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UNIT
Seven
Alterations in the
Gastrointestinal System
26
CHAPTER
Structure and Function of
the Gastrointestinal System
Digestion and Absorption
Carbohydrates
Fats
Proteins
Anorexia, Nausea, and Vomiting
Anorexia
Nausea
Vomiting
Structure and Organization of the
Gastrointestinal Tract
Upper Gastrointestinal Tract
Esophagus
Stomach
Middle Gastrointestinal Tract
Lower Gastrointestinal Tract
Gastrointestinal Wall Structure
Innervation and Motility
Innervation
Enteric Nervous System
Autonomic Nervous System
Swallowing and Esophageal Motility
Gastric Motility
Small Intestinal Motility
Colonic Motility
Defecation
Hormonal and Secretory Function
Gastrointestinal Hormones
Gastrointestinal Secretions
Salivary Secretions
Gastric Secretions
Intestinal Secretions
S
tructurally, the gastrointestinal tract is a long, hollow tube
with its lumen inside the body and its wall acting as an
interface between the internal and external environments.
The wall does not normally allow harmful agents to enter the
body, nor does it permit body fluids and other materials to es-
cape. The process of digestion and absorption of nutrients re-
quires an intact and healthy gastrointestinal tract epithelial
lining that can resist the effects of its own digestive secretions.
The process also involves movement of materials through the
gastrointestinal tract at a rate that facilitates absorption, and it
requires the presence of enzymes for the digestion and absorp-
tion of nutrients.
As a matter of semantics, the gastrointestinal tract also is re-
ferred to as the
digestive tract
, the
alimentary canal
, and at times,
the
gut
. The intestinal portion also may be called the
bowel
. For
the purposes of this text, the salivary glands, the liver, and the
pancreas, which produce secretions that aid in digestion, are
considered
accessory organs
.
459
460
Unit Seven: Alterations in the Gastrointestinal System
the first three parts of the gastrointestinal tract. The liver and
pancreas are discussed in Chapter 28.
STRUCTURE AND ORGANIZATION
OF THE GASTROINTESTINAL TRACT
Upper Gastrointestinal Tract
The mouth forms the entryway into the gastrointestinal tract
for food; it contains the teeth, used in the mastication of food,
and the tongue and other structures needed to direct food to-
ward the pharyngeal structures and the esophagus.
In the digestive tract, food and other materials move slowly
along its length as they are systematically broken down into
ions and molecules that can be absorbed into the body. In the
large intestine, unabsorbed nutrients and wastes are collected
for later elimination. Although the gastrointestinal tract is lo-
cated inside the body, it is a long, hollow tube, the lumen (
i.e.
,
hollow center) of which is an extension of the external envi-
ronment. Nutrients do not become part of the internal envi-
ronment until they have passed through the intestinal wall and
have entered the blood or lymph channels.
For simplicity and understanding, the digestive system can
be divided into four parts (Fig. 26-1). The upper part—the
mouth, esophagus, and stomach—acts as an intake source and
receptacle through which food passes and in which initial di-
gestive processes take place. The middle portion consists of the
small intestine—the duodenum, jejunum, and ileum. Most di-
gestive and absorptive processes occur in the small intestine.
The lower segment—the cecum, colon, and rectum—serves as
a storage channel for the efficient elimination of waste. The
fourth part consists of the accessory organs—the salivary
glands, liver, and pancreas. These structures produce digestive
secretions that help dismantle foods and regulate the use and
storage of nutrients. The discussion in this chapter focuses on
Esophagus
The esophagus is a tube that connects the oropharynx with
the stomach. The esophagus begins at the lower end of the
pharynx. It is a muscular, collapsible tube, approximately
25 cm (10 in) long, that lies behind the trachea. The muscu-
lar walls of the upper third of the esophagus are skeletal-type
striated muscle; these muscle fibers are gradually replaced by
smooth muscle fibers until, at the lower third of the esopha-
gus, the muscle layer is entirely smooth muscle.
The esophagus functions primarily as a conduit for passage
of food from the pharynx to the stomach, and the structures of
its walls are designed for this purpose: the smooth muscle lay-
ers provide the peristaltic movements needed to move food
along its length, and the epithelial layer secretes mucus, which
protects its surface and aids in lubricating food. There are
sphincters at either end of the esophagus: an upper esophageal
Soft palate
Hard palate
Nasopharynx
Oral cavity
Tongue
Sublingual gland
Submandibular
gland
Parotid gland
Oropharynx
Pharynx
Trachea
Esophagus
Diaphragm
Liver (cut)
Stomach
Gall bladder
Duodenum
Common
bile duct
Pancreas
Ascending
colon
Spleen
Transverse
colon
Jejunum
Small
intestine
Descending
colon
Cecum
Vermiform
appendix
Sigmoid colon
Ileum
Anus
Rectum
■
FIGURE 26-1
■
The digestive system.
461
Chapter 26: Structure and Function of the Gastrointestinal System
called the
body
, the orifice encircled by a ringlike muscle that
opens into the small intestine is called the
pylorus
, and the
portion between the body and pylorus is called the
antrum
(Fig. 26-2). The presence of a true pyloric sphincter is a matter
of controversy. Regardless of whether an actual sphincter exists,
contractions of the smooth muscle in the pyloric area control
the rate of gastric emptying.
