Digestive System
• The digestive system can be divided into two
major parts:
Digestive System
The process of chemically and
physically breaking down foods into
simpler forms that can be absorbed is
called digestion.
Digestive tract
The digestive tract is the tube that extends
from the mouth to the anus, and it
consists of the
– (1) mouth,
– (2) pharynx,
– (3) esophagus,
– (4) stomach,
– (5) small intestine, and
– (6) large intestine.
– (1) the alimentary canal (gastrointestinal, or digestive
tract) and
– (2) the accessory organs. The digestive tract is the
tube that extends from the mouth to the anus, and it
consists of the (1) mouth, (2) pharynx, (3) esophagus,
(4) stomach, (5) small intestine, and (6) large
intestine. The accessory organs include the (1) teeth,
(2) tongue, (3) salivary glands, (4) liver, (5)
gallbladder, and (6) pancreas.
Accessory organs
The accessory organs include the
– (1) teeth,
– (2) tongue,
– (3) salivary glands,
– (4) liver,
– (5) gallbladder, and
– (6) pancreas.
Digestive System Overview
• The oral cavity (mouth) contains the tongue and
the teeth.
• Salivary glands produce saliva, a mixture of
mucus and enzyme (amylase), and empty into
the oral cavity.
• Located behind the mouth is the throat
(pharynx). The pharynx is divided into three
parts, and the two digestive components are the
(1) oral pharynx and the (2) laryngopharynx,
which function in swallowing and the passage of
food.
Figure 26.1
Illustration showing the design and structures of the digestive system.
1
Digestive System Overview
Digestive System Overview
• Swallowing forces food into the esophagus, the tube that
descends from the pharynx to the stomach.
• The small intestine is a long twisted tube that extends
from the stomach to the large intestine.
• The large intestine is divided into the (1) cecum,
(2) appendix, (3) colon, (4) rectum, and (5) anal
canal.
– The duodenum receives a liquid mixture of food from the
stomach called chyme and secretions from the liver and the
pancreas.
– The liver produces bile which contains bile salts for the
emulsification of fats.
– The pancreas produces pancreatic juice which contains two
major components, (1) enzymes for digestion and (2)
bicarbonate ions for adjusting the acidic chyme toward neutral.
– The ileocecal valve regulates the emptying of the small intestine.
Digestive System Overview
– The sigmoid colon joins the rectum, which terminates
at the anal canal. The short anal canal terminates at
the opening to the outside called the anus.
• The wall of the alimentary canal is organized
from the esophagus to the anal canal into four
distinctive layers. Located from the inside to the
outside, the layers of the wall are called the
–
–
–
–
(1) mucosa,
(2) submucosa,
(3) muscularis externa, and
(4) serosa.
MUCOSA
• The mucosa of the digestive tract is its
innermost layer, a mucous membrane that
opens to the body’s exterior.
– The mucosa consists of an epithelial lining, under
which is attached a layer of loose connective tissue
called the lamina propria. Under the lamina propria is
a layer of smooth muscle called the muscularis
mucosae.
• Regions of the alimentary canal show variations
in the structure of the mucosa that relate to its
functions, which include protection, secretion,
and absorption.
Figure 26.2 Illustration showing the
organization of the layers of the
digestive tract.
Figure 26.3 Photograph of a crosssection of the small intestine showing
the organization of the layers of the
digestive tract.
MUCOSA
• Stratified Squamous Epithelium
– The mucosal epithelium of the mouth, esophagus,
and the anal canal is stratified squamous epithelium
which functions in protection.
Figure 26.4 Illustration of
stratified squamous epithelium
of the esophagus.
Figure 26.5 Low power photograph of
stratified squamous epithelium of the
esophagus. Stratified squamous epithelium
functions as a protective epithelium.
MUCOSA
• Simple Columnar Epithelium
The mucosal epithelium of the stomach, small
intestine, large intestine, and rectum is simple
columnar epithelium.
– Often the exposed (apical) plasma membranes of the
columnar cells are modified into projections called
microvilli. Microvilli function to increase the surface
area of the exposed (apical) plasma membranes.
– The apical plasma membranes of the columnar cells
contain digestive enzymes and function in the
absorption of the end products of digestion.
Associated with the epithelium are numerous goblet
cells.
– A goblet cell functions as a mucous gland.
2
•Simple Columnar Epithelium
Modifications of the Mucosa
Figure 26.6 Illustration of simple
columnar epithelium of the small
intestine.
Figure 26.7 High power photograph of
simple columnar epithelium of the
small intestine. In addition to forming a
protective membrane, the epithelium’s
cells have microvilli that contain
digestive enzymes and function in
absorption.
In addition to having a layer of
epithelium, the mucosae of the stomach,
small intestine, and large intestine are
modified for digestive functions.
