Chapter 24

Copyright © John Wiley & Sons, Inc. All rights reserved.
Chapter 24
The
Digestive
System

The Digestive System
Gastroenterology is the study of the gastrointestinal
system.
 The digestive system
functions are ingestion,
secretion, digestion
(chemical and mechanical),
mixing and propulsion,
absorption and
defecation.

The digestive system, also called the gastrointestinal
system, is composed of the alimentary canal (GI tract),
and the accessory organs.
 The alimentary canal extends from the mouth to the
anus through the ventral body cavity (approximately 9
m, or 30 ft.).
 The accessory organs include the teeth, tongue, salivary
glands, liver, gallbladder, and pancreas.

The GI tract, and accessory organs like the liver and
pancreas, are responsible for facilitating the body’s
metabolic processes.
 Catabolism: Larger molecules are broken into smaller
molecules (mouth, stomach, duodenum).
◦ In the GI tract, this is called digestion and can occur
by either mechanical or chemical means.
 Anabolism: Smaller molecules are used as building
blocks for larger molecules (liver).

Mechanical digestion includes all movements that
facilitate catabolic processes:
 Mastication
 Swallowing
 Mixing
◦ Increase contact of food with digestive chemicals
 Peristalsis
◦ Movement of muscles within the GI tract that
facilitates movement of food

Chemical digestion is mainly accomplished by using
water to break chemical bonds (hydrolysis).
 Fats are broken down into fatty acids and glycerol.
 Carbohydrates are broken down from polysaccharides
into monosaccharides.
 Proteins are broken down into polypeptides and amino
acids.

(Interactions Animation)
Enzyme mediated hydrolysis
You must be connected to the internet to run this animation

Anatomy Overview
The wall of the GI tract from the lower esophagus to the
anal canal has the same basic, four-layered arrangement
of tissues.
 The four layers of the tract, from deep to
superficial, are the mucosa, submucosa,
muscularis, and serosa/adventitia.
 The lumen is
the inside of
the tube.

Anatomy Overview
The mucosa is a mucous membrane made of various types
of epithelium sitting on a loose connective tissue called the
lamina propria.
 Nonkeratinized stratified squamous epithelium (for
protection) lines the pharynx, esophagus, and anus.
 Simple columnar epithelium (for secretion/absorption)
lines the stomach and intestines.
◦ Located among the epithelial cells are various
glandular cells that secrete mucus and fluid into the
lumen of the tract (exocrine).

The lamina propria contains a prominent lymphoid tissue
(mucosa-associated lymphatic tissue or “MALT”) that
protect against disease. Underneath it is the muscularis
mucosae, a thin layer of smooth muscle that throws
the lining of the stomach
and small intestines into
tiny folds (increases
surface area to aid
digestion/absorption).
Anatomy Overview

Anatomy Overview
The submucosa is composed of loose connective tissue
that binds the mucosa to the muscularis.
 It contains blood and lymphatic vessels
(to receive absorbed substances)
and an extensive network
of neurons known as
the submucosal
plexus.

The muscularis of the mouth, pharynx, superior and
middle parts of the esophagus, and anal sphincter contains
skeletal muscle that allows for voluntary swallowing and
control of defecation. Throughout the rest of the
tract, it is smooth muscle, arranged in
inner circular and outer longitudinal
sheets, with the myenteric
nerve plexus continuing
between them.
Anatomy Overview

Anatomy Overview
The serosa/adventitia is the outermost layer.
 If attached to surrounding tissues (e.g. around the
esophagus), it is called adventitia - a fibrous connective
tissue arranged around the organ which it supports.
 If contained in the peritoneal cavity, it is called serosa ,
which has a slippery mesothelium surface layer.
◦ Serosa covers the intra-abdominal organs as the
visceral peritoneum.