KEY CONCEPTS
STRUCTURE AND FUNCTION OF
THE GASTROINTESTINAL TRACT
The gastrointestinal tract is a long, hollow tube that
extends from the mouth to the anus; food and fluids
that enter the gastrointestinal tract do not become
part of the internal environment until they have
been broken down and absorbed into the blood
or lymph channels.
■
Middle Gastrointestinal Tract
The small intestine, which forms the middle portion of the di-
gestive tract, consists of three subdivisions: the duodenum, the
jejunum, and the ileum. The duodenum, which is approxi-
mately 22 cm (10 in) long, connects the stomach to the jeju-
num and contains the opening for the common bile duct and
the main pancreatic duct. Bile and pancreatic juices enter the
intestine through these ducts. It is in the jejunum and ileum,
which together are approximately 7 m (23 ft) long and must be
folded onto themselves to fit into the abdominal cavity, that
food is digested and absorbed.
The wall of the gastrointestinal tract is essentially a
five-layered tube: an inner mucosal layer; a support-
ing submucosal layer of connective tissue; a fourth
and fifth layer of circular and longitudinal smooth
muscle that functions to propel its contents in a
proximal-to-distal direction; and an outer, two-
layered peritoneum that encloses and prevents
friction between the continuously moving
segments of the intestine.
■
Lower Gastrointestinal Tract
The large intestine, which forms the lower gastrointestinal tract,
is approximately 1.5 m (4.5 to 5 ft) long and 6 to 7 cm (2.4 to
2.7 in) in diameter. It is divided into the cecum, colon, rectum,
and anal canal. The cecum is a blind pouch that projects down
at the junction of the ileum and the colon. The ileocecal valve
lies at the upper border of the cecum and prevents the return of
feces from the cecum into the small intestine. The appendix
arises from the cecum approximately 2.5 cm (1 in) from the
ileocecal valve. The colon is further divided into ascending,
transverse, descending, and sigmoid portions. The ascending
colon extends from the cecum to the undersurface of the liver,
where it turns abruptly to form the right colic (hepatic) flexure.
The transverse colon crosses the upper half of the abdominal
cavity from right to left and then curves sharply downward
beneath the lower end of the spleen, forming the left colic
The nutrients contained in ingested foods and fluids
must be broken down into molecules that can be
absorbed across the wall of the intestine. Gastric
acids and pepsin from the stomach begin the diges-
tive process: bile from the liver, digestive enzymes
from the pancreas, and brush border enzymes break
carbohydrates, fats, and proteins into molecules that
can be absorbed from the intestine.
■
sphincter and a lower esophageal sphincter. The upper eso-
phageal, or pharyngoesophageal, sphincter consists of a circu-
lar layer of striated muscle. The lower esophageal, or gastro-
esophageal, sphincter is an area approximately 3 cm above the
junction with the stomach. The circular muscle in this area
normally remains tonically contracted, creating a zone of high
pressure that serves to prevent reflux of gastric contents into the
esophagus. During swallowing, there is “receptive relaxation”
of the lower esophageal sphincter, which allows easy propul-
sion of the esophageal contents into the stomach. The lower
esophageal sphincter passes through an opening, or hiatus, in
the diaphragm as it joins with the stomach, which is located in
the abdomen. The portion of the diaphragm that surrounds the
lower esophageal sphincter helps to maintain the zone of high
pressure needed to prevent reflux of stomach contents into the
esophagus.
Fundus
Esophagus
Body
Pylorus
Stomach
The stomach is a pouchlike structure that lies in the upper part
of the abdomen and serves as a food storage reservoir during
the early stages of digestion. Although the residual volume of
the stomach is only approximately 50 mL, it can increase to
almost 1000 mL before the intraluminal pressure begins to rise.
The esophagus opens into the stomach through an opening
called the
cardiac orifice
, so named because of its proximity to
the heart. The part of the stomach that lies above and to the left
of the cardiac orifice is called the
fundus
, the central portion is
Duodenum
Antrum
■
FIGURE 26-2
■
Structures of the stomach, showing the pace-
maker area and the direction of chyme movement resulting from
peristaltic contractions.
462
Unit Seven: Alterations in the Gastrointestinal System
(splenic) flexure. The descending colon extends from the colic
flexure to the rectum. The rectum extends from the sigmoid
colon to the anus. The anal canal passes between the two me-
dial borders of the levator ani muscles. Powerful sphincter
muscles guard against fecal incontinence.
Jejunum
Gastrointestinal Wall Structure
The digestive tract is essentially a five-layered tube (Fig. 26-3).
The inner luminal layer, or
mucosal layer
, is so named because
its cells produce mucus that lubricates and protects the inner
surface of the alimentary canal. The epithelial cells in this layer
have a rapid turnover rate and are replaced every 4 to 5 days.
Approximately 250 g of these cells are shed each day in the
stool. Because of the regenerative capabilities of the mucosal
layer, injury to this layer of tissue heals rapidly without leaving
scar tissue. The
submucosal layer
consists of connective tissue.
