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
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 Digestive System
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).
The Digestive System
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
The Digestive System
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.
The Digestive System
(Interactions Animation)
Enzyme mediated hydrolysis
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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).
Anatomy Overview
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
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.
Anatomy Overview
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
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.
Anatomy Overview
• 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
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).
The Mouth
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
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
(Interactions Animation)
Carbohydrate Digestion in the Mouth
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The Mouth
(Interactions Animation)
Lipid Digestion in the Mouth
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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.
The Mouth
(Interactions Animation)
Mastication
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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
(Interactions Animation)
Deglutition
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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 Esophagus
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
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
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
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
The Stomach
The stomach has four main regions:
 the cardia
 the fundus
 the body
 the pylorus
Note the additional oblique layer of smooth
muscle in the gastric muscularis, which is
limited primarily to the body of the stomach
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
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
(Interactions Animation)
Stomach Peristalsis
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The Stomach
(Interactions Animation)
Chemical Digestion - Gastric Acid
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The Stomach
(Interactions Animation)
Protein Digestion in the Stomach
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