Anatomy Overview

Anatomy Overview
The peritoneum is the body’s largest serous membrane,
and it wraps around most abdominopelvic organs.
 The visceral peritoneum forms the “serosa” of the
alimentary canal and
covers other intra-abdominal
organs.
 It then continues around
the abdominal wall as the
parietal peritoneum.

Anatomy Overview
Unlike the pericardium and pleurae, which smoothly
cover the heart and lungs, the peritoneum contains large
folds that bind the organs to one another and to the
cavity walls. There are five
major peritoneal folds:
◦ greater omentum
◦ falciform ligament
◦ lesser omentum
◦ mesentery
◦ mesocolon

Anatomy Overview
The greater omentum is the largest peritoneal fold.
 It drapes over the transverse colon and the anterior coils
of the small intestine like a “fatty apron”.
◦ It contains many lymph nodes that help combat
and contain infections
of the GI tract.
◦ The large amount of adipose tissue can greatly
expand (as seen in people with “beer bellies”).

Anatomy Overview
The falciform ligament attaches the liver to the anterior
abdominal wall
and diaphragm.

The lesser omentum is a peritoneal fold that suspends the
stomach and duodenum from the inferior edge of the liver.
 It forms a pathway for blood
vessels (hepatic portal
vein and common
hepatic artery)
to enter the liver,
and it contains the
common bile duct.
Anatomy Overview

Together, the mesentery (of the small intestine) and
mesocolon (of the large intestine) attach the bowel to the
posterior abdominal wall,
holding the intestines
loosely in place as
muscular contractions
mix and move the
luminal contents
along the GI tract.
Anatomy Overview

Anatomy Overview
Some abdominopelvic organs are covered by visceral
peritoneum only on their anterior surfaces. The portion of
the organ that lies behind the peritoneum is said to be
“retroperitoneal”. Organs in the
retroperitoneal space include:
 The kidneys and ureters
 Most of the pancreas
 The adrenal glands
 The aorta and inferior vena cava

Physiology Overview
Digestive activities of the gastrointestinal tract occur in
three overlapping phases:
1. The cephalic phase
2. The gastric phase
3. The intestinal phase
As we study the glands of the
mouth, stomach, and small
intestine – as well as the
secretions of the accessory organs the liver and pancreas –
we will learn about the specifics of all 3 phases.

Physiology Overview
During the cephalic phase of digestion, the smell,
sight, thought, or initial taste of food activates neural
centers in the cerebral cortex, hypothalamus, and
brain stem to prepare for digestion.
 The brain stem activates the facial (CN VII) and
glossopharyngeal (CN IX) nerves to stimulate
secretion of saliva, while the vagus nerves (CN X)
stimulate secretion of gastric juice.

Physiology Overview
Once food reaches the stomach, the gastric phase of
digestion begins.
 Neural and hormonal mechanisms (the hormone
gastrin is a key player) promote secretion of gastric
juice and increase gastric motility.
The intestinal phase of digestion begins once food enters
the small intestine.
 Neural and hormonal responses promote the continued
digestion of foods that have reached the small intestine.

The Mouth
The oral or buccal cavity, is formed by the cheeks, hard
and soft palates, and the tongue.
 Mechanical digestion of food through mastication
(chewing) enables it to be mixed with saliva to form a
soft flexible bolus that can be easily swallowed.
 Saliva starts the process of chemical digestion of food
◦ Saliva is 99.5% water, with tiny amounts of dissolved
ions, IgA, lysozyme (a bacteriolytic enzyme), and
salivary amylase (a digestive enzyme that acts on
starch).

Three large salivary glands secrete most of the saliva: the
parotid, submandibular, and sublingual glands. The
smaller glands are found on the lips (labial), cheeks
(buccal), palate
(palatal), and tongue
(lingual).
Daily salivary
secretions average
1–1½ liters.
The Mouth

The Mouth
Salivary regulation is under the control of the ANS
 Parasympathetic stimulation promotes secretion of a
moderate amount of saliva. Salivary centers are located
in the brain stem and efferent nerve impulses are
transmitted by the facial (VII) and glossopharyngeal (IX)
nerves.
◦ Touch (pressoreceptors), smell, taste (taste buds), and
psychological factors are also salivary stimulators.
 Sympathetic stimulation decreases saliva secretions.

