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Saladin 5e Extended Outline
Chapter 25
The Digestive System
I. General Anatomy and Digestive Processes (pp. 966–969)
A. The study of the digestive tract and its disorders is called gastroenterology. (p. 966)
B. The digestive system is the organ system that processes food, extracts nutrients, and eliminates
the residue. (p. 966)
1. Its function is accomplished in five stages.
a. Ingestion is the selective intake of food.
b. Digestion is the mechanical and chemical breakdown of food into usable
form.
c. Absorption is the uptake of nutrient molecules into the epithelial cells of the
digestive tract and then into the blood or lymph.
d. Compaction is the absorbing of water and consolidation of indigestible
residue into feces.
e. Defecation is the elimination of feces.
2. The digestive phase has two facts, mechanical and chemical.
a. Mechanical digestion is the physical breakdown of food into smaller particles.
i. Action of the teeth and the churning contractions of the stomach and
small intestine are parts of mechanical digestion.
ii. It exposes more food surface to the action of enzymes.
b. Chemical digestion is a series of hydrolysis reactions that break
macromolecules into monomers (residues).
i. These monomers are monosaccharides, amino acids, monoglycerides
and fatty acids, and nucleic acids.
ii. Chemical digestion is carried out by digestive enzymes of the
salivary glands and stomach, pancreas, and small intestine.
3. Some nutrients are already present in usable form without digestion: vitamins, free
amino acids, minerals, cholesterol, and water.
C. The digestive system has two anatomical subdivisions, the digestive tract and the accessory
organs. (pp. 966–968) (Fig. 25.1)
1. The digestive tract is a muscular tube extending from mouth to anus.
a. It is about 9 m (30 ft.) long in a cadaver.
b. It is also known as the alimentary canal.
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c. It includes the mouth, pharynx, esophagus, stomach, small intestine and large
intestine.
d. Part of this, the stomach and intestines, constitute the gastrointestinal (GI)
tract.
2. The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder, and
pancreas.
3. The digestive tract is open to the environment at both ends, and material in it is
considered external to the body until absorbed by epithelial cells.
4. Most of the digestive tract follows a basic structural plan, with a wall composed of the
following tissue layers, in order from inner to outer surface. (Fig. 25.2)
a. Mucosa, with epithelium, lamina propria, and muscularis mucosae.
b. Submucosa.
c. Muscularis externa, with inner circular layer and outer longitudinal layer.
d. Serosa, with areolar tissue and mesothelium.
5. Variations are found in different regions of the tract.
6. The mucosa (mucous membrane) lining the lumen consists of three layers: epithelium,
lamina propria, and muscularis mucosae.
a. The epithelium is simple columnar in most of the digestive tract, but stratified
squamous from the oral cavity through the esophagus and in the lower anal
canal, where there is more abrasion.
b. The muscularis mucosae tenses the mucosa, creating grooves and ridges that
enhance contact with food.
c. The mucosa has an abundance of lymphocytes and lymphatic nodules, the
mucosa-associated lymphatic tissue (MALT)
7. The submucosa is a thicker layer of loose connective tissue containing blood vessles,
lymphatic vessels, a nerve plexus, and glands in some places that secrete mucus.
a. The MALT extends into the submucosa in some regions of the GI tract.
8. The muscularis externa consists of usually two layers of muscle near the outer surface.
a. Cells of the inner layer encircle the tract, while those of the outer layer run
longitudinally.
b. Sphincters (valves) form in some places to regulate passage of material.
c. The longitudinal layer is responsible for motility that moves food and residue.
9. The serosa is composed of a thin layer of areolar tissue topped by a simple squamous
mesothelium.
a. It begins in the lower 3 to 4 cm of the esophagus and ends just before the
rectum.
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b. The pharynx, most of the esophagus, and the rectum are surrounded by a
fibrous connective tissue called the adventitia.
10. The esophagus, stomach, and intestines have a nervous network called the enteric
nervous system.
a. This system regulates digestive tract motility, secretion, and blood flow.
b. It is thought to have over 100 million neurons, more than the spinal cord.
c. It can function independently of the CNS although the CNS has a significant
influence over it.
d. The enteric nervous system is usually regarded as part of the autonomic
nervous system, but opinions vary.
e. It is composed of two networks of neurons: the submucosal (Meissner)
plexus, and the myenteric (Auerbach) plexus of parasympathetic ganglia and
nerve fibers between the layers of the muscularis externa.
i. Parasympathetic preganglionic fibers terminate in the ganglia of the
myenteric plexus.
ii. Postganglionic fibers arising in the is plexus not only innervate the
muscularis externa but also pass through its inner layer to contribute to
the submucosal plexus.
iii. The myenteric plexus controls peristalsis and other contractions of
the muscularis externa.
iv. The submucosal plexus controls movements of the muscularis
mucosae and glandular secretion of the mucosa.
f. The enteric nervous system also includes sensory neurons.
D. The peritoneum is a serous membrane that lines the wall of the abdominal cavity and gives rise
to connective tissue sheets called mesenteries. (pp. 968–969)
1. The stomach and intestines must be free to move in the abdominal cavity and are not
tightly bound to the abdominal wall, but are suspended from it by the mesenteries.
a. The mesenteries hold abdominal viscera in their proper relationship and
prevent the intestines from becoming twisted.
b. The mesenteries also provide passage for blood vessels and nerves as well as
containing lymphatic nodes and vessels.
2. Along the posterior (dorsal) midline, the parietal peritoneum turns inward and forms
the dorsal mesentery.
a. This is a translucent, two-layered membrane.
b. Upon reaching a digestive tract organ, the two layers of the mesentery
separate and pass around opposite sides of the organ, forming the serosa.
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3. In some places, the two layers come together again on the far side and continue as
another sheet, the ventral mesentery.
a. The ventral mesentery may hang freely in the abdominal cavity or attach to
the ventral abdominal wall or other organs.
4. The lesser omentum, a ventral mesentery, extends from the right superior margin
(lesser curvature) of the stomach to the liver. (Fig. 25.3)
5. Another membrane, the greater omentum, hangs from the left inferior margin (greater
curvature) of the stomach and loosely covers the small intestine like an apron.
a. At its inferior margin, the greater omentum turns back on itself and passes
upward, forming a deep pouch between its deep and superficial layers.
b. At its superior margin, it forms serous membranes around the spleen and
transverse colon.
c. Beyond the transverse colon, it continues as a mesentery called the
mesocolon; it anchors the colon to the posterior abdominal wall.
6. The omenta have a loosely organized, lacy appearance due partly to gaps in the
membranes and partly to irregularly distributed adipose tissue.
a. They also contain many lymph nodes and vessels, blood vessels, and nerves.
b. The omenta adhere to perforations of inflamed areas of the stomach or
intestines, contribute immune cells to the site, and isolate infections.
7. An organ enclosed by mesentery (serosa) on both sides is considered to be
intraperitoneal.
8. An organ lying against the posterior body wall and covered by peritoneum anteriorly
only is said to be retroperitoneal.
a. The duodenum, most of the pancreas, and parts of the large intestine are
retroperitoneal.
b. The stomach, liver, and other parts of the small and large intestines are
intraperitoneal.
E. The motility and secretion of the digestive tract are regulated by neural, hormonal, and
paracrine mechanisms. (p. 969)
1. Neural controls include short and long autonomic reflexes.
a. In short (myenteric) reflexes, stretching or chemical stimulation acts through
the myenteric nerve plexus to stimulate contractions in the nearby regions of
muscularis externa—as in the peristaltic contractions of swallowing.
b. Long (vagovagal) reflexes act through autonomic nerve fibers that carry
sensory signals to the CNS and convey motor commands back.
c. Parasympathetic fibers of the vagus nerves are especially important in
stimulating digestive motility and secretion.
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2. Numerous hormones, such as gastrin and secretin, are produced in the digestive tract
and stimulate relatively distant parts of the tract.
3. Paracrine secretions, such as histamine and prostaglandins, diffuse through tissue
fluids to stimulate nearby target cells.
II. The Mouth Through Esophagus (pp. 970–977)
A. The mouth is also known as the oral or buccal cavity. (pp. 970–973)
1. Its functions include ingestion, taste and sensory responses to food, chewing, chemical
digestions, swallowing, speech, and respiration.
2. The mouth is enclosed by the cheeks, lips, palate, and tongue. (Fig. 25.4)
a. its anterior opening is the oral fissure and its posterior opening is the fauces.
b. It is lined with stratified squamous epithelium that is keratinized in areas
subject to abrasion, such as gums and hard palate, and is nonkeratinized in areas
such as the floor of the mouth, the soft palate, and the inside of the cheeks and
lips.
