Phases of Swallowing
Figure 26.11
Stomach
• Upper left quadrant of abdomen
• It continues the mechanical and chemical digestion of the
bolus
• The bolus eventually is processed into a pastelike soup
called chyme
• Possesses three layers of muscle to aid in the mechanical
processing of ingested materials
Regions of the Stomach
•
The stomach is composed of four regions:
•
•
•
•
•
•
cardia
fundus
body
pyloris
The inferior border of the stomach is the greater
curvature and the superior border is the inferior
curvature.
The internal surface of the stomach is thrown into
folds called gastric folds (rugae).
Structure of Stomach
Figure 26.12
Gastric Folds Structure of Stomach
Figure 26.12
Wall of the Stomach
• Lined by simple columnar epithelium although
little absorption occurs in the stomach
• Stomach lining is indented by numerous
depressions called gastric pits
Gastric Pits
Figure 26.13
Gastric Secretions
• Along and at the
base of the gastric
pits are the
openings of gastric
glands that secrete
products into the
stomach.
Figure 26.13
Gastric Gland
Five types of secretory cells form the gastric epithelium:
1. Surface mucous cells
2. Mucous neck cells
3. Parietal cells
4. Chief cells: PEPSIN
5. Enteroendocrine cells: GASTRIN
Gastric Gland
Figure 26.13
Small Intestine
• Finishes the chemical digestion process and is responsible for
absorbing most of the nutrients
• Ingested materials spend at least 12 hours in the small intestine as
chemical digestion and absorption are completed
• About 6 meters (20 feet) long in an unembalmed cadaver but much
shorter in a living individual due to muscle tone
Small Intestine
Consists of three specific segments (from proximal to distal from the
stomach):
• duodenum—25 cm (10 inches)
• jejunum—2.5 m (7.5 feet)
• ileum—3.6 m (10.8 feet)
Small Intestine
Figure 26.14
Duodenum
• C-shaped
• Upper right quadrant
• Becomes continuous with jejunum at the duodenojejunal flexure
(midline)
• The major duodenal papilla is the site where bile and pancreatic
secretions enter the duodenum
Jejunum
• Middle portion of the small intestine
• Primary region for chemical digestion and nutrient absorption
Ileum
• Last segment of the small intestine
• Distal end terminates at the ileocecal valve, a sphincter that controls
the entry of materials into the large intestine
Histology of the Small Intestine
• Internally, the mucosal and submucosal tunics are thrown into folds
called the circular folds (plicae circularis).
• Microscopic fingerlike projections called villi can be seen on the
surface of the circular folds.
• Microscopic fingerlike projections called microvilli can be seen on the
surface of the villi.
Histology of the Small Intestine
Figure 26.15
Histology of the Small Intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Simple columnar
epithelium with microvilli
(absorbs nutrients)
Circular folds
Mucosa
Submucosa
Muscularis
Inner circular layer
Outer longitudinal layer
Capillary network
Goblet cells
Lacteal
Enteroendocrine cells
(secrete hormones)
Circular fold
Serosa
(a)
Intestinal gland
Lymphatic nodule
Intestinal villi
Muscularis mucosae
Venule
Lymph vessel
Arteriole
Submucosa
(c) Intestinal villus
Inner circular layer
Muscularis
Outer longitudinal layer
Serosa
(b) Section of small intestine
Simple columnar cell
Intestinal lumen
Microvilli
Villi
Simple columnar
epithelium
Intestinal
lumen
Lamina propria
TEM 18,000x
(e) Microvilli
Goblet cells
LM 70x
Figure 26.15
(d) Intestinal villi
d: © The McGraw-Hill Companies, Inc./Photo by Dr. Alvin Telser; e: © Dr. Lee Peachey
Small Intestine
Large Intestine
• The large intestine forms a three-sided perimeter around the
centrally located small intestine
• Diameter is 6.5 cm vs. 2.5 cm of the small intestine, thus the name
large intestine