The Mouth
Carbohydrate Digestion in the Mouth

The Mouth
Lipid Digestion in the Mouth

The Mouth
The tongue is composed of skeletal muscle under
voluntary somatic motor control - it forces the moistened
food bolus into position for swallowing (deglutition) and
places the bolus into contact with the teeth for chewing.
 The extrinsic muscles of the tongue attach to bones in
the area and move the tongue from side to side.
 Intrinsic muscles originate within the tongue and alter
its shape and size for speech and swallowing

The Mouth

The Mouth
The teeth or dentes are located in sockets of the alveolar
processes of the mandible and maxillae. The sockets are
lined by the periodontal ligament - a dense fibrous
connective tissue that anchors the teeth to the socket
walls and acts as a shock absorber during chewing.
 A typical tooth has three major external regions: the
crown, root, and neck.
◦ The neck of each tooth is covered by the gingivae , or
gums, which extend slightly into each socket.

The Mouth
Dentin is a calcified connective tissue that forms most of
the tooth.
 The dentin of the crown is
covered by enamel, a
harder-than-bone
calcified material and
encloses the pulp cavity,
a space filled with pulp
(a connective tissue
containing blood vessels,
nerves, and lymphatic vessels).

The Mouth
Humans have two dentitions or sets of teeth.
 There are 20 deciduous or “baby teeth” which begin to
erupt around 6 months of age.
◦ All are lost between 6–12 yrs. of ages
 There are 32 permanent teeth
numbered from right to left, top
(1–16) to bottom (17–32).
◦ The third molars (teeth 1,
16, 17, and 32) are the
wisdom teeth.

Mastication
The Mouth

Deglutition
Deglutition is the act of swallowing food. The oropharynx
and laryngopharynx have digestive as well as respiratory
functions, and swallowed food must transit them both on
the way to the esophagus – and it must do so while not
going into the nasal cavity or the airway.
 This complex process involves many muscles subject to a
number of voluntary and involuntary controls.
◦ Deglutition has 3 stages: voluntary, pharyngeal, and
esophageal.

Deglutition
The first stage is the voluntary act of the tongue forcing the
bolus to the back of the oral cavity and into the oropharynx.
The involuntary pharyngeal stage begins as the bolus passes
into the oropharynx - receptors send impulses to the
deglutition center in the medulla and pons.
 Returning impulses cause the soft palate to move superiorly
and posteriorly to close the nasopharynx.
 The epiglottis moves slightly inferiorly to close the glottis.

Deglutition
The esophageal stage of swallowing begins once the bolus
enters the esophagus.
 Peristalsis, a progression of coordinated contractions
and relaxations of the circular and longitudinal layers
of the muscularis, push the bolus onward.

Deglutition
Deglutition

The Esophagus
The only digestive system function that occurs in the
esophagus is propulsion (moving food into the stomach).
The esophagus is a muscular tube that
begins inferior to the laryngopharynx,
and positioned posterior to the trachea.
 Leaving the neck, it traverses the
thoracic cavity down the posterior
mediastinum before piercing the
diaphragm through the esophageal
hiatus.

The mucosa of the esophagus is nonkeratinzed stratified
squamous epithelium
 The type of muscle in the
muscularis of the esophagus
varies by region
◦ the superior 1/3 is
skeletal muscle
◦ the intermediate 1/3 is
skeletal and smooth muscle
◦ the inferior 1/3 is
smooth muscle
The Esophagus

The Esophagus
Upper and lower esophageal sphincters (UES and LES)
are situated at each end of the tube.
 The LES regulates the movement of food from the
esophagus into the stomach.
◦ Incompetence of the LES
results in Gastroesophageal
Reflux Disease (GERD),
which manifests as
“heart burn”.