3. The cheeks and lips retain food and push it between the food for chewing.
a. They are essential for articulate speech and for sucking and blowing actions.
b. Their fleshiness is due mainly to subcutaneous fat, buccinator muscles of the
cheeks, and the orbicularis oris of the lips.
c. A median fold called the labial frenulum attaches each lip to the gum between
the anterior incisors.
d. The vestibule is the space between the cheeks or lips and the teeth.
e. The lips are divided into three areas.
i. The cutaneous area is colored like the rest of the face and has hair
follicles and sebaceous glands.
ii. The red area (vermilion) is the hairless region where the lips meet;
here blood capillaries and nerve endings come closer to the epidermal
surface.
iii. The labial mucosa is the inner surface of the lip, facing the gums
and teeth.
4. The tongue, although muscular, is a remarkably agile and sensitive organ.
a. Its surface is covered with nonkeratinized stratified squamous epithelium and
has bumps and projects called lingual papillae, where the taste buds reside. (Fig.
25.5)
b. The anterior two-thirds of the tongue, called the body, occupies the oral
cavity, while the posterior one-third, the root, occupies the oropharynx.
c. The boundary between body and root is marked by a V-shaped row of vallate
papillae, and behind these, a groove called the terminal sulcus.
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d. The body is attached to the floor of the mouth by a median fold called the
lingual frenulum.
e. The muscles of the tongue compose most of its mass.
i. The intrinsic muscle, entirely within the tongue, produce the
movements of speech.
ii. The extrinsic muscles, with origins other than in the tongue, produce
the stronger movements of food manipulation and include the
genioglossus, hyoglossus, palatoglossus, and styloglossus.
f. Amid the muscles are serous and mucous lingual glands that secrete some
saliva.
g. The lingual tonsils are contained in the root.
5. The palate separates the oral cavity from the nasal cavity, making it possible to breath
while chewing food.
a. The anterior portion, the hard (bony) palate, is supported by the palatine
processes of the maxillae and the smaller palatine bones.
i. Transverse ridges called palatine rugae aid the tongue in holding and
manipulating food.
b. Posteriorly, the soft palate has a more spongy texture and is composed mainly
of skeletal muscle and glandular tissue.
i. It has a conical medial projection, the uvula, visible at the rear of the
mouth.
ii. The uvula helps retain food in the mouth until one is ready to
swallow.
c. A pair of muscular arches on each side of the oral cavity begin superiorly near
the uvula and follow the wall of the cavity to its floor.
i. The anterior one is the palatoglossal arch.
ii. The posterior one is the palatopharyngeal arch, which marks the
beginning of the pharynx.
iii. The palatine tonsils are located on the wall between the arches.
6. The teeth are collectively called the dentition; they serve to masticate the food,
breaking it into smaller pieces.
a. Adults normally have 16 teeth in the mandible and 16 in the maxilla.
b. From midline to the rear of each jaw, there are two incisors, a canine, two
premolars, and up to three molars. (Fig. 25.6b)
i. The incisors are chisel-like cutting teeth.
ii. The canines are more pointed and act to puncture and shred food.
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iii. The premolars and molars have relatively broad surfaces adapted for
crushing and grinding.
c. Each tooth is embedded in a socket called an alveolus, forming a joint called a
gomphosis between the tooth and bone. (Fig. 25.7)
i. The alveolus is lined by a periodontal ligament, a modified
periosteum that anchors the tooth firmly but allows for slight
movement when chewing.
d. The gum, or gingiva, covers the alveolar bone.
e. Parts of the tooth are defined by their relationship to the gingiva.
i. The crown is the portion above the gum.
ii. The root is the portion below the gum, embedded in alveolar bone.
iii. The neck is the point where the crown, root, and gum meet.
iv. The space between the tooth and gum is the gingival sulcus, and
hygiene of this sulcus is important to dental health.
Insight 25.1 Tooth and Gum Disease
f. Most of a tooth consists of hard yellowish tissue called dentin, covered with
enamel in the crown and neck and cementum in the root.
i. Dentin and cementum are living connective tissues with a calcified
matrix.
ii. Cells of the cementum (cementocytes) are scattered more or less
randomly and occupy tiny cavities similar to lacunae of bone.
iii. Cells of the dentin (odontoblasts) line the pulp cavity and have
slender processes that travel through parallel tunnels in the dentin.
iv. Enamel is not a tissue but a cell-free secretion produced before the
tooth erupts above the gum.
v. Damaged dentin and cementum can regenerate, but damaged enamel
cannot.
g. Internally, a tooth has a dilated pulp cavity in the crown and a narrow root
canal in the lower root.
i. These spaces are occupied by pulp, a mass of loose connective tissue,
blood, and lymphatic vessels, and nerves.
ii. The nerves and vessels enter the tooth through the apical foramen, at
the basal end of each root canal.
h. The meeting of the teeth when the mouth closes is called occlusion; the
surfaces where they meet are called the occlusal surfaces.
i. The occlusal surface of a premolar has two rounded bumps called
cusps; thus premolars are known as bicuspids.
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ii. The molars have four to five cusps.
iii. The cusps of upper and lower premolars and molars mesh when the
jaws are closed and slide over each other as the jaw makes lateral
chewing motions.
i. Teeth develop beneath the gums and erupt (emerge) in predictable order.
i. 20 deciduous teeth (milk teeth or baby teeth) erupt from the ages of 6
to 30 months, beginning with the incisors. (Fig. 25.6a)
ii. Between 6 and 25 years of age, these are replaced by the 32
permanent teeth.
iii. As a permanent tooth grows, the root of the deciduous tooth
dissolves and leaves little more than the crown by the time it falls out.
iv. The third molars (wisdom teeth) erupt around ages 17 to 25, if at all.
v. Wisdom teeth often become impacted—crowded and unable to erupt.
B. Mastication (chewing) is the first step in mechanical digestion; it breaks food into pieces small
enough to be swallowed and exposes more surface to digestive enzymes. (p. 974)
1. Mastication requires little thought; food stimulates oral receptors that trigger an
involuntary chewing reflex.
2. The tongue, buccinator, and orbicularis oris muscles manipulate food, pushing it
between the teeth.
3. The masseter and temporalis muscles produce the up-and-down crushing action of the
teeth, and the lateral and medial pterygoid muscles and masseters produce side-to-side
grinding.
C. Saliva moistens the mouth, digests some starch and fat, cleanses the teeth, inhibits bacterial
growth, dissolves molecules for sensing by taste receptors, and moistens and binds food particles.
(pp. 973–975)
1. It is a hypotonic solution of 97.0% to 99.5% water with the following solutes.
a. Salivary amylase is an enzyme that begins starch digestion.
b. Lingual lipase is an enzyme activated by stomach acid that digests fat after
food is swallowed.
c. Mucus binds and lubricates the food and aids in swallowing.
d. Lysozyme is an enzyme that kills bacteria.
e. Immunoglobulin A (IgA) is an antibody that inhibits bacterial growth.
f. Electrolytes include sodium, potassium, chloride, phosphate, and bicarbonate
salts.
2. Saliva has a pH of 6.8 to 7.0; some salivary components work better at one pH than at
another.
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a. Salivary amylase works well at neutral pH and is deactivated by the low pH of
the stomach.
b. Lingual lipase does not act in the mouth but is activated by the acidity of the
stomach.
3. There are two kinds of salivary glands, intrinsic and extrinsic.
a. The intrinsic salivary glands are an indefinite number of small glands
dispersed amid the other oral tissues.
i. They include the lingual glands in the tongue, labial glands on the
inside of the lips, and buccal glands on the inside of the cheeks.
ii. They secrete small amounts of saliva at a constant rate whether we
are eating or not.
iii. This saliva contains lingual lipase and lysozyme and serves to
moisten the mouth and inhibit bacterial growth.
b. The extrinsic salivary glands are three pairs of larger, more discrete organs
located outside the oral mucosa that communicate with the oral cavity by way of
ducts.
i. The parotid gland is located just beneath the skin anterior to the
earlobe; its duct opens into the mouth opposite the second upper molar.
ii. The submandibular gland is located halfway along the body of the
mandible deep to the mylohyoid muscle; its duct empties into the
mouth on the side of the lingual frenulum, near the lower central
incisors.
iii. The sublingual gland is located on the floor of the mouth and has
multiple ducts that empty into the mouth posterior to the papilla of the
submandibular duct.
c. All three extrinsic glands are compound tubuloacinar glands with a treelike
arrangement of branching ducts ending in acini.
i. Some acini have only mucous cells, some only serous, and some a
mixture. (Fig. 25.10)
ii. Mucous cells secrete salivary mucus and serous cell secrete a thinner
fluid rich in amylase and electrolytes.