Mucosa of the Stomach
• A modification of the stomach’s mucosa is the presence
of gastric glands.
Figure 26.8 The mucosa of the stomach is modified to contain gastric
glands, which produce gastric juice and hormones.
Mucosa of the Small Intestine
• Two modifications of the small intestine’s
mucosa are the presence of villi and intestinal
glands (crypts).
Figure 26.9 The mucosa of the small intestine is modified to form villi and
contains intestinal glands. Villi increase the mucosal surface area and gastric
glands produce intestinal juice.
Mucosa of the Large Intestine
A modification of the large intestine’s mucosa is
the presence of intestinal glands which function
in absorption and secretion of mucus.
SUBMUCOSA
•
The submucosa consists of loose
connective tissue. It contains abundant
blood vessels, lymphatics, and nerves.
Only slight variations are seen in the
various regions of the alimentary canal.
Figure 26.10 The mucosa of the large intestine is modified to contain intestinal
glands which function in absorption and secretion of mucus.
3
MUSCULARIS EXTERNA
•
MUSCULARIS EXTERNA
The muscularis externa is the muscular layer located
to the outside of the submucosa. It functions in the
mixing and propulsion movements of the contents of the
alimentary canal.
– The mouth, pharynx, and upper esophagus have various
percentages of skeletal muscle tissue for voluntary movements
such as chewing and swallowing.
– From the lower esophagus to the anus, the muscularis externa
consists of smooth muscle, under control of the autonomic
nervous system.
– Except for the stomach, the muscularis is arranged in two layers,
an inner circular layer and an outer longitudinal layer. The
stomach has an additional layer, located inside of the circular
layer, called the oblique layer.
Figure 26.11 The muscularis externa is the muscular layer located to the outside
of the submucosa. Except for the stomach, the muscularis externa consists of
two muscular layers that function in the mixing and propulsion movements of the
contents of the alimentary canal.
MUSCULARIS EXTERNA
MUSCULARIS EXTERNA
• Peristalsis
– Peristalsis is the wavelike muscular contractions that
produce forward movement of the organ’s internal
contents. Peristalsis of the digestive tract is produced
by the sequential contraction of the smooth muscles
of the muscularis externa. Contraction of the circular
layer constricts the lumen (cavity) and contraction of
the longitudinal layer dilates the lumen (cavity).
• Segmentation
– Segmentation is the process where non-sequential
segments of the digestive tract contract resulting in
the mixing of the internal contents. The contraction of
each segment pushes a portion of the contents
forward and a portion backward. Thus, the contents
are mixed, rather than pushed in a forward direction.
Figure 26.12 Peristalsis is the wavelike muscular contractions that produce
forward movement of the organ’s internal contents. Segmentation is the nonsequential contraction of segments of the tract that results in the forward and
backward movements (mixing) of the contents.
MUSCULARIS EXTERNA
Serosa
• Except for the esophagus, the outermost
layer of the alimentary canal is called the
serosa (visceral peritoneum).
– The serosa consists of loose connective
tissue and an epithelium that produces a
lubricating serous fluid.
– The esophagus is lined by a layer of
connective tissue called the adventitia. The
adventitia connects the esophagus to
surrounding tissues in the thoracic cavity.
Figure 26.13 An X-ray of the upper digestive tract shows peristaltic waves of
the stomach. Peristaltic waves of the stomach move the contents of the
stomach (chyme) into the duodenum.
4
Serosa
The Peritoneum and its Cavity
The peritoneum is the serous
membrane that lines the abdominal
cavity and continues inward to cover its
viscera, the internal organs.
Figure 26.14 The serosa is the outer layer of the digestive tract (except for the
esophagus). The serosa, also called the visceral peritoneum, functions in the
maintenance of the serous fluid in the abdominopelvic cavity.
The Peritoneum and its Cavity
• The peritoneum consists of a layer of simple
squamous epithelium (mesothelium) and forms
two layers.
• The outer layer lines the abdominal wall and is
called the parietal peritoneum.
• The inner layer covers the organs of the
abdominal cavity and is called the visceral
peritoneum.
• The space between the two layers, the
peritoneal cavity, contains a film of serous fluid.
Mesenteries
• When organs extend into the peritoneal cavity,
the peritoneum is formed into the mesenteries,
two layers of peritoneum that house blood
vessels and nerves.
– Some organs of the abdomen, such as the kidneys,
pancreas, ascending colon, and duodenum, are
located to the outside of the peritoneum (between the
peritoneum and the abdominal wall) and are called
retroperitoneal organs.
• Mesenteries include the mesenteries of the
small intestine (mesenteries of jejunum and
ileum) and the transverse colon (mesocolon),
which connect their respective viscera to the
posterior abdominal wall.
Omentum
•
The omentum is a double layer sheet or fold
of the peritoneum and connects the stomach to
other abdominal organs.