The stomach is a J-shaped enlargement of the GI tract situated between the esophagus
and the first part of the small intestine (the duodenum).
The position and size of the stomach varies continually - the diaphragm pushes it
inferiorly with each inhalation and pulls it superiorly with each exhalation.
 Rugae are large folds in the
mucosa of the empty stomach
which enable gastric
distension, depending
on the amount of stomach
contents.
The Stomach

The Stomach
Empty, the stomach is about the size of a large sausage,
but it is the most distensible part of the GI tract and can
expand to accommodate a large quantity of food.
 Because a meal can be eaten much more quickly than
the intestines can digest and absorb it, one of the
functions of the stomach is to serve as a mixing
chamber and holding reservoir.
 As a functional adaptation, the gastric muscularis
contains an additional 3rd inner oblique layer of muscle
to facilitate the mixing action of mechanical digestion.

Note the additional oblique layer of smooth
muscle in the gastric muscularis, which is
limited primarily to the body of the stomach
The Stomach
The stomach has four main regions:
 the cardia
 the fundus
 the body
 the pylorus

The Stomach
Simple columnar epithelial cells (surface mucous cells)
line the mucosal surface and secrete a protective mucous.
 Columns of secretory
cells extend down into
the lamina propria
forming gastric glands.
 Several gastric glands open
into the bottom of narrow
channels called
gastric pits.

The Stomach
A variety of specialized exocrine and endocrine cell types
populate the gastric glands and pits.
 Exocrine gland cells include:
◦ mucous neck cells which produce mucus
◦ parietal cells which produce intrinsic factor and HCl
◦ chief cells which secrete the protease pepsinogen and
gastric lipase
 Enteroendocrine G cells, located mainly in the pyloric
antrum, secrete the hormone gastrin into the
bloodstream.

The Stomach
The secretions of the mucous, parietal, and chief cells
form gastric juice, which totals 2–3 l/d.
The stomach is protected
from its own gastric
juice by a 1–3 mm
thick layer of alkaline
mucus secreted by
surface mucous cells
and mucous neck cells.

The Stomach
The strongly acidic nature of gastric juice kills many
microbes, partially denatures proteins in food, and
converts pepsinogen into pepsin.
 Pepsin is the only proteolytic enzyme in the stomach.
 Gastric lipase splits triglycerides.
 Intrinsic factor (IF) is needed for absorption of vitamin
B12 in the terminal ileum.
◦ Vitamin B12 is needed for RBC production.

The Stomach
Disturbing the balance between hydrochloric acid
production, pepsin secretion, and mucosal defenses can
lead to erosion of the stomach's epithelial lining.
 This graphic shows an
endoscopic view of
a gastric erosion, possibly
caused by consumption of
too much alcohol or use
of an NSAID drug such as
aspirin or ibuprofen.

The Stomach
Gentle, rippling, peristaltic movements called mixing
waves pass over the stomach every 15 to 25 seconds.
 These waves macerate food, mix it with secretions of
the gastric glands, and reduce it to a soupy liquid
called chyme.

The Stomach
Stomach Peristalsis

Chemical Digestion - Gastric Acid
The Stomach

Protein Digestion in the Stomach
The Stomach

Lipid Digestion in the Stomach
The Stomach

The Stomach

The Stomach
Although digestion is a major function of the stomach, its
epithelial cells are impermeable to most materials, and
very little absorption takes place.
Within 2 to 4 hours after eating a meal, the stomach has
emptied its contents into the duodenum.
 Foods rich in carbohydrate spend the least time.
 High-protein foods remain somewhat longer.
 Emptying is slowest after a fat-laden meal containing
large amounts of triglycerides.