4. Salivation is the production of saliva.
a. The extrinsic salivary glands secrete 1 to 1.5 L of saliva per day.
b. Food stimulates tactile, pressure, and taste receptors in the mouth, which
transmit signals to salivatory nuclei in the medulla oblongata and pons.
i. These nuclei also receive input from higher brain centers, so that
even the odor, sight, or thought of food can stimulate salivation.
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ii. Irritation of the stomach and esophagus also stimulates salivation.
c. The salivatory nuclei send signals to the glands by way of autnomic fibers in
the facial and glossopharyngeal nerves.
i. Stimuli such as aroma, taste, and texture of food cause
parasympathetic stimulation that produces abundant, thin saliva rich in
enzymes.
ii. Sympathetic stimulation briefly enhances salivation, but its primary
effect is to produce thicker saliva with more mucus; the mouth may feel
sticky or dry under conditions of stress.
d. Salivary amylase begins to digest starch as the food is chewed, while the
mucus in the saliva binds food into a mass called a bolus.
D. The pharynx is a muscular funnel that connects the oral cavity to the esophagus as well as
connecting the nasal cavity to the larynx. (p. 975)
1. It has a deep layer of longitudinally oriented skeletal muscle and a superficial layer of
circular skeletal muscle.
a. The circular muscle is divided into superior, middle, and inferior pharyngeal
constrictors that force food downward during swallowing.
b. When food is not being swallowed, the inferior constrictor remains contracted
to exclude air from the esophagus; this constriction is considered the upper
esophageal sphincter although it is not an anatomical feature of the esophagus.
c. The upper esophageal sphincter disappears at the time of death when the
muscle relaxes; thus it is considered a physiological sphincter rather than
anatomical.
E. The esophagus is a straight muscular tube 25 to 30 cm long. (p. 975) (Figs. 25.1, 25.2)
1. It begins at a level between vertebra C6 and the cricoid cartilage, inferior to the larynx
and posterior to the trachea.
2. After passing downward through the mediastinum, the esophagus penetrates the
diaphragm at the esophageal hiatus and continues another 3–4 cm to meet the stomach at
the level of T7.
3. The opening into the stomach is called the cardiac orifice (because of its position in the
body, relatively near the heart).
a. Food pauses briefly at this point because of the lower esophageal sphincter
(LES).
b. The LES prevents stomach contents from regurgitating into the esophagus.
c. “Heartburn” has nothing to do with the heart, but is the burning sensation
caused by acid reflux.
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4. The wall of the esophagus is organized into the typical digestive system layers with
some regional specializations.
a. The mucosa has a nonkeratinized stratified squamous epithelium.
b. The submucosa contains esophageal glands that secrete lubricating mucus
into the lumen.
c. When the esophagus is empty, the mucosa and submucosa are deeply folded
into longitudinal ridges.
d. The muscularis externa is composed of skeletal muscle in the upper 1/3, a
mixture of skeletal and smooth muscle in the middle 1/3, and only smooth
muscle in the lower 1/3.
e. Most of the esophagus in in the mediastinum, where it is covered with a
connective tissue adventitia, which merges with that of the trachea and thoracic
aorta; the short segment below the diaphragm is partially covered by a serosa.
F. Swallowing, or deglutition, involves over 22 muscles in the mouth, pharynx, and esophagus,
and is coordinated by the swallowing center in the medulla oblongata. (pp. 975–977)
1. The swallowing center, a pair of nuclei, communicates with muscles of the pharynx
and esophagus through the trigeminal, facial, glossopharyngeal, and hypoglossal nerves
(CN V, VII, IX, and XII).
2. Swallowing occurs in two phases. (Fig. 25.11)
a. The first phase, or buccal phase, is under voluntary control.
i. The tongue collects food, presses it against the palate to form a bolus,
and pushes it posteriorly.
ii. The food accumulates in the oropharynx, in front of the “blade” of
the epiglottis.
iii. When the bolus reaches a critical size, the epiglottis tips posteriorly
and the food bolus slides around it through a space on each side of the
glottis (laryngeal opening)
iv. As the bolus enters the laryngopharynx, it stimulates tactile
receptors and activates the next phase.
b. The pharyngo–esophageal phase is involuntary, and three actions prevent
food and drink from reentering the mouth or entering the nasal cavity or larynx.
i. The root of the tongue blocks the oral cavity.
ii. The soft palate rises and blocks the nasophraynx.
iii. The infrahyoid muscles pull the larynx up to meet the epiglottis
while the vestibular folds adduct to close the airway.
iv. The food bolus is driven downward by constriction of the upper,
then middle, and finally lower pharyngeal constrictors.
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v. As the bolus enters the esophagus, it stretches and triggers
peristalsis. (Fig. 25.11b)
vi. Esophageal motility is entirely involuntary although the upper
muscle is skeletal.
c. Esophageal peristalsis is moderated partly by a short reflex through the
myenteric nerve plexus.
i. The bolus stimulates a stretch receptor, which sends signals to the
muscularis externa behind and ahead of the bolus.
ii. The circular muscle behind the bolus constricts and pushes it
downward.
iii. Ahead of the bolus the circular muscle relaxes while the
longitudinal muscle contracts, pulling the wall of the esophagus upward
and making it wider.
d. Most food passes by gravity when we are standing or sitting upright, but
peristalsis ensures that food can be swallowed regardless of body position.
III. The Stomach (pp. 977–986)
A. The stomach is a muscular sac in the upper left abdominal cavity that receives food, stores it,
breaks it up, and begins digestion to produce chyme. (p. 977)
B. In terms of gross anatomy, it is a J-shaped and relatively vertical in tall people, and more nearly
horizontal in short people. (pp. 977–979) (Fig. 25.12)
1. The lesser curvature of the stomach extends about 10 cm from esophagus to duodenum
along the medial to superior aspect.
2. The greater curvature extends the longer distance, about 40 cm, from esophagus to
duodenum on the lateral to inferior aspect.
3. The stomach is divided into four regions.
a. The cardiac region (cardia) is a small area within about 3 cm of the cardiac
orifice.
b. The fundic region (fundus) is the dome-shaped portion superior to the
esophageal attachment.
c. The body (corpus) makes up the greatest part of the stomach distal to the
cardiac orifice.
d. The pyloric region is a slightly narrower pouch at the inferior end that is
subdivided into the antrum and the pyloric canal.
i. The pyloric canal terminates at the pylorus, a narrow passage into the
duodenum.
ii. The pylorus is surrounded by a thick ring of smooth muscle, the
pyloric (gastroduodenal) sphincter.
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C. In terms of innervation and circulation, the stomach receives parasympathetic nerve fibers from
the vagus nerves and sympathetic fibers from the celiac ganglia; it is supplied with book from the
celiac trunk, and all blood drained from the stomach and intestines enters the hepatic portal
circulation before returning to the heart. (p. 979)
D. Microscopic anatomy shows that the stomach wall has tissue layers similar to those of the
esophagus with some variation. (p. 979)
1. The mucosa is covered with a simple columnar glandular epithelium. (Fig. 25.13)
a. The apical regions of its surface cells are filled with mucin, which swells with
water and becomes mucus upon secretion.
2. The mucosa and submucosa are flat and smooth when the stomach is full, but form
longitudinal gastric rugae when emptied.
3. The lamina propria is almost entirely occupied by tubular glands.
4. The muscularis externa has three layers rather than two; an outer longitudinal layer, a
middle circular layer, and an inner oblique layer. (Fig. 25.12)
5. The gastric mucosa is pocked with depressions called gastric pits, also lined with
columnar epithelium. (Fig. 25.13)
a. Two or three tubular glands open into the bottom of each pit and span the rest
of the lamina propria.
b. In the cardiac and pyloric regions, they are called cardiac glands and pyloric
glands.
c. In the rest of the stomach, they are called gastric glands.
d. The three types of glands differ in composition but contain the following five
cell types.
i. Mucous cells secrete mucus and predominate in cardiac and pyloric
glands; in gastric glands they are called mucous neck cells because they
are located in the neck of the gland.
ii. Regenerative (stem) cells are found in the base of the pit and neck of
the gland; they divide rapidly and produce a continual supply of new
cells.
iii. Parietal cells are found mostly in the upper half of the gland and
secrete hydrochloric acid, intrinsic factor, and ghrelin; they are mostly
found in gastric glands.
iv. Chief cells, which are the most numerous, are found in the lower
half of gastric glands; they secrete gastric lipase and pepsinogen.
v. Enteroendocrine cells secrete hormones and paracrine messengers
and are found mostly in gastric and pyloric glands; there are at least
eight kinds, each of which produces a different messenger.