• The omentum is organized into the greater
omentum and the lesser omentum.
– The lesser omentum mostly connects the stomach to
the liver.
– The greater omentum mostly connects the stomach to
the transverse colon.
– From the stomach the greater omentum extends
downward over the abdominal viscera and then loops
back upon itself to attach to the transverse colon.
– The transverse colon is attached by the mesentery,
the mesocolon, to the posterior abdominal wall.
– A large peritoneal ligament, the falciform ligament
attaches the liver to the anterior abdominal wall.
Figure 26.15 Sagittal section of the abdomen showing the peritoneum, its cavity,
and relationship to the abdominal viscera.
5
ORGANS OF THE
DIGESTIVE SYSTEM
MOUTH
The mouth (oral cavity, or buccal cavity) is the
area posterior the lips, anterior to the
pharynx, medial to the cheeks, inferior to the
palate, and superior to the tongue.
MOUTH
– The anterior region of the mouth is the vestibule. The vestibule is
the area between by the lips and cheeks, and the gums and
teeth.
– The cavity of the mouth, the oral cavity proper, is located within
the boundary of the teeth and gums, the palate, the tongue, and
the posterior pharynx.
– The pharynx (commonly called the throat) is the portion of the
digestive tract that extends from the mouth and the superior
nasal cavity to the larynx (commonly called the voice box). The
pharynx is divided into the nasopharynx, oropharynx, and
laryngopharynx.
– The roof of the mouth, the palate, consists of the hard palate and
the soft palate. Hanging from the posterior boundary of the soft
palate is the uvula. During swallowing, the soft palate and uvula
close the entrance to the nasal pharynx to prevent the entrance
of food.
Figure 26.16 Illustration of major structures of the head and neck.
Tongue
• Internally, the tongue consists of bundles of skeletal
muscle fibers (intrinsic muscles) arranged in various
directions which are mostly used to change the shape of
the tongue. Extrinsic muscles move the tongue for the
Tongue
Internally, the tongue consists of bundles of
skeletal muscle fibers (intrinsic muscles)
arranged in various directions with its surface
structured as a modified mucous membrane
– (1) manipulation of food during chewing, drinking, and
swallowing and
– (2) movements for the production of sound.
• The surface of the tongue is a mucous membrane
containing abundant sensory receptors and is lined with
protective stratified squamous epithelium.
• The superior (top) surface of the tongue contains three
types of small protuberances called papillae:
– (1) filiform, the (2) circumvallate, and (3) fungiform papillae.
• The inferior surface of the tongue is attached to the floor
of the oral cavity by the midline lingual frenulum.
6
Figure 26.17 The illustration of the tongue shows the general distribution
of its surface projections, the papillae. The two low power photographs
show the structure of the papillae.
Figure 26.20 High power photograph of a taste bud from a circumvallate
papilla. Taste buds contain receptor cells that communicate with the
surface by way of “taste hairs” (microvilli) that pass through a pore.
Salivary Glands
The major salivary glands are the paired
Salivary Glands
The salivary glands empty saliva
into the oral cavity.
– (1) parotid - located superficially, slightly
inferior, and anterior to the ears
– (2) sublingual - located beneath the tongue.
– (3) submandibular glands - located medial to
the mandible at each mandibular angle
The glands may be classified according to the
type of secretion they produce or their structural
organization.
PHARYNX
The pharynx is the tube that extends
inferiorly from behind the nose to the
base of the larynx (voice box).
Figure 26.21 High power photograph of the submandibular salivary gland.
The submandibular salivary gland contains both mucous and serous cells.
7
PHARYNX
• The pharynx is divided into three divisions:
– (1) the nasopharynx - does not function as
part of the digestive system.
– (2) the oropharynx - extends from the level of
the soft palate to the level of the hyoid bone
– (3) the laryngopharynx - extends from the
level of the hyoid bone downward to the
cricoid cartilage of the larynx and to the origin
of the esophagus
Esophagus
Esophagus
The esophagus is located
between the laryngopharynx and
the cardiac region of the stomach
Esophagus
• The esophagus passes through an opening in
the diaphragm, the esophageal hiatus, into the
abdomen where it joins the cardiac orifice of the
stomach.
• A sphincter, the cardiac (or gastroesophageal)
sphincter surrounds the cardiac orifice and
functions to prevent reflux of the stomach’s
contents back into the esophagus.
• The lumen (inner cavity) of the esophagus is
closed by the folding of the mucosa and
submucosa, except when in contact with passing
food.
Figure 26.22 X-ray showing radiopaque substance passing through the
esophagus into the stomach. The position of the cardiac sphincter is shown
by the constriction where the esophagus meets the stomach.
Esophagus
STOMACH
Figure 26.24 A low power photograph of
Figure 26.23 A scanning power photograph the esophagus showing the details of its
four layers.
of the esophagus in cross-section.