The Stomach
At appropriate intervals, the stomach allows a small
amount of chyme to pass through the pyloric sphincter
and enter the duodenum to begin the intestinal phase
of digestion.
Completion of digestion
is a collective effort of
pancreatic juice,
bile, and intestinal juice
in the small intestine.

The Pancreas
Digestion and absorption in the small intestine depend
heavily on secretions from the pancreas and gallbladder
(liver).
 The pancreas is an oblong gland located posterior to the
stomach in the retroperitoneal space.
◦ It is connected to the duodenum by the
hepatopancreatic ampulla and accessory ducts.
◦ It secretes enzymes, which digest food in the small
intestine, and sodium bicarbonate, which buffers the
acidic pH of chyme.

The Pancreas

The Pancreas
About 99% of pancreatic acini (glandular clusters)
participate in exocrine secretion – only 1% of the clusters,
called pancreatic islets,
form the endocrine
portion of the gland
(secreting the hormones
glucagon, insulin, and
somatostatin and
pancreatic polypeptide).

The Pancreas
About 1-1.5 liters of alkaline pancreatic juice is secreted
into the duodenum each day. It creates the proper pH for
the following digestive enzymes in the small intestine:
 A starch digesting enzyme called pancreatic amylase
 Several enzymes that cleave polypeptides into dipeptides
and single amino acids: trypsin, chymotrypsin,
carboxypeptidase, and elastase
 Pancreatic lipase, the major triglyceride (fat) digesting
enzyme in adults

Carbohydrate Digestion – The Pancreas
The Pancreas

Lipid Digestion - Bile Salts and Pancreatic Lipase
The Pancreas

The liver is the body’s largest gland and second largest
organ. It has 2 main lobes
(right and left –
divided by the falciform
ligament) and is covered
by visceral peritoneum.
The liver is made up of
repeating functional units
called liver lobules.
The Liver and Gallbladder

Hepatocytes are the major functional cells of the liver. As
the body’s “chemical factories”, their metabolic versatility
is truly remarkable. Hepatocytes participate in a number
of digestive and non-digestive functions.
 Important digestive functions include:
◦ the synthesis, transformation, and
storage of proteins, carbohydrates,
and fats
◦ detoxification, modification, and excretion
of a variety of exogenous and endogenous substances

The heparin molecule
Non-digestive liver functions include:
 Phagocytosis of old or worn-out cells
 Making heparin (anticoagulant) and other plasma
proteins (prothrombin, fibrinogen, and albumin)
 Modifying vitamin D to its active form
Human albumin
Vitamin D3, the active
form of the molecule

Venous blood (from the hepatic portal vein) and arterial
blood (from the hepatic artery) feed the lobule from the
triad on its outer margin.
 The blood mixture percolates through endothelial-lined
spaces called
sinusoids
(a specialized
capillary)
towards the
central vein.

Path of blood in hepatic sinusoid
Microstructure of the liver lobule

Fixed macrophages within the sinusoids called Kupffer
cells destroy red cells, white cells, and
bacteria in blood draining
from the GI tract.
An important function of lobule
hepatocytes is to secrete bile, an
excretory product that helps emulsify fats for the watery
environment of small intestine digestive juices.
Hepatocytes secrete about 1 liter of bile per day.

Bile is an alkaline solution consisting of water, bile salts,
cholesterol, and bile pigments. It is both an excretory
product and a digestive secretion.
 Bile salts are used in the small intestine for the
emulsification and absorption of lipids.
◦ Without bile salts, most of the lipids in food would
be passed out in feces, undigested.
 The dark pigment in bile is called bilirubin and comes
from the catabolism of old red blood cells.

Bile secreted into the canaliculi (located between the
hepatocytes) exits the liver in the common hepatic duct.
 This duct joins the
cystic duct from the
gallbladder to form
the common bile
duct (CBD).

The CBD works its way towards the duodenum and joins
with the pancreatic duct to form
the hepatopancreatic
ampulla just proximal
to the second part of the
duodenum.
 The duodenal papilla
(“nipple”) pierces the
intestinal mucosa to
deliver its contents.