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D. The gastric secretions of the gastric glands produce 2 to 3 L of gastric juice per day, composed
mainly of water, hydrochloric acid, and pepsin. (pp. 979–981)
1. Gastric juice has a high concentration of hydrochloric acid and a pH as low as 0.8.
a. Parietal cells contain carbonic anhydrase (CAH), which catalyzes the first step
in the following reaction:
i. CO2 + H2O  H2CO3  HCO3– + H+
b. The H+ is pumped into the lumen of a gastric gland by an active transport
protein called H+–K+ ATPase, an antiport.
c. HCl secretion does not affect the pH within the parietal cell.
d. Bicarbonate ions (HCO3–) are exchanged for chloride ions (Cl–) from the
blood plasma, the same chloride shift that occurs in the renal tubules and red
blood cells, and the Cl– is pumped into the lumen as well.
e. Blood leaving the stomach has a higerh pH when digestion is occurring than
when the stomach is empty (the alkaline tide).
f. Stomach acid has several functions.
i. It activates the enzymes pepsin and lingual lipase.
ii. It breaks up connective tissues and plant cell walls.
iii. It converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+), which
can be absorbed and used.
iv. It contributes to nonspecific disease resistance by destroying
ingested pathogens.
2. Pepsin is one of the digestive enzymes secreted as an inactive protein called a
zymogen, which is then converted to active form by removal of certain amino acids.
a. In the stomach, chief cells secrete pepsinogen, and hydrochloric acid removes
some amino acids to convert it to pepsin.
b. Because pepsin digests protein and pepsinogen is a protein, pepsin has an
autocatalytic effect—as some pepsin is formed, it converts pepsinogen to pepsin.
(Fig. 25.15)
c. The ultimate function of pepsin is to digest dietary proteins to shorter chains
that are then completely digested in the small intestine.
3. Gastric lipase from chief cells, along with lingual lipase, digest 10% to 15% of the
dietary fat.
4. Intrinsic factor, a glycoprotein secreted by parietal cells, is essential to the absorption
of vitamin B12 in the small intestine.
a. Without vitamin B12, hemoglobin cannot be synthesized and pernicious
anemia develops.
Saladin Outline Ch.25
Page 15
b. The secretion of intrinsic factor is the only indispensable function of the
stomach.
c. As people age, the gastric mucosa atrophies, and the risk of pernicious anemia
rises.
5. As many as 20 chemical messengers are produced by the gastric and pyloric glands’
enteroendocrine cells.
a. Most are hormones that travel in the blood to distant target cells.
b. Some also behave as paracrine secretions.
c. Several are peptides produced in both the digestive tract and the central
nervous system and are thus called gut–brain peptides.
i. These include substance P, vasoactive intestinal peptide (VIP),
secretin, gastric inhibitory peptide (GIP), cholecystokinin, and
neuropeptide Y (NPY).
ii. Gastric secretions are summarized in Table 25.1.
E. Gastric motility turns ingested food into chyme. (p. 981–983)
1. Upon swallowing, the swallowing center of the medulla oblongata signals the stomach
to relax.
2. The arriving food stretches the stomach and activates the receptive-relaxation response
by which the stomach relaxes to accept more food.
3. The stomach soon shows rhythmic, peristaltic contractions about every 20 seconds that
are governed by pacemaker cells in the longitudinal layer of the muscularis externa of the
greater curvature.
4. After 30 minutes, these contractions become quite strong.
5. The antrum holds about 30 mL of chyme; as a peristaltic wave passes down, the
antrum squirts about 3 mL of chyme into the duodenum.
a. When the wave reaches the pyloric sphincter, it squeezes the sphincter shut.
b. Chyme that does not get through is turned back into the antrum.
6. Receiving only small amounts of chyme at a time allows the duodenum to neutralize
the stomach acid and digest nutrients little by little.
7. A typical meal is emptied from the stomach in about 4 hours.
F. Vomiting is the forceful ejection of stomach and intestinal contents (chyme) from the mouth.
(p. 983)
1. Vomiting involves multiple muscular actions integrated by the emetic center of the
medullar oblongata.
2. Vomiting is commonly induced by overstretching of the stomach or duodenum;
chemical irritants such as alcohol and bacterial toxins; visceral trauma; intense pain; or
psychological and sensory stimuli.
Saladin Outline Ch.25
Page 16
3. Vomiting is usually preceded by nausea and retching.
a. In retching, thoracic expansion and abdominal contraction create a pressure
difference that dilates the esophagus.
i. The lower esophageal sphincter relaxes while the stomach and
duodenum contract spasmodically.
ii. Chyme enters the esophagus but then drops back into the stomach as
the muscles relax; it does not get past the upper esophageal sphincter.
iii. Retching is often accompanied by tachycardia, profuse salivation,
and sweating.
b. Vomiting occurs when abdominal contraction and rising thoracic pressure
force the upper esophageal sphincter open, the esophagus and body of the
stomach relax, and chyme is driven out of the stomach and mouth by strong
abdominal contraction along with reverse peristalsis.
c. Projectile vomiting is sudden vomiting with no prior nausea or retching.
i. It may be caused by neurological lesions but is also common in
infants after feeding.
4. Chronic vomiting can cause dangerous fluid, electrolyte, and acid–base imbalances.
a. In cases of frequent vomiting, as in the eating disorder bulimia, tooth enamel
becomes severely eroded by acid in the chyme.
b. Aspiration of acid chyme is very destructive to the respiratory tract, and many
have died from aspiration of vomit when unconscious or semiconscious.
c. Surgical anesthesia must be preceded by fasting until the stomach and small
intestine are empty.
G. Most digestion and nearly all nutrient absorption occur in the small intestine. (p. 983)
1. The stomach does not absorb significant nutrients but does absorb aspirin and some
lipid-soluble drugs.
2. Alcohol is absorbed mainly by the small intestine, so its intoxicating effects depends
partly on how rapidly the stomach is emptied.
H. Protection of the stomach from self-digestion is accomplished by three means.
1. Mucous coat. The thick, highly alkaline mucus that coats the stomach resists the action
of acid and enzymes.
2. Tight junctions. The epithelial cells are joined by tight junctions that prevent gastric
juice from seeping between them and digesting the underlying tissues.
3. Epithelial cell replacement. The stomach’s epithelial cells live only 3 to 6 days and are
then sloughed off; they are rapidly replaced by cell division in the gastric pits.
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I. Gastric function is regulated by the nervous and endocrine systems; gastric activity is divided
into three stages called the cephalic, gastric, and intestinal phases, which overlap and may even
occur simultaneously. (pp. 983–986) (Fig. 25.17)
1. The cephalic phase is the stage in which the stomach responds to the mere sight, smell,
taste, or thought of food.
a. These sensory and mental inputs converge on the hypothalamus, which relays
signals to the medulla oblongata.
b. Vagus nerve fibers from the medulla stimulate the enteric nervous system of
the stomach.
2. The gastric phase is a period in which swallowed food and semidigested protein
activate gastric activity.
a. About 2/3 of gastric secretion occurs during this phase.
b. Ingested food stimulates gastric activity in two ways: by stretching the
stomach and by raising the pH of its contents.
c. Gastric secretion is stimulated chiefly by three chemicals: acetylcholine
(ACh), histamine, and gastrin.
i. ACh is secreted by parasympathetic nerve fibers of both short and
long reflex pathways.
ii. Histamine is a paracrine secretion from enteroendocrine cells in
gastric glands.
iii. Gastrin is a hormone produced by enteroendocrine G cells in the
pyloric glands.
d. All three stimulate parietal cells to secrete hydrochloric acid and intrinsic
factor.
e. As dietary protein is digested, it breaks down into smaller peptides and amino
acids, which directly stimulate G cells to secrete more gastrin—a positive
feedback loop. (Fig. 25.18)
f. As digestion continues and peptides are emptied from the stomach, the pH
drops; below pH 2, the parietal cells and G cells are inhibited—a negative
feedback loop.
Insight 25.2 Peptic Ulcer (Fig. 25.16)
3. The intestinal phase is a stage in which the duodenum responds to arriving chyme and
moderates gastric activity through hormones and nerve reflexes.
a. The duodenum initially enhances gastric secretion, but soon inhibits it.
i. Stretching of the duodenum accentuates vagovagal reflexes that
stimulate the stomach.
Saladin Outline Ch.25
Page 18
ii. Amino acids in the chyme stimulate G cells of the duodenum to
secrete more gastrin, which further stimulates the stomach.
b. Increased acid and semidigested fats in the duodenum trigger the enterogastric
reflex by which inhibitory signals are sent to the stomach by way of the enteric
nervous system.
c. Signals are also sent to the medulla oblongata that accomplish two actions.
i. The vagal nuclei are inhibited, reducing vagal stimulation of the
stomach.
ii. Sympathetic neurons are stimulated, sending inhibitory signals to the
stomach.
d. Chyme also stimulates duodenal enteroendocrine cells to release secretin and
cholecystokinin (CCK), which stimulate pancreas and gallbladder but also
suppress gastric secretion and motility.
e. The enteroendocrine cells also secrete glucose-dependent insulinotropic
peptide (GIP), originally called gastric-inhibitory peptide.
i. This peptide now appears to be more concerned with stimulating
insulin secretion.