Located between the esophagus
and the first portion of the small
intestine, the duodenum
8
STOMACH
STOMACH
• The functions of the stomach include
– (1) food storage, the
– (2) mechanical and enzymatic digestion of foods, and
– (3) the production of intrinsic factor for the intestinal
absorption of vitamin B12.
• The stomach is divided into four regions, the
–
–
–
–
–
(1) cardia,
(2) fundus,
(3) body, and
(4) pylorus.
The lateral curved surface of the stomach is called
the greater curvature, and the medial curved surface
is the lesser curvature
Figure 26.25 Illustration showing
the general structure of the
stomach.
STOMACH
Stomach Mucosa
•
Figure 26.27 X-ray of upper digestive
tract showing the stomach and the
small intestine.
Figure 26.28 Scanning power
photograph of the wall of the
stomach.
Stomach Mucosa
Figure 26.26 Illustration showing the
internal detail of the stomach and the
relationship of the stomach to the liver
and small intestine.
The mucosa of the stomach is modified to allow for expansion
(rugae) and to produce gastric juice (gastric glands).
Figure 26.29 Illustration of the mucosa of
the stomach. The mucosa of the stomach
is modified to contain gastric glands, which
produce gastric juice, intrinsic factor, and
hormones.
Figure 26.30 Low power
photograph of the mucosa of the
stomach. The gastric glands contain
parietal, chief, and enteroendocrine
cells.
Stomach Mucosa
• Rugae
– The rugae are folds in the mucosa and submucosa that allow for
expansion when the stomach fills.
• Simple columnar epithelium
– The surface epithelium of the mucosa consists of simple
columnar epithelium.
• Gastric pits
– The gastric pits are the openings to the gastric glands. They are
lined with the mucous neck cells of the gastric glands.
• Mucous neck cells
– The cuboidal to columnar mucous neck cells are the cells of the
gastric glands that line the gastric pits.
• Gastric glands
– The gastric glands are glands in the mucosa that mostly produce
(1) gastric juice, (2) intrinsic factor, and (3) hormones (by the
enteroendocrine cells). The two types of cells that contribute to
gastric juice are the (1) parietal cells and the (2) chief
(zymogenic) cells.
Figure 26.31 High power view of the parietal and chief cells of the gastric glands.
Parietal cells produce hydrochloric acid and intrinsic factor. The chief cells produce
the inactive enzyme called pepsinogen.
9
Parietal cells
• The parietal cells are mostly located in the proximal
portions of the gastric glands. Parietal cells function in
the production of hydrochloric acid and intrinsic factor.
• Hydrogen ions (H+) produced within the parietal cells
(CO2 + H2O -> H2CO3 -> H+ + HCO3) are pumped out
of the cells into the lumen of the gland. In the lumen, the
hydrogen ions combine with chloride ions (from the
parietal cells) to form hydrochloric acid (HCl).
Hydrochloric acid functions to
– (1) activate the enzyme pepsinogen, which is produced by the
chief cells, and
– (2) denatures proteins of ingested foods and many
microorganisms (bacteria and fungi).
• Intrinsic factor is a substance produced by the parietal
cells that is essential for the intestinal absorption of
vitamin B12. Vitamin B12 is essential for the production
of normal red blood cells (lack of B12 results in
pernicious anemia).
Chief (zymogenic) cells
• The chief (zymogenic) cells produce an
inactive enzyme called pepsinogen.
– Hydrochloric acid activates pepsinogen into
pepsin, an enzyme that digests proteins to
smaller fragments called peptides.
– Pepsin begins protein digestion by breaking
certain peptide bonds. Further protein
digestion occurs in the small intestine.
– The chief (zymogenic) cells are mostly
located in the distal portions of the gastric
glands.
Enteroendocrine cells
Control of Gastric Activity
• Enteroendocrine cells are mostly located in the
distal portions of the gastric glands located in the
stomach’s pylorus.
• The activity of the gastric mucosa is under
neural and hormonal control and can be
divided into three phases, the
– Enteroendocrine cells of the stomach produce
hormones mostly involved in regulation of the activity
of the stomach’s gastric glands and muscularis.
– The release of the hormone gastrin is triggered by the
arrival of partially digested proteins into the pylorus.
Gastrin enters circulation and targets the gastric
glands, which increases
– (1) the production of gastric juice and
– (2) gastric mixing.
Cephalic phase
• The cephalic (head) phase begins with the
thought, sight, and smell of food.
• These stimuli by way of the hypothalamus
increase parasympathetic neural output to
the stomach by way of the vagus nerves.
• The cephalic phase functions to begin the
stimulation of the gastric glands to
increase gastric juice and hormone
production. The neural stimulation of the
muscularis externa increases gastric
churning.