Between meals, the
sphincter of the
hepatopancreatic ampulla is
closed – bile “backs-up” into
the gall bladder where it is
stored and concentrated up
to ten-fold through the
absorption of water and
ions.

Under the influence of the hormone cholecystokinin
(CCK), the gallbladder contracts and ejects stored bile.
Although not necessary for life, normal gall bladder
function is highly desirable.
 After surgical
removal of the
gall bladder (called a
cholecystectomy), a person
would experience severe indigestion
if they ate a large meal high in fat content.

Chemical Digestion – Bile

The Small Intestine
The small intestine is divided into 3 regions:
 The duodenum (10 in)
 The jejunum (8 ft)
 The ileum (12 ft)
◦ If measured in a cadaver, the intestines are longer
than if measured in a live person due to the loss of
smooth muscle contraction.
In the small intestine, digestion continues, even while the
process of absorption begins.

The Small Intestine
Mechanical digestion in the small intestine is a localized
mixing contraction called segmentations.
 Segmentations is a type of peristalsis used to mix chyme
and bring it in contact with the mucosa for absorption.
 It begins in the lower portion of the stomach and
pushes food forward along a small stretch of small
intestine.
◦ It is governed by the myenteric plexus.

The Small Intestine
Segmentation Animation

The Small Intestine
Circular folds called the plicae circulares are permanent
ridges of the mucosa and
submucosa that encourage
turbulent flow of chyme.

The Small Intestine
Villi are multicellular structures that can barely be seen
by the naked eye. They form finger-like projections that
are covered with a simple columnar epithelium.

Microvilli are microscopic folds in the apical surface of
the plasma membrane on each simple columnar cell
(about 200 million/mm2
).
The plicae circulares,
villi, and microvilli all
contribute to increase
the surface area of the
small intestine, allowing
for maximum reabsorption of nutrients.
The Small Intestine

The small intestinal mucosa contains many deep crevices
lined with glandular epithelium (intestinal glands) that
secrete intestinal juice. Its function is to complete the
digestive process begun by
pancreatic juice.
 Trypsin exists in pancreatic
juice in the inactive form
trypsinogen - it and other
enzymes are activated by
intestinal juice.
The Small Intestine

The Small Intestine
Most of the enzymatic digestion in the small intestine
occurs inside the epithelial cells or
on their surfaces (rather than in
the lumen of the tube) as
intestinal juice comes in
contact with the brush
border of the villi.

The Small Intestine
Digestion on the Brush Border

The Small Intestine
Before discussing the absorption of nutrients, the events of
gastric and intestinal digestion are reviewed in this
animation.
Hormonal Control of Digestive Activities

The Small Intestine
Intestinal absorption is the passage of digested nutrients
into the blood or lymph: 90% of all intestinal absorption
occurs in the small intestine.
 Proteins (amino acids), nucleic acids, and sugars
(monosaccharides) are absorbed into blood capillaries
by facilitated diffusion or active transport.
 Triglycerides (fats) aggregate into globules along with
phospholipids and cholesterol and become coated with
proteins. These large spherical masses are called
chylomicrons.

The Small Intestine
Chylomicrons, too large to enter blood capillaries, enter
specialized lymphatic vessels called lacteals and
eventually drain
into the superior
vena cava and
mix with blood.
All dietary
lipids are absorbed
by simple diffusion.

The Small Intestine
Carbohydrate Absorption in the Small Intestine

Protein Absorption in the Small Intestine
The Small Intestine

Nucleic Acid Absorption in the Small Intestine
The Small Intestine

Lipid Absorption in the Small Intestine
The Small Intestine

The large intestine is about 5 feet in length. Starting at the
ileocecal valve, the large intestine has 4 parts:
 The cecum
 The colon
◦ ascending
◦ transverse
◦ descending
◦ sigmoid
 The rectum
 The anal canal
The Large Intestine

The Large Intestine
There are no circular folds or villi in the large intestine.
 The mucosa is mostly an absorptive epithelium (mainly
for water), and microvilli are plentiful.
 Interspersed goblet
cells produce mucous,
but no digestive
enzymes are secreted.