IV. The Liver, Gallbladder, and Pancreas (pp. 986–992)
A. The liver is a reddish brown gland located inferior to the diaphragm and filling most of the
right hypochondriac and epigastric regions. (pp. 986–992) (Fig. 25.19)
1. It is the body’s largest gland, weighing about 1.4 kg (3 lbs).
2. It has many functions, of which only one, the secretion of bile, contributes to digestion.
3. In terms of gross anatomy, the liver has four lobes: the right, left, quadrate, and
caudate lobes.
a. Anteriorly, the right and left lobes are visible and are separated from each
other by the falciform ligament.
b. The round ligament (ligamentum teres), also visible anteriorly, is a remnant of
the umbilical vein.
c. From the inferior view, the quadrate lobe is seen next to the gallbladder, and
the tail-like caudate lobe is posterior to that.
i. An irregular opening between these lobes, the porta hepatis, is a point
of entry for the hepatic portal vein and proper hepatic artery.
ii. It is also the point of exit for the bile passages, all of which travel in
the lesser omentum.
d. The gallbladder adheres to a depression on the inferior surface of the liver
between the right and quadrate lobes.
Saladin Outline Ch.25
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e. Posteriorly, the liver exhibits a deep groove (sulcus) that accommodates the
inferior vena cava.
f. The superior surface has a bare area where it is attached to the diaphragm.
g. A serosa covers the rest of the liver.
2. In terms of microscopic anatomy, the interior of the liver is filled with tiny cylinders
called hepatic lobules, about 2 mm long by 1 mm in diameter.
a. A lobule consists of a central vein surrounded by radiating sheets of cuboidal
cells called hepatocytes. (Fig. 25.20)
b. Each plate of hepatocytes is an epithelium one or two cells thick.
c. The spaces between the plates are blood-filled channels called hepatic
sinusoids, lined by fenestrated endothelium.
i. The hepatocytes have a brush border of microvilli that project into the
sinusoids.
ii. Blood that filters through the sinusoids comes directly from the
stomach and intestines; the hepatocytes absorb nutrients from this and
also remove and degrade hormones, toxins, bile pigments, and drugs.
iii. The hepatocytes also secrete albumin, lipoproteins, clotting factors,
angiotensinogen, and other factors into the blood.
iv. Between meals, they also break down stored glycogen.
d. The sinusoids also contain hepatic microphages (Kupffer cells) that remove
bacteria and debris.
e. The hepatic lobules are separated by a sparse connective tissue stroma that is
especially visible in the triangular area where three or more lobules meet.
i. The hepatic triad of two blood vessels and a bile duct are often found
in these areas.
ii. The blood vessels are small branches of the proper hepatic artery and
the hepatic portal vein, both of which supply blood to the sinusoids.
iii. Blood that filters through the sinusoids collects in the central vein
and ultimately into the right and left hepatic veins.
f. The liver secretes bile into the narrow channels called bile canaliculi between
the back-to-back layers of hepatocytes within each plate.
i. Bile passes from the canaliculi into the bile ductules of the triads and
ultimately into the right and left hepatic ducts.
ii. The hepatic ducts converge on the inferior side of the liver to form
the common hepatic duct.
iii. A short distance on, this is joined by the cystic duct coming from
the gall bladder. (Fig. 25.21)
Saladin Outline Ch.25
Page 20
g. The union of the common hepatic duct and the cystic duct forms the
bile duct. i. The bile duct joins the pancreatic duct near the duodenum
to form an expanded chamber called the hepatopancreatic ampulla.
ii. The ampulla terminates at a fold of tissue, the major duodenal
papilla, on the duodenal wall.
iii. This papilla contains a muscular hepatopancreatic sphincter
(sphincter of Oddi) that regulates passage of pile and pancreatic juice
into the duodenum.
B. The gallbladder is a pear-shaped sac on the underside of the liver that stores and concentrates
bile. (pp. 989–990)
1. It is about 10 cm long and is lined by a highly folded mucosa with a simple columnar
epithelium.
2. Its head (fundus) usually projects slightly beyond the inferior margin of the liver; its
neck (cervix) leads into the cystic duct.
3. Bile is a yellow-green fluid containing minerals, cholesterol, neutral fats,
phospholipids, bile pigments, and bile acids.
a. The principal pigment is bilirubin, derived from the decomposition of
hemoglobin.
i. Bacteria of the large intestine metabolize bilirubin to urobilinogen,
which give feces a brown color.
ii. In the absence of bile secretion, the feces are grayish white and
marked with streaks of undigested fat (acholic feces).
b. Bile acids (bile salts) are steroids synthesized from cholesterol that aid in fat
digestion and absorption.
c. All other components of the bile are wastes destined for excretion; if these
wastes become excessively concentrated, they may form gallstones.
Insight 25.3 Gallstones
4. Bile gets into the gallbladder by first filling the bile duct, then overflowing into the
gallbladder.
a. Between meals, the gallbladder absorbs water and electrolytes from the bile
and concentrates it by a factor of 5 to 10 times.
b. The liver secretes about 500 to 1,000 mL of bile per day.
5. About 80% of bile acids are reabsorbed in the ilium, the last portion of the small
intestine, and are returned to the liver, where hepatocytes absorb and resecrete them, a
process called the enterohepatic circulation.
a. This process reuses the bile acids two or more times during the digestion of a
meal.
Saladin Outline Ch.25
Page 21
b. The 20% that is not reabsorbed is excreted in feces.
c. This is the body’s only way of eliminating excess cholesterol.
d. New bile acids are synthesized from cholesterol to replace the quantity lost in
feces.
C. The pancreas is a spongy retroperitoneal gland posterior to the greater curvature of the stomach,
about 12 to 15 cm long and 2.5 cm thick. (pp. 990–991) (Fig. 25.21)
1. It has a globose head encircled by the duodenum, a midportion called the body, and a
blunt, tapered tail on the left.
2. It is both and endocrine and exocrine gland; the endocrine part involves the pancreatic
islets, which secrete insulin and glucagon.
3. About 99% of the pancreas is exocrine tissue, which secretes 1,200 to 1,500 mL of
pancreatic juice per day. (Table 25.2)
a. The cells of the secretory acini exhibit a high density of rough ER and
zymogen granules, which are vesicles filled with secretion. (Fig. 25.22)
b. The acini open into a system of larger ducts that eventually converge on the
main pancreatic duct.
c. The main pancreatic duct joins the bile duct at the hepatopancreatic ampulla.
d. The accessory pancreatic duct, however, branches from the main pancreatic
duct and opens independently into the duodenum at the minor duodenal papilla,
proximal to the major papilla.
e. The accessory duct allows pancreatic juice to be released even when bile is
not.
4. Pancreatic juice is an alkaline mixture of water, enzymes, zyomogens, sodium
bicarbonate, and other electrolytes.
a. The acini secrete the enzymes and zymogens, while the ducts secrete the
sodium bicarbonate.
b. The bicarbonate serves to buffer the hydrochloric acid arriving from the
stomach.
5. The pancreatic zymogens are trypsinogen, chymotrypsinogen, and
procarboxypeptidase.
a. When secreted into the intestinal lumen, trypsinogen is converted to trypsin
by enterokinase, which is secreted by the intestinal mucosa. (Fig. 25.23)
b. Trypsin is autocatalytic and converts trypsinogen into more trypsin.
c. Tripsin also converts the two other zymogens into chymotrypsin and
carboxypeptidase.
d. Tripsin’s primary role is the digestion of dietary protein.
Saladin Outline Ch.25
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6. Other pancreatic enzymes include pancreatic amylase, which digests starch; pancreatic
lipase, which digests fat; and ribonuclease and dexoyribonuclease, which digest RNA and
DNA.
D. Secretion is regulated by responses that cause release of pancreatic juice and bile upon three
types of stimuli.
1. Acethylcholine (ACh) coming from the vagus and enteric nerves stimulate pancreatic
acini to secrete enzymes even before food has been swallowed.
a. The enzymes remain stored in the pancreatic acini and ducts, however, for
release when chyme enters the duodenum.
2. Cholecystokinin (CCK) is secreted by the mucosa of the duodenum and proximal
jejunum in response to fats in the small intestine.
a. It stimulates the pancreatic acini to secrete enzymes.
b. It has a strongly stimulatory effect on the gallbladder, from which it gets its
name; it induces contractions of the gallbladder and relaxation of the
hepatopancreatic sphincter.