– (1) cephalic phase,
– (2) gastric phase, and
– (3) intestinal phase.
Gastric Phase
• The gastric (stomach) phase begins when food enters
the stomach. Three primary stimuli for increased gastric
secretion are
– (1) distension (stretch) of the stomach,
– (2) an increase of the stomach’s pH (contents become more
basic), and
– (3) the presence of proteins.
• The primary hormone released by the gastric mucosa is
gastrin, which targets the gastric glands resulting in
increased release of gastric juice.
• As the stomach’s pH decreases to at about a pH of 3,
the gastric glands begin to decrease secretion.
• Gastrin also
– (1) targets the small intestine and increases its motility,
– (2) targets the valve between the small and large intestine, the
ileocecal valve, resulting in its relaxation, and
– (3) targets the large intestine and increases motility.
10
Intestinal phase
• The intestinal phase is regulated by chyme
entering the duodenum.
• As chyme enters the duodenum, it
– stretches the duodenum producing a neural
response, and
– its chemical contents (the presence of lipid,
protein, carbohydrate, and pH) trigger a
hormonal response.
Intestinal phase
Distension of the duodenum
• The duodenum’s primary neural response
to being distended (stretched) results in
the inhibition of gastric secretions, mixing,
and propulsion.
Intestinal phase
Response to lipid and protein
• The duodenum’s response to the presence of lipid and
protein is the secretion of the hormone cholecystokinin
(CCK).
• Cholecystokinin targets
– (1) the gastric glands and inhibits their secretion of gastric juice.
Cholecystokinin also targets the
– (2) pancreas to produce enzyme rich pancreatic juice, targets
– (3) gallbladder stimulating its contraction and release of bile, and
targets the
– (4) hepatopancreatic sphincter causing its relaxation for the
entrance of pancreatic juice and bile into the duodenum.
Intestinal phase
Response to protein
• The duodenum’s response to the
presence of proteins is the release of the
hormone intestinal gastrin from its
mucosa.
• Intestinal gastrin targets the gastric glands
to increase gastric juice production and
gastric mixing.
• Pancreatic juice facilitates the digestion of the arriving
lipid, protein, and carbohydrate, and buffers the acidic
chyme. Bile functions to change lipid into small globules
(emulsification) to facilitate their digestion.
Intestinal phase
Response to decreased pH
• The duodenum’s response to the arrival of
chyme with a pH of below 4.5 is the secretion of
the hormone secretin.
• Secretion
– (1) targets the stomach and inhibits the gastric
glands, gastric mixing, and propulsion,
– (2) targets the pancreas and increases the release of
bicarbonate ions into the pancreatic juice, and
– (3) targets the liver and increases the secretion of
bile.
• Bicarbonate ions bind hydrogen ions and
increase the pH (to more alkaline) of the
duodenum’s contents.
Submucosa, Muscularis, Serosa
• Submucosa
– The submucosa is located beneath the mucosa. It
consists of loose connective tissue and is highly
vascular.
• Muscularis Externa
– The muscularis externa contains three layers of
smooth muscle: (1) an inner oblique layer, (2) a
middle circular layer, and (3) an outer longitudinal
layer. The muscular contractions aid the churning of
food and the emptying of the contents into the small
intestine.
• Serosa
– The serosa is the outer layer and is covered with
simple squamous epithelium. The serosa is also
called the visceral peritoneum.
11
SMALL INTESTINE
SMALL INTESTINE
The small intestine is divided into
three regions, the (1) duodenum,
the (2) jejunum, and the (3) ileum.
SMALL INTESTINE
• The duodenum begins at the pyloric sphincter
and curves to the left around the head of the
pancreas.
• The duodenum receives chyme from the
stomach, bile from the liver and gallbladder, and
pancreatic juice from the pancreas.
– The common bile duct, which delivers bile from the
liver and gallbladder, merges with the pancreatic duct
in the wall of the duodenum forming the
hepatopancreatic ampulla (pancreaticohepatic
ampulla, or duodenal ampulla).
– The duodenal ampulla enters the duodenum at a
small projection called the duodenal papilla. A
sphincter, the hepatopancreatic sphincter
(pancreaticohepatic sphincter, or sphincter of Oddi)
controls the entrance of bile and pancreatic juice into
the duodenum.
SMALL INTESTINE
• The hepatopancreatic sphincter is mostly controlled by
the hormone cholecystokinin (CCK) released from the
duodenum in response to the arrival of lipid and/or
carbohydrate rich chyme.
• The jejunum functions as the primary site for intestinal
digestion and absorption.
• In the ileum, digestion and the absorption of the end
products are finalized.
– The ileocecal valve regulates the movement of materials from
the ileum to the cecum, the first region of the large intestine.