• Absorbs fluids and ions and compacts undigestible wastes and
solidifies them into feces
• Stores the feces until defecation (expulsion of the feces)
Large Intestine
The large intestine is comprised of the
following structures:
• cecum
• ascending colon
• transverse colon
• descending colon
• sigmoid colon
• rectum
• anal canal
Large Intestine
Figure 26.16
Cecum
•
•
•
First part of large intestine
Blind sac located in lower right quadrant of abdomen
Ileocecal valve represents junction between small intestine and
large intestine
Large Intestine
Figure 26.16
Ascending Colon
• Originates at the ileocecal valve and ascends right side of abdomen
• As it approaches the inferior border of the liver, it makes a 90-degree
turn toward the left side of the abdominal cavity; this bend in the
colon is called the right colic flexure (hepatic flexure)
Large Intestine
Figure 26.16
Transverse Colon
• Originates at the right colic flexure and approaches the spleen in the
upper left abdominal quadrant
• Suspended by the transverse mesocolon
• It makes a 90-degree turn inferiorly at the spleen; this bend in the
colon is called the left colic flexure (splenic flexure)
Large Intestine
Figure 26.16
Descending Colon
• Originates at the left colic flexure
• Found along the left side of the abdomen
• Makes contact with iliac fossa and terminates into the sigmoid colon
Large Intestine
Figure 26.16
Sigmoid Colon
• Its shape resembles the letter S
• Turns inferomedially and is suspended by the sigmoid mesentery
• Terminates as the rectum
Large Intestine
Figure 26.16
Rectum
• Muscular tube that readily expands to store accumulated fecal
material prior to defecation
• Three thick, transverse folds of the rectum, called rectal valves,
ensure that fecal material is retained during the passage of gas
• The rectum terminates at the anal canal
Large Intestine
Figure 26.16
Anal Canal
• Terminal few centimeters of the large intestine
• Passes through an opening in the levator ani muscles of the pelvic
floor
• Anal columns line the internal surface of the anal canal
• Anal sinuses secrete mucin for lubrication during defecation
• Internal and external anal sphincters open and close the anal canal
during defecation
Anal Canal
Figure 26.16
Histology of the Large Intestine
• Lined with simple columnar epithelium and goblet
cells, which secrete mucin to lubricate the fecal
material
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Goblet cells
Opening to intestinal gland
Opening to
intestinal gland
Simple columnar epithelium
Goblet cells
Simple columnar
epithelium
Mucosa
Intestinal gland
Intestinal gland
Lamina propria
Submucosa
Lymphatic nodule
Muscularis mucosae
Muscularis
Muscularis
mucosae
Inner circular
layer of
muscularis
Figure 26.17
Serosa
(or adventitia)
Nerves
(a) Large intestine tunics
Arteriole
Venule
Outer
longitudinal
layer of
muscularis
(tenia coli)
LM 80x
(b) Large intestine mucosa and submucosa
b: © The McGraw-Hill Companies, Inc./Photo by Dr. Alvin Telser
Muscular Wall of Large Intestine
• Longitudinal muscle is incomplete, forming bundles called teniae coli
• They bunch up the large intestines into many sacs, collectively called
haustra
• Extending off the external surface of the haustra are lobules of fat
called omental appendices or epiploic appendages
Large Intestine
Figure 26.16
Accessory Digestive Organs
Includes the following organs:
• liver
• gall bladder
• pancreas
• biliary apparatus
Liver
•
•
Located in right quadrant of abdomen
Comprised of four incompletely separated lobes:
•
•
•
•
right lobe
left lobe
caudate lobe
quadrate lobe
Liver
Figure 26.18
Liver
Along the inferior surface of the liver, several structures
collectively make the shape of the letter H:
• inferior vena cava and ligamentum venosum
form the inferior vertical parts