3. Secretin is produced in the small intestine in response to the acidity of chyme from the
stomach.
a. It stimulates ducts of the liver and pancreas to secrete sodium bicarbonate to
offset HCl and protect the intestinal mucosa from stomach acid.
b. This action also raises the intestinal pH to the optimum level for pancreatic
and intestinal digestive enzymes.
V. The Small Intestine (pp. 992–995)
A. Nearly all chemical digestion and nutrient absorption occur in the small intestine, so called
because of its small diameter (2.5 cm); it is the longest part of the digestive tract (2.7 to 4.5 m long
in life). (p. 992)
B. In terms of gross anatomy, the small intestine is a coiled mass filling most of the abdominal
cavity inferior to the stomach and liver; it is divided into three regions: duodenum, jejunum, and
ileum. (pp. 992–993) (Fig. 25.24)
1. The duodenum constitutes the first 25 cm (10 in.), beginning at the pyloric valve,
arcing around the head of the pancreas, and ending at a sharp bend called the
duodenojejunal flexure.
a. The name duodenum refers to its length, about that of 12 fingers.
b. Slightly distal to the pyloric valve, it exhibits wrinkles called the major and
minor duodenal papillae, where the pancreatic duct and accessory pancreatic
duct enter.
c. Most of the duodenum, along with the pancreas, is retroperitoneal.
Saladin Outline Ch.25
Page 23
d. Here stomach acid is neutralized, fats are emulsified by bile acids, pepsin is
inactivated by elevated pH, and pancreatic enzymes take over chemical
digestion.
2. The jejunum is the first 40% of the small intestine beyond the duodenum (1.0 to 1.7 m
in a living person).
a. Its name refers to the fact that early anatomists typically found it to be empty.
b. It begins in the upper left quadrant of the abdomen, but lies mostly within the
umbilical region.
c. It has large, tall, closely spaced circular folds, with relatively thick and
muscular walls and a rich blood supply giving it a relatively red color.
d. Most digestion and nutrient absorption occur here.
3. The ileum forms the last 60% of the postduodenal small intestine (1.6 to 2.7 m).
a. It occupies mainly the hypogastric region and part of the pelvic cavity.
b. Compared with the jejunum, its wall is thinner, less muscular, and less
vascular, and it has a paler pink color.
c. On the side opposite from its mesenteric attachment, the ileum has prominent
lymphatic nodules in clustered called Peyer patches.
4. The end of the small intestine is the ileocecal junction, where the ileum joins the
cecum of the large intestine.
a. The muscularis of the ileum is thickened here to form a sphincter, the
ileocecal valve, which protrudes into the cecum and regulates passage of food
residue.
5. Both the jejunum and ileum are intraperitoneal and thus covered with a serosa that is
continuous with the folded mesentery that suspends the small intestine from the posterior
abdominal wall.
C. In terms of microscopic anatomy, the tissue layers of the small intestine are reminiscent of
those in the esophagus and stomach with modifications for digestion and absorption. (pp. 993–
995)
1. The lumen is lined with simple columnar epithelium.
2. The muscularis externa is notable for a thick inner circular layer and a thinner outer
longitudinal layer.
3. A large internal surface area is produced by the small intestine’s relatively great length
and by three kinds of internal folds or projections: the circular folds, villi, and microvilli.
a. If the mucosa were smooth, its surface area would be only about 0.3 to 0.5 m2,
but with these features, the actual surface area is about 200 m2.
b. The circular folds (plicae circulares) are the largest folds and are transverse to
spiral ridges up to 10 mm high. (Fig. 25.21)
Saladin Outline Ch.25
Page 24
i. They occur from the duodenum to the middle of the ileum, altering
the path of chyme into a spiral and promoting mixing and absorption.
ii. They are not found in the distal half of the ileum, but most
absorption is complete by the time chyme reaches that point.
c. The projections called villi are about 0.5 to 1.0 mm high, with tonguelike or
fingerlike shapes. (Fig. 25.25)
i. They are largest in the duodenum and become smaller in more distal
regions.
ii. A villus is covered with two kinds of epithelial cells: columnar
absorptive cells (enterocytes) and mucus-secreting goblet cells.
iii. The cells are joined by tight junctions that prevent digestive
enzymes from seeping between them.
iv. The core of a villus is filled with areolar tissue of the lamina
propria; embedded are an arteriole, a capillary network, a venule, and a
lymphatic capillary called a lacteal.
v. Blood capillaries absorb most nutrients, but the lacteal absorbs most
lipids, giving its contents a milky appearance.
vi. The core of the villus also contains a few smooth muscle cells that
contract periodically to mix chyme and to move lymph down the
lacteal.
d. Each absorptive cell of a villus has a fuzzy brush border of microvilli about 1
μm high.
i. The brush border increases surface area and contains brush border
enzymes in the plasma membrane.
ii. These enzymes are not released into the lumen, but digest the chyme
contents as it contacts the brush border, a process called contact
digestion.
4. On the floor of the small intestine, between the bases of the villi, are numerous pores
that open into tubular glands called intestinal crypts (crypts of Lieberkühn).
a. These crypts are similar to gastric glands and extend as far as the muscularis
mucosae.
b. In the upper half they consist of enterocytes and goblet cells like those of the
villi.
c. The lower half is dominated by dividing stem cells, which replace epithelial
cells; these have a lifespan of 3 to 6 days.
d. A few Paneth cells, clustered at the base of each crypt, secretes lysozyme,
phospholipase, and definsins.
Saladin Outline Ch.25
Page 25
5. The duodenum has prominent duodenal (Brunner) glands in the submucosa that secrete
bicarbonate-rich mucus.
6. Throughout the small intestine, the lamina propria and submucosa have a large
population of lymphocytes that intercept pathogens.
D. The intestinal crypts secrete 1 to 2 L of intestinal juice per day, especially in response to acid,
hypertonic chyme and distension of the intestine.
1. This fluid has a pH of 7.4 to 7.8 and contains water and mucus but relatively little
enzyme.
2. Most enzymes that function in the small intestine are found in pancreatic juice and in
the brush border.
E. Motility of the small intestine serves three functions: (1) to mix chyme with bile and with
intestinal and pancreatic secretions; (2) to churn chyme and bring it into contact with the mucosa;
and (3) to move residue toward the large intestine.
1. Segmentation is a movement in which stationary, ringlike constrictions appear at
several places along with intestine and then relax as new constrictions form elsewhere.
(Fig. 25.26a)
a. Pacemaker cells of the muscularis externa set the rhythm of segmentation at
about 12 times per minute in the duodenum and 8 to 9 times per minute in the
ileum.
b. The slower rate in the distal region assists with progression toward the colon.
c. The intensity of contractions is modified by nervous and hormonal influences,
but not the frequency.
2. Peristalsis begins as segmentation declines, after most nutrients have been absorbed.
a. A peristaltic wave begins in the duodenum, travels 10 to 70 cm, and then dies
out, only to be followed by another wave that begins a little farther down the
tract. (Fig. 25.26b)
b. These successive, overlapping waves are called a migrating motor complex.
c. Over a period of about two hours, they move the chyme toward the colon; a
second complex then expels residue and bacteria from the small intestine,
thereby helping to limit bacterial colonization.
d. Refilling of the stomach at the next meal suppresses peristalsis and reactivates
segmentation.
3. The ileocecal valve is usually closed, but food in the stomach triggers both the release
of gastrin and the gastroileal reflex, both of which enhance segmentation in the ileum and
relax the valve.
4. As the cecum fills, the pressure pinches the valve shut and prevents reflux of cecal
contents into the ileum.
Saladin Outline Ch.25
Page 26
VI. Chemical Digestion and Absorption (pp. 996–1001)
A. Most of the digestible carbohydrate is starch; cellulose is indigestible. (pp. 996–997)
1. Starch is digested first to oligosaccharides up to eight glucose units long, then into the
disaccharide maltose, and finally to glucose, which is absorbed by the small intestine.
a. Beginning in the mouth, salivary amylase hydrolyzes starch to
oligosaccharides.
i. Salivary amylase functions best at pH 6.8 to 7.0, typical of the mouth.
ii. It is denatured upon contact with stomach acid, but if it is protected
in the middle of a food mass, it can continue to digest for as long as 1
to 2 hours.
b. About 50% of dietary starch is digested before it reaches the small intestine,
where pancreatic amylase continues the process. (Fig. 25.27)
c. In the small intestine, starch is entirely converted to oligosaccharides and
maltose within 10 minutes.
d. Digestion is completed as chyme contacts the brush border and is exposed to
enzymes.
i. Two enzymes, dextrinase and glucoamylase, hydrolyze
oligosaccharides that are three or more residues long.
ii. A third enzyme, maltase, hydrolyzes maltose to glucose.
e. The major dietary disaccharides are sucrose and lactose.
i. They are digested by the brush border enzymes sucrase and lactase,
and the resulting monosaccharides are immediately absorbed.
ii. Sucrose consists of glucose plus fructose; lactose consists of glucose
plus galactose.
iii. In most of the world population, lactose becomes indigestible after
about age 4.