– The relaxation of the ileocecal valve is mostly controlled by the
hormone gastrin, which is secreted by the stomach’s mucosa.
Gastrin results in
• increased gastric gland secretion and gastric motility, and
• relaxes the ileocecal valve allowing the small intestine to begin
emptying its materials into the large intestine.
Figure 26.32 Illustration showing the relationships of the duodenum, liver, and
pancreas.
SMALL INTESTINE
SMALL INTESTINE - MUCOSA
• The mucosa of the small intestine is modified for
– (1) increasing surface area for absorption and the
– (2) secretion of intestinal juice.
• Modifications for increasing surface area include
– (1) villi - small fingerlike vascular projections formed
by the mucosa
– (2) microvilli - microvilli are minute “hairlike”
projections of the surface (apical) plasma membranes
of the columnar cells.
Figure 26.33 Scanning power
photograph of a cross-section of the
small intestine (jejunum). The wall of
the small intestine consists of four
layers.
Figure 26.34 Low power photograph of the
wall of the small intestine (jejunum). The
photograph shows details of the intestine’s
four layers.
12
SMALL INTESTINE - MUCOSA
Figure 26.35 Illustration showing the
modification of the mucosa of the small
intestine into villi and intestinal glands.
Figure 26.36 Low power photograph
showing the mucosa of the small intestine.
Villi increase the surface area and
intestinal glands produce intestinal juice.
SMALL INTESTINE - MUCOSA
SMALL INTESTINE - MUCOSA
Figure 26.38 Except for fats, the end-products of digestion enter the blood
capillaries immediate beneath the epithelium. The capillaries enter the hepatic
portal circulation and are transported to the liver. Fats enter the lacteal and are
transported into systemic circulation.
SMALL INTESTINE
MUSCULARIS EXTERNA
• The muscularis externa contains two distinctive layers of
smooth muscle.
– The outer layer of smooth muscle is called the longitudinal layer
as the cells are located parallel to the long axis of the organ.
Contraction of the longitudinal layer shortens the organ and
causes a dilation of its internal cavity.
– The inner layer of smooth muscle is called the circular layer as
the cells are located around the circumference of the organ.
Contraction of the circular layer lengthens the organ and causes
a constriction of its internal cavity.
• Contraction of the smooth muscle produces
segmentation and peristalsis.
Figure 26.39 High power photograph of intestinal glands. Intestinal
glands produce intestinal juice, a watery mixture of mucus.
– Segmentation is the process of dividing and mixing the intestinal
contents.
– Peristalsis is the wavelike muscular contractions that produce
forward movement of the intestinal contents.
SMALL INTESTINE
MUSCULARIS EXTERNA
REGIONS OF THE SMALL
INTESTINE
Figure 26.40 Scanning and high power photographs of the muscularis externa of
the small intestine. The muscularis externa consists of an inner circular and an
outer longitudinal layer. Intestinal movements include peristalsis and segmentation.
Duodenum - Duodenal (Brunner’s) Glands
and
Ileum - Peyer’s Patches
13
Duodenum - Duodenal (Brunner’s) Glands
Figure 26.41
• The duodenum, is specialized for the mixing of chyme,
bile, and pancreatic juice.
• The acidic chyme entering the duodenum is prevented
from damaging the mucosa by the thick alkaline
mucous coating produced by the duodenal (Brunner’s)
glands. Duodenal glands are located in the submucosa
and empty onto the surface of the duodenum through
ducts.
Peyer’s Patches
• Peyer’s patches are
lymphatic nodules located
in the submucosa of the
ileum. When found in
groups, or aggregated,
they are called Peyer’s
patches.
• Peyer’s patches are sites
of lymphatic tissue which
defend against bacteria
and antigenic substances
in the intestinal contents.
Peyer’s patches become
more numerous at the
distal portion of the ileum.
Figure 26.41
PANCREAS
PANCREAS
The pancreas is located beneath the
stomach in a transverse position along the
fold of the duodenum. The pancreas functions
as both an endocrine gland and as an
exocrine gland.
PANCREAS
• The most abundant cells of the pancreas (about
99%) are exocrine in function and are organized
into groups called acini. Each pancreatic acinus
is formed by a group of exocrine cells that
surrounds a central lumen (cavity).
• The acini produce pancreatic juice which enters
into small ducts. The small ducts merge into the
pancreatic duct which leaves the pancreas,
unites with the common bile duct, and enters the
duodenum at a common region called the
hepatopancreatic ampulla.
• The least abundant cells of the pancreas (about
1%) are endocrine in function and are organized
into the islets of Langerhans.
• The islets of Langerhans are highly vascular,
and their secretions (hormones) enter the blood.
Two of the hormones released from the islets
are
– insulin and
– glucagon,
– Both function in the regulation of blood glucose levels.