The Large Intestine
The large intestine is attached to the posterior
abdominal wall by its mesocolon peritoneal membrane.
Teniae coli are 3 separate longitudinal ribbons of
smooth muscle that run the length of the colon.
 Because the teniae coli is shorter than the intestine,
the colon becomes sacculated into small pouches
called haustra (giving it a segmented appearance).
◦ As one haustrum distends, it stimulates muscles to
contract, pushing the contents to the next
haustrum.

The Large Intestine
Hanging inferior to the ileocecal valve is the cecum, a
small pouch about 2.5 in long.
 Attached to the cecum is a 3 in coiled tube called the
appendix.
The open end of the cecum merges with a long tube
called the colon, with its various parts.
 Both the ascending and descending colon are
retroperitoneal; the transverse and sigmoid colon
are not.

The Large Intestine
The rectum is the last 8 in of the GI tract and lies anterior
to the sacrum and coccyx.
 The terminal 1 in of the rectum is
called the anal canal . The mucous
membrane of the anal canal is
arranged in longitudinal folds
called anal columns that contain
a network of arteries and veins.
◦The opening of the anal canal
to the exterior is called the anus.

The Large Intestine

The Large Intestine
Including the 2 liters we drink,
about 9 liters of fluid enter the
small intestine each day.
 The small intestine absorbs
about 8 liters; the remainder
passes into the large intestine,
where most of the rest of it is
also absorbed.
◦ Only 100 mL/d of water is
excreted in the feces.

The Large Intestine
Feces are the waste leftover after digesting and absorbing
all the nutrients we can from eaten material. Though it
is lower in energy than the food it came from, feces may
still contain a large amount of energy, often 50% of that
of the original food.
 The characteristic brown coloration comes from a
combination of bile and bilirubin.
 The distinctive odor is due to bacterial action - both
aerobic and anaerobic bacteria participate.

The Large Intestine
Though the human body consists of about 100 trillion
cells, we carry about ten times as many microorganisms
in the intestines. Bacteria make up most of the flora in
the colon and about 60% of the dry mass of feces.
As these bacteria digest/ferment left-over food, they
secrete beneficial chemicals such as vitamin K, biotin (a B
vitamin), and some amino acids (they are our main
source of some of these nutrients.)

The Large Intestine
The mechanical events associated
with defecation include localized
haustral churning and peristalsis.
Two autonomic nervous system
reflexes that initiate strong bouts
of mass peristalsis are the
gastroileal reflex and the
gastrocolic reflex.
◦ Both reflexes occur with
distension of the stomach. Gastric distension initiates mass
peristalsis by the ANS

The Large Intestine
The gastroileal reflex causes relaxation of the ileocecal
valve, intensifies peristalsis in the ileum, and forces any
chyme into the cecum.
The gastrocolic reflex intensifies strong peristaltic waves
that begin at about the middle of the transverse colon and
quickly drive the contents of the colon into the rectum.
 This mass peristalsis takes place three or four times a
day during or immediately after a meal, and may lead to
defecation.

The Large Intestine
The defecation reflex is activated by stretch receptors
stimulated by filling of the rectum.
 The events leading to defecation include:
◦ Food in the stomach stimulates mass peristalsis.
◦ Food moves through the intestine into the rectum.
◦ Rectal pressoreceptors respond to distention and
longitudinal muscles shorten the rectum.
◦ ANS releases the internal anal sphincter and gives a
conscious awareness of distention.
◦ Release of external sphincter is under conscious
control.

Mechanical Digestion in the Large Intestine
The Small Intestine

Chapter 24

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Chapter 24