Insight 25.4 Lactose Intolerance
2. The plasma membrane of absorptive cells contains transport proteins that absorb
monosaccharides. (Fig. 25.28)
a. About 80% of the absorbed sugar is glucose.
b. Glucose is taken up by a sodium–glucose transport protein (SGLT) like that
of the kidney tubules.
c. The glucose is then transported out the base of the cell and into the ECF.
i. Sugar entering the ECF increases its osmolarity, and water is drawn
from the lumen of the intestine through the now-leaky tight junctions.
ii. Water carried more glucose and other nutrients by solvent drag; as
much as two or three times as much glucose as carried by the SGLT.
Saladin Outline Ch.25
Page 27
d. The SGLT also absorbs galactose; fructose, in contrast, is absorbed by
facilitated diffusion using a separate carrier that does not depend on Na +.
e. Inside the epithelial cell, most fructose is converted to glucose.
f. Glucose, galactose, and remaining fructose are transported out the base of the
cell by facilitated diffusion and absorbed by blood capillaries.
g. The monosaccharides are then delivered to the liver by the hepatic portal
system.
B. Amino acids come from three protein sources: dietary proteins, digestive enzymes digested by
each other, and sloughed epithelial cells. (pp. 997–999)
1. Endogenous amino acids from the last two sources total about 30 g/day, compared to
44 to 60 g/day from food.
2. Enzymes that digest proteins are called proteases (peptidases).
3. In the stomach, pepsin hydrolyzes peptide bonds between tyrosine and phenylalanine,
breaking 10% to 15% of dietary protein into shorter polypeptides and some free amino
acids. (Fig. 25.29)
a. Pepsin has an optimal pH of 1.5 to 3.5, and is inactivated when it passes into
the duodenum and mixes with alkaline pancreatic juice (pH 8).
4. In the small intestine, trypsin and chymotrypsin from the pancreatic enzymes continue
hydrolysis.
5. Shorter oligopeptides are then taken apart by three more enzymes.
a. Carboxypeptidase, a pancreatic secretion, removes amino acids from the –
COOH end of the chain.
b. Aminopeptidase, a brush border enzyme, removes them from the –NH2 end.
c. Dipeptidase, also a brush border enzyme, splits dipeptides in the middle,
releasing the last two amino acids.
6. Amino acid absorption is similar to that of monsaccharides.
a. Several sodium-dependent amino acid cotransporters carry different classes of
amino acids across the membrane.
b. Dipeptides and tripeptides may also be absorbed, but are hydrolyzed within
the epithelial cells.
c. At the basal surfaces, amino acids leave the cell by facilitated diffusion, enter
the capillaries, and are carried away in the hepatic portal circulation.
7. The absorptive cells of infants can take up intact proteins by pinocytosis and release
them to the blood by exocytosis.
a. This allows IgA from breast milk to confer passive immunity.
b. Intact proteins entering the infant’s blood may be treated as foreign antigens
and sometimes trigger food allergies.
Saladin Outline Ch.25
Page 28
c. The ability of the intestine to pinocytose protein declines but never
completely ceases.
C. The hydrophobic quality of lipids makes their digestion and absorption more complicated that
that of carbohydrates and proteins (p. 999) (Fig. 25.30)
1. Fats are digested by enzymes called lipases.
2. Lingual lipase, secreted by the intrinsic salivary glands of the tongue, digests a small
amount of fat while food is still in the mouth, but becomes more active in the acidic pH
of the stomach.
3. Gastric lipase makes a much larger contribution to preduodenal fat digestion.
4. Most fat digestion occurs in the small intestine through the action of pancreatic lipase.
a. Fat in chyme is in large globules as it enters the duodenum.
b. It is broken up into smaller emulsification droplets by components of bile, the
lecithin and bile acids.
c. Intestinal segmentation also breaks the fat up into droplets as small as 1 μm in
diameter, and a coating of lecithin and bile acids keeps it broken up, exposing its
surface to enzymatic action.
5. There is enough pancreatic lipase in the small intestine after a meal to digest the
average daily fat intake in 1 or 2 minutes.
a. Lipase removes the first and third fatty acids of a triglyceride and leaves the
middle one, producing two free fatty acids (FFAs) and a monoglyceride.
6. Absorption of fatty acids, monoglycerides, and other lipids depends on minute droplets
in the bile called micelles.
a. Micelles consist of 20 to 40 bile acid molecules aggregated with hydrophilic
side groups facing outward and hydrophobic steroid rings facing inward.
b. Bile phospholipids and cholesterol diffuse into the center of the micells to
form its core.
c. The micelles pass down the bile duct into the duodenum, where they absorb
fat-soluble vitamins, more cholesterol, and the products of fat digestion.
d. Their hydrophilic surfaces allow them to remain suspended in water.
e. They transport lipids to the surfaces of the intestinal absorptive cells, where
the lipids leave the micelles and diffuse into the cells.
f. The micelles are then reused.
g. Without micelles, the small intestine absorbs only about 40% to 50% of
dietary fat and almost no cholesterol.
7. Within the intestinal cell, FFAs and monoglycerides are transported into the smooth
endoplasmic reticulum and resynthesized into triglycerides.
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a. The Golgi complex combines these triglycerides with a small amount of
cholesterol and coats the complex with a film of phospholipids and protein to
form droplets called chylomicrons.
b. It packages these into secretory vesicles that migrate to the basal surface of
the cell and release their contents into the core of the villus.
8. Chylomicrons are too large to penetrate the endothelium of blood capillaries and are
instead taken up by the more porous lacteals and into the lymph.
9. The fatty intestinal lymph (chyle) eventually flows through the cisterna chyli to the
thoracic duct, and then into the bloodstream at the left subclavian vein.
D. Nucleic acids are present in only small quantities compared to other dietary macromolecules.
(p. 999)
1. The nucleases (ribonuclease and deoxyribonuclease) of pancreatic juice hydrolyze
these to nucleotides.
2. Nucleosidases and phosphatases of the brush border then break these into phosphate
ions, ribose or deoxyribose, and nitrogenous bases.
3. These are transported by membrane carriers and enter the capillary blood.
E. Vitamins are absorbed unchanged. (p. 999)
1. The fat-soluble vitamins A, D, E, and K are absorbed with other lipids; if they are
ingested without fat-containing food, they are not absorbed at all.
2. Water-soluble vitamins (B complex and vitamin C) are absorbed by simple diffusion,
with the exception of B12.
a. This large molecule can only be absorbed if it binds to intrinsic factor from
the stomach.
b. The B12–intrinsic factor complex then binds to receptors on cells of the distal
ileum where it is taken up by receptor-mediated endocytosis.
F. Minerals (electrolytes) are absorbed along the entire length of the small intestine. (p. 1001)
1. Sodium ions are cotransported with sugars and amino acids.
2. Chloride ions are actively transported in the distal ileum by a pump that exchanges
them for bicarbonate ions, the reverse of the exchange that takes place in the stomach.
3. Potassium ions are absorbed by simple diffusion.
a. The potassium concentration of chyme rises as water is absorbed, creating a
gradient that favors potassium absorption.
b. In diarrhea, when water absorption is hindered, potassium absorption is
hindered also; this can lead to hypokalemia.
4. Iron and calcium are unusual in that they are absorbed in proportion to the body’s
need, whereas other minerals are absorbed at a constant rate.
5. Iron absorption is stimulated by the liver hormone hepcidin.
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a. The absorptive cells bind ferrous ions (Fe2+) but are unable to absorb ferric
ions (Fe3+).
b. Stomach acid reduces most ferric ions to ferrous.
c. Ferrous ions are transported to the basal surface of the cell and are taken up
by the extracellular protein transferrin.
i. The transferrin–iron complex diffuses into the blood and is carried to
bone marrow for hemoglobin synthesis, muscle tissue for myoglobin
synthesis, and the liver for storage.
ii. Excess dietary iron binds irreversibly to ferritin in the epithelial cells
and is held there until the cell sloughs off.