Pancreatic Juice
• The release of pancreatic juice (and bile) from
the hepatopancreatic ampulla is controlled by
the hepatopancreatic sphincter.
• The hormone cholecystokinin produced at the
duodenum targets the hepatopancreatic
sphincter and results in its relaxation.
• Pancreatic juice neutralizes the acidic chyme
from the stomach, and it contains digestive
enzymes for the digestion of carbohydrate,
protein, and lipid.
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Secretin
Pancreatic Juice
• The secretion of pancreatic juice is under
hormonal and neural control.
– Neurally, the pancreas is controlled by the
parasympathetic division of the autonomic
nervous system by way of the vagus nerves.
– The dominate control mechanism for
secretion of pancreatic juice is by the
hormones secretin and cholecystokinin, which
are released by the enteroendocrine cells of
the duodenum.
Secretin
•
The hormone secretin is released when
the enteroendocrine cells of the duodenum
are stimulated by the arrival of acidic
chyme.
• Secretin targets the pancreas and
stimulates the release of bicarbonate rich
pancreatic juice. Bicarbonate functions as
a hydrogen ion acceptor and neutralizes
the acidity.
Cholecystokinin (CCK)
• The hormone cholecystokinin (CCK) is
released when the enteroendocrine cells
of the duodenum are stimulated by the
arrival of chyme rich in fats and proteins.
• Cholecystokinin targets the pancreas and
stimulates the release of enzyme rich
pancreatic juice. Enzymes function in the
intestinal digestion of incoming fats,
proteins, and carbohydrates.
Figure 26.43 Illustration showing the function of the hormone secretin.
Cholecystokinin (CCK)
PANCREAS
Figure 26.45 Scanning power
photograph of the pancreas. The
pancreas is organized into exocrine
(acini) and endocrine (islets) cells.
Figure 26.46 High power photograph of
the pancreas showing an islet and groups
of acini. Pancreatic acini function in the
secretion of pancreatic juice.
Figure 26.43 Illustration showing the function of the hormone cholecystokinin (CCK).
15
Liver
LIVER
The liver is located in the upper
right part of the abdominal cavity
immediately below the diaphragm.
Liver
• Two major vessels enter the liver, the
hepatic artery and the hepatic portal vein.
– The hepatic artery delivers oxygen rich blood
to the liver.
– The hepatic portal vein delivers oxygen-poor
blood rich in nutrients from the digestive tract
to the liver.
• The liver functions to process the nutrient
rich blood from hepatic portal circulation
before it enters systemic circulation.
Liver – Bile Secretion
• The liver consists of four divisions, or lobes, the (1) right
lobe, (2) left lobe, (3) quadrate lobe, and (4) the caudate
lobe.
• The falciform ligament, a large ligament that attaches the
liver to the diaphragm and the anterior abdominal wall,
separates the right and left lobes. The larger of these
two lobes is the right lobe and at its medial inferior
surface is the much smaller quadrate lobe. The small
caudate lobe is found at the liver’s superior posterior
surface between the right and left lobes.
• The remnant of the fetal umbilical vein, the ligamentum
teres (or round ligament) is located at the anterior free
margin of the falciform ligament.
• The gallbladder is located in a shallow depression on the
inferior surface of the right lobe.
Liver
• The liver also functions to produce bile. Increased
secretion of bile results from stimulation of the liver by
– (1) increased blood levels of bile salts and the hormone
– (2) secretin.
• Bile salts are the components of bile that function in the
emulsification of fats.
• When acidic, protein and fat rich chyme enters the
duodenum the enteroendocrine cells release the
hormones secretin and cholecystokinin (CCK).
– Cholecystokinin targets the gallbladder resulting in contraction,
and targets the hepatopancreatic sphincter resulting in
relaxation.
– Bile then enters into the small intestine where its bile salts
function in the emulsification of fats. The bile salts are
reabsorbed from the small intestine into the blood.
Liver – Lobules
• The functional units of the liver are called
lobules. A lobule consists of hepatic cells
(hepatocytes) that form rows (cords or plates) in
a radial pattern around a central vein.
• Between the cords are modified capillaries
(sinusoids) that receive blood from both the
hepatic artery (highly oxygenated) and the
hepatic portal vein (contains products of
digestion).
• The sinusoids are lined with macrophages
(Kupffer cells) that destroy worn-out blood cells
and bacteria. The blood leaves the sinusoids
and enters the central vein.
Figure 26.46 Illustration showing the control of the secretion of bile.
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Liver – Lobules
Liver – Lobules
• The central veins of the lobules ultimately merge
and route blood into systemic circulation through
the hepatic veins.
• Bile ducts collect bile from small channels, the
bile canaliculi, located at the basal surface of the
hepatocytes. The bile ducts lead to the right and
left hepatic ducts, which exit the liver.
Figure 26.48 Illustration showing the structure of the
functional units of the liver, the lobules.