• gall bladder and round ligament form the superior
vertical parts
• porta hepatis represents the horizontal crossbar;
this is where the blood vessels (hepatic portal vein and
hepatic artery proper) and lymphatic vessels, bile
ducts, and nerves enter and leave the liver
Liver
Figure 26.18
Histology of the Liver
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Hepatic sinusoid
Central vein
Hepatocytes
Bile canaliculi
Hepatic
lobule
Reticuloendothelial
cell
Central vein
Hepatic sinusoid
Bile canaliculi
Hepatocyte
Portal triad
Branch of
bile duct
(a) Hepatic lobules
Branch of
hepatic portal vein
Branch of
hepatic artery
Portal triad
Branch of
bile duct
Branch of
hepatic
portal vein
Hepatic
sinusoid
Branch of
hepatic artery
Hepatocytes
(b) Hepatocytes and sinusoids
LM 220x
(c) Portal triad
Figure 26.19
c: © The McGraw-Hill Companies, Inc./Photo by Dr. Alvin Telser
Gall Bladder
• Embedded on the inferior surface of the liver
• Functions to collect and concentrate bile
• The cystic duct connects the gall bladder to the common bile duct
Gall Bladder
Figure 26.18
Pancreas
• Has both endocrine (Chapter 20) and
exocrine functions
• Exocrine functions involve secreting
digestive enzymes and bicarbonate,
collectively called pancreatic juices, into
the duodenum via the pancreatic duct
into the duodenum
Pancreas
Figure 26.20
Biliary Apparatus
• Network of thin ducts that transport bile from liver and gall bladder
to duodenum
• Function of the gall bladder is storage (not production) of bile
Biliary Apparatus
• Left and right lobes of the liver drain bile into the left
and right hepatic ducts.
• Left and right hepatic ducts merge to form the
common hepatic duct.
• The cystic duct and the common hepatic duct merge
to form the common bile duct.
• The common hepatic duct and the main pancreatic
duct enter the duodenum through the
hepatopancreatic ampulla.
Bilia
ry
App
arat
us
Figure 26.21
Development of the
Digestive System Foregut
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Ventral
mesentery
Esophagus
Dorsal
mesentery
Gallbladder
(cystic bud)
Liver
Falciform
ligament
Liver buds
Cystic bud
Foregut
Vitelline duct
Ventral
pancreatic bud
Dorsal
pancreatic bud
Vitelline duct
Midgut
Lesser curvature
of stomach
Greater curvature
of stomach
Dorsal pancreatic bud
Ventral pancreatic bud
Primary intestinal
loop
Hindgut
(a) Week 4: Liver, gallbladder, and pancreatic buds develop
(b) Week 5: Greater and lesser curvatures of stomach form
Direction
of stomach
rotation
Gallbladder
Falciform
ligament
Falciform ligament
Esophagus
Liver
Liver
Greater
omentum
Lesser omentum
Gallbladder
Direction
of duodenum
rotation
Ventral pancreatic bud
Direction of pancreas,
bile duct rotation
Figure 26.22
Stomach
Common bile duct
Pancreas
Dorsal
pancreatic bud
(c) Weeks 6–7: Rotation of stomach, pancreatic buds
Duodenum
(d) Week 8: Postnatal position of organs attained
Development of the
Digestive System Midgut
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Ventral
mesentery
Liver
Dorsal
mesentery
Vitelline
duct
Stomach
Primary intestinal
loop (midgut)
Descending
abdominal
aorta
Dorsal
mesentery
Cranial loop
Caudal loop
90° counterclockwise
Superior
rotation
mesenteric artery
(b) Week 6: Herniation of loop; 90º counterclockwise rotation
Caudal loop
(forms much of large intestine)
Direction of rotation 180º
counterclockwise
Vitelline duct
Caudal loop
Cranial loop
Hindgut
(a) Week 5: Primary intestinal loop forms
Ventral
mesentery
Duodenum
Transverse colon
Colon
Cranial loop
(forms most of
small intestine)
Small intestine
Ascending
colon
Descending colon
(c) Weeks 10–11: Retraction of intestines back into abdominal cavity;
180º counterclockwise rotation
Cecum
Sigmoid colon
Appendix
Figure 26.23
(d) Postnatal position
Rectum
Primitive Gut Tube