6. Calcium is absorbed throughout the intestine, some by active transport in the
duodenum, and more by diffusion between epithelial cells in the jejunum and ileum.
a. In the duodenum, Ca2+ enters the absorptive cells through calcium channels in
the apical cell membrane.
b. Inside the cell, it binds to a protein, calbindin, keeping the intracellular
concentration low and maintaining a gradient favorable to uptake.
c. Free calcium in the cytoplasm is pumped out the basal side by calcium–
ATPase as well as a Na+–Ca2+ antiport and from there to the blood capillaries.
d. Parathyroid hormone is secreted in response to a drop in blood calcium level,
stimulating the kidneys to synthesize vitamin D; this affects the absorptive cells
of the duodenum in three ways.
i. The number of calcium channels increases.
ii. The amount of calbindin in the cytoplasm increases.
iii. The number of calcium-ATPase pumps in the basal membrane
increases.
e. Vitamin D has no effect on the paracellular diffusion of calcium in the
jejunum and ileum, where most calcium is absorbed.
G. The digestive system is one of the systems involved in water balance. (p. 1001)
1. The digestive tract receives about 9 L of water per day from all sources.
2. About 8 L of this is absorbed by the small intestine, and 0.8 L by the large intestine,
leaving 0.2 L voided in the daily fecal output.
3. Water is absorbed by osmosis following the absorption of salts and organic nutrients
that pass into the ECF.
4. Diarrhea occurs when the large intestine reabsorbs too little water.
a. This may occur when the intestine is irritated by bacteria and feces pass
through too quickly.
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b. It may also result from abnormally high concentrations of a solute such as
lactose that opposes osmotic absorption of water.
5. Constipation occurs when fecal movement is slow, too much water is reabsorbed, and
the feces become hardened.
a. It can result from lack of dietary fiber, lack of exercise, emotional upset, or
long-term laxative abuse.
VII. Chemical Digestion and Absorption (pp. 1002–1006)
A. The large intestine is about 1.5 m (5 ft) long and 6.5 cm (2.5 in.) in diameter in a cadaver; it
receives about 500 mL of food residue per day and reduces it to about 150 mL. (p. 1002) (Fig.
25.31)
1. It begins with the cecum, a blind pouch in the lower right abdominal quadrant inferior
to the ileocecal valve.
2. Attached to the lower end of the cecum is the appendix, a blind tube 2 to 7 cm long; it
is a significant source of immune cells.
3. The colon is that part of the large intestine between the ileocecal junction and rectum;
it is divided into three regions.
a. The ascending colon begins at the ileocecal valve and passes up the right side
of the abdominal cavity, making a 90° turn at the right colic (hepatic) flexure
near the right lobe of the liver.
b. This is now the transverse colon; it passes horizontally across the upper
abdominal cavity and turns 90° downward at the left colic (splenic) flexure near
the spleen.
c. Here it becomes the descending colon, which passes down the left side of the
abdominal cavity.
4. The pelvic cavity is narrower than the abdominal cavity, so at the pelvic inlet the colon
turns medially and downward forming an S-shaped portion called the sigmoid colon.
a. An instrument called a sigmoidoscope is used to visually examine the sigmoid
colon.
5. In the pelvic cavity, the large intestine continues as the rectum, about 15 cm long.
a. The rectum has three infoldings called transverse rectal folds (rectal valves)
which allow passing of gas while retaining feces.
6. The final 3 cm of the large intestine is the anal canal. (Fig. 25.31b)
7. The anal canal passes through the levator ani muscle and terminates at the anus.
a. The mucosa at the anus forms longitudinal ridges called anal columns with
depressions between them called anal sinuses.
b. Passage of feces causes the sinuses to extrude extra mucus to lubricate the
anal canal during defecation.
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8. Large hemorrhoidal veins form superficial plexuses in the anal columns and around the
orifice.
a. They lack valves and are subject to distension and venous pooling.
b. Hemorrhoids are permanently distended veins that protrude into the anal
canal or form bulges external to the anus.
9. The muscularis externa of the colon is unusual in that although it completely encircles
the colon, its longitudinal fibers are concentrated in three thickened, ribbonlike strips.
a. Each strip is called a taenia coli.
b. The muscle tone contracts the colon lengthwise and causes its wall to bulge,
forming pouches called haustra.
10. In the rectum and anal canal, however, the longitudinal muscle forms a continuous
sheet without haustra.
11. The anus, like the urethra, is regulated by two sphincters.
a. The internal anal sphincter is composed of smooth muscle of the muscularis
externa.
b. The external anal sphincter is composed of skeletal muscle of the pelvic
diaphragm.
11. The ascending and descending colon are retroperitoneal, whereas the transverse and
sigmoid colon are covered with serosa and anchored to the posterior abdominal wall by
the mesocolon.
a. The serosa of these regions often has omental (epiploic) appendages, fatty
pouhces of peritoneum of unknown function.
B. In terms of microscopic anatomy, the mucosa of the large intestine has a simple columnar
epithelium in all regions except the lower half of the anal canal, where it has nonkeratinized
stratified squamous epithelium.
1. There are no circular folds or villi in the large intestine, but there are intestinal crypts.
2. These crypts are deeper than in the small intestine and have a greater density of goblet
cells for increased mucus secretion.
3. The lamina propria and submucosa have an abundance of lymphatic tissue.
C. The large intestine harbors about 800 species of bacteria collectively called the bacterial flora.
1. The relationship with the bacteria is mutually beneficial.
2. They provide us with additional nutrients that we cannot extract otherwise.
3. Some also synthesize B vitamins and vitamin K, which are absorbed by the colon.
4. Bacteria produce intestinal gas, which is a less desirable effect.
a. The large intestine contains about 7 to 10 L of gas, expelling about 500
mL/day as flatus and reabsorbing the rest.
b. Most is swallowed air, but the flora add to it.
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c. Cramping can result when undigested nutrients pass into the colon and furnish
an abnormal substrate that leads to increased gas production.
d. Flatus is composed of nitrogen, carbon dioxide, hydrogen, methane, hydrogen
sulfide, and two amines: indole and skatole.
e. Indole, skatole, and hydrogen sulfide produce the odor of flatus and feces.
D. The large intestine takes about 12 to 24 hours to reduce the residue of a meal to feces.
1. The intestine does not chemically change the residue, but reabsorbs water and
electrolytes from it.
2. Feces are 75% water and 25% solids.
a. The solids are 30% bacteria, 30% undigested dietary fiber, 10% to 20% fat,
and smaller amounts of protein, sloughed cells, salts, mucus, and other
secretions.
b. The fat is not from the diet but from broken down epithelial cells and bacteria.
3. The most common type of colonic motility is segmentation called haustral
contractions, which occur about every 30 minutes.
a. Distension of a haustrum with feces stimulates it to contract, which churns
and mixes the residue, promotes absorption, and passes the residue distally.
4. Stronger contractions called mass movements occur one to three times a day, last about
15 minutes, and move residue for several centimeters at a time.
a. Mass movements are often triggered by the gastrocolic and duodenocolic
reflexes, in which filling of the stomach and duodenum stimulates motility of
the colon.
b. Mass movements occur especially in the transverse colon and often within an
hour after breakfast.
E. Stretching of the rectum stimulates the two defecation reflexes, which are often felt soon after a
meal.
1. The intrinsic defecation reflex operates entirely within the myenteric nerve plexus.
a. Stretch signals travel through the plexus to the muscularis of the descending
and sigmoid colon and rectum.
b. A peristaltic wave is activate that drives feces downward and relaxes the
internal anal sphincter.
c. This reflex is relatively weak and usually requires the second reflex.
2. The parasympathetic defecation reflex is a spinal reflex. (Fig. 25.32)
a. Stretch signals are transmitted to the spinal cord.
b. Motor signals return by way of the pelvic nerves to intensify peristalsis in the
descending and sigmoid colon and rectum and to relax the internal anal
sphincter.
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3. These reflexes are involuntary and are the sole means of controlling defecation in
infants and people with transecting spinal cord injuries.
4. The external anal sphincter, like the external urethral sphincter, is under voluntary
control; defecation normally occurs only when this sphincter is voluntarily relaxed.
5. Defecation is also aided by the Valsalva maneuver.
6. If the defecation urge is suppressed, contractions cease in a few minutes and the
rectum relaxes; the reflexes occur again a few hours later.
7. Some common digestive disorders are listed in Table 25.3
Insight 25.5 The Man with a Hole in His Stomach (Fig. 25.33)
Connective Issues: Digestive System Interactions
Cross Reference
Additional information on topics mentioned in Chapter 25 can be found in the chapters listed below.
Atlas A: Abdominal organs and their relationships
Chapter 10: Muscles of the tongue
Chapter 16: Types of papillae and sense of taste
Chapter 17: Endocrine secretions of the pancreas
Chapter 21: Mucosa-associated lymphatic tissue (MALT)
Chapter 21: Peyer patches
Chapter 22: Anatomy of the pharynx
Chapter 24: Renal excretion of H+
Chapter 26: Role of gut–brain peptides in appetite regulation
Chapter 26: Importance of cellulose as fiber in the diet