Liver – Lobules
Figure 26.49 Scanning power
photograph of the liver’s lobules with
Masson’s stain. Masson’s stain
differentiates the connective tissue
that surrounds the lobules.
Figure 26.50 High power photograph of
a lobule. The lobules are the functional
units of the liver.
Liver - Glucose
Figure 26.51 Low and high power
photographs of the liver showing
hepatocytes with glycogen. Storage
of glycogen is promoted by the
hormone insulin, which is secreted
by the pancreatic islets when blood
levels of glucose increase.
Figure 26.52 Low and high power
photographs of hepatocytes with their
glycogen removed. The tissue is
treated with amylase which digests the
glycogen into glucose.
Liver - Glucose
• The liver has several metabolic functions
that relate to the processing of blood
received from the digestive system. One
function is the processing of glucose.
• The liver functions as the primary glucose
storage organ for the maintenance of
blood glucose levels. The liver is targeted
by the blood sugar regulating hormones
insulin and glucagon, which are secreted
by the pancreas.
Liver - Kupffer Cells
• The liver functions as a
site for macrophages
which remove
substances such as
bacteria and worn out
red blood cells from the
blood. The macrophages
of the liver are called
Kupffer cells and are
fixed macrophages
because they remain in
the sinusoids of the liver
Figure 26.53 Kupffer cells are the fixed
macrophages of the liver. Located within
the sinusoids, the Kupffer cells function to
remove foreign materials found within the
blood.
17
LARGE INTESTINE
LARGE INTESTINE
• The large intestine extends from the end of the
ileum to the anus and is mostly positioned
around the small intestine.
• The gross anatomy of the large intestine differs
from the small intestine in three major
differences,
– (1) in its diameter, the
– (2) presence of haustra, and
– (3) epiploic appendages.
The large intestine functions mainly as the
(1) final absorption site for water and remaining electrolytes,
(2) compacts remaining fecal material, and is a
(3) storage site for waste materials prior to defecation.
LARGE INTESTINE
• As its name indicates, the diameter of the large
intestine is greater than that of the small
intestine.
– The longitudinal muscle of the large intestine is not
organized into an encircling layer as found in the
small intestine. Instead, the longitudinal muscle is
organized into three ribbon-like bands, the teniae coli.
Muscle tone of the teniae coli shortens the large
intestine forming pouch-like regions called haustra.
• Projecting from the surface of the large intestine
are numerous fat-filled sacs, the epiploic
appendages. The epiploic appendages are
formed from the serosa (visceral peritoneum).
LARGE INTESTINE
LARGE INTESTINE
•
The large intestine is divided into five
regions, the
– (1) cecum,
– (2) appendix,
– (3) colon,
– (4) rectum, and
– (5) anal canal.
LARGE INTESTINE
• Cecum
– The large intestine begins with a pouch-like portion, the cecum.
The distal region of the small intestine, the ileum, joins the
cecum at the ileocecal valve. The ileocecal valve is relaxed by
the hormone gastrin, produced during the gastric phase of the
stomach’s response to arriving food. Attached to the inferior
aspect of the cecum is the appendix.
• Appendix
– The walls of the appendix house lymphatic tissue.
• Colon
–
–
–
–
–
The colon is divided into four major divisions, the
(1) ascending colon, the
(2) transverse colon, the
(3) descending colon, and the
(4) sigmoid colon.
• The short anal canal terminates at the opening to the
outside called the anus.
Figure 26.54 The large intestine functions mainly as the final absorption site for
water and remaining electrolytes, compacts remaining fecal material, and is a
storage site for waste materials prior to defecation. The large intestine is divided into
five regions, the (1) cecum, (2) appendix, (3) colon, (4) rectum, and (5) anal canal.
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LARGE INTESTINE
LARGE INTESTINE - Mucosa
Figure 26.56 The mucosa of the large
intestine is modified for absorption
(simple columnar epithelium) and the
secretion of mucus (intestinal glands).
Figure 26.55 Scanning power photograph
of the large intestine showing its four
layers.
Figure 26.57 The mucosa of the large
intestine contains numerous goblet
cells, which function in the secretion of
lubricating mucus.
LARGE INTESTINE -
Muscularis Externa
The muscularis externa consists of
(1) an inner circular muscle layer, and
(2) the teniae coli.
• The teniae coli are the three ribbon-like bands of
longitudinally arranged smooth muscle of the large
intestine. Muscle tone of the teniae coli puckers the large
intestine into pouch-like areas called haustra.
• The muscularis externa functions in the mixing and the
movement of the intestinal contents. Mixing by
segmentation occurs as some haustra undergo
contractions and move the contents into adjacent
haustra. Mass movements occur as peristaltic waves
travel long distances to move the intestinal contents
forward and into the rectum.
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