This layer contains blood vessels, nerves, and structures re-
sponsible for secreting digestive enzymes. The third and fourth
layers, the
circular
and
longitudinal muscle layers
, facilitate move-
ment of the contents of the gastrointestinal tract. The outer
layer, the
peritoneum
, is loosely attached to the outer wall of the
intestine.
The peritoneum is the largest serous membrane in the body,
having a surface area approximately equal to that of the skin.
The peritoneum consists of two continuous layers—the parietal
and the visceral peritoneum. The
parietal peritoneum
comes in
contact with and is loosely attached to the abdominal wall,
whereas the
visceral peritoneum
invests the viscera such as the
stomach and intestines. A thin layer of serous fluid separates
the parietal and visceral peritoneum, forming a potential space
called the
peritoneal cavity
. The serous fluid forms a moist and
slippery surface that prevents friction between the continu-
ously moving abdominal structures. In certain pathologic
states, the amount of fluid in the potential space of the peri-
toneal cavity is increased, causing a condition called
ascites
.
The jejunum and ileum are suspended by a double-layered
fold of peritoneum called the
mesentery
(Fig. 26-4). The mesen-
tery contains the blood vessels, nerves, and lymphatic vessels
Double layer
of peritoneum
Mesentery
Arterial arcades in mesentery
■
FIGURE 26-4
■
The attachment of the mesentery to the small
bowel. (Thomson J.S. [1977].
Core textbook of anatomy.
Philadel-
phia: J.B. Lippincott)
that supply the intestinal wall. The mesentery is gathered in
folds that attach to the dorsal abdominal wall along a short line
of insertion, giving a fan-shaped appearance, with the in-
testines at the edge. A filmy, double fold of peritoneal mem-
brane called the
greater omentum
extends from the stomach to
cover the transverse colon and folds of the intestine (Fig. 26-5).
Liver
Lesser omentum
Pancreas
Stomach
Duodenum
Peritoneum
Transverse
colon
Longitudinal
muscle
Mesentery
Greater
omentum
Circular
muscle
Small intestine
Parietal
peritoneum
Submucosa
Uterus
Lumen of gut
Visceral
peritoneum
Bladder
Rectum
Mucosa
(mucous
membrane)
■
FIGURE 26-3
■
Transverse section of the digestive system.
(Thomson J.S. [1977].
Core textbook of anatomy.
Philadelphia:
J.B. Lippincott)
■
FIGURE 26-5
■
Reflections of the peritoneum as seen in sagittal
section.
463
Chapter 26: Structure and Function of the Gastrointestinal System
Enteric Nervous System
The intramural neurons (
i.e.
, those contained within the wall
of the gastrointestinal tract) consist of two networks: the
myenteric and submucosal plexuses. Both plexuses are aggre-
gates of ganglionic cells that extend along the length of the
gastrointestinal wall. The myenteric (Auerbach’s) plexus is lo-
cated between the circular muscle and longitudinal muscle
layers, and the submucosal (Meissner’s) plexus between the
mucosal layer and the circular muscle layers (Fig. 26-6). The
activity of the neurons in the myenteric and submucosal
plexuses is regulated by local influences, input from the ANS,
and by interconnecting fibers that transmit information
between the two plexuses.
The myenteric plexus consists mainly of a linear chain of
interconnecting neurons that extend the full length of the gas-
trointestinal tract. Because it extends all the way down the in-
testinal wall and because it lies between the two muscle layers,
it is concerned mainly with motility along the length of the gut.
The submucosal plexus, which lies between the mucosal and
circular muscle layers of the intestinal wall, is mainly con-
cerned with controlling the function of each segment of the in-
testinal tract. It integrates signals received from the mucosal
layer into local control of motility, intestinal secretions, and
absorption of nutrients.
Intramural plexus neurons also communicate with receptors
in the mucosal and muscle layers. Mechanoreceptors monitor
the stretch and distention of the gastrointestinal tract wall, and
chemoreceptors monitor the chemical composition (
i.e.
, osmo-
lality, pH, and digestive products of protein and fat metabo-
lism) of its contents. These receptors can communicate directly
with ganglionic cells in the intramural plexuses or with visceral
afferent fibers that influence ANS control of gastrointestinal
function.
The greater omentum protects the intestines from cold. It
always contains some fat, which in obese persons can be a con-
siderable amount. The omentum also controls the spread of in-
fection from gastrointestinal contents. In the case of infection,
the omentum adheres to the inflamed area so that the infection
is less likely to enter the peritoneal cavity. The lesser omentum
extends between the transverse fissure of the liver and the lesser
curvature of the stomach.
In summary,
the gastrointestinal tract is a long, hollow
tube, the lumen of which is an extension of the external envi-
ronment. The digestive tract can be divided into four parts:
an upper part, consisting of the mouth, esophagus, and
stomach; a middle part, consisting of the small intestine; a
lower part, consisting of the cecum, colon, and rectum; and
the accessory organs, consisting of the salivary glands, the
liver, and the pancreas. Throughout its length, except for the
mouth, throat, and upper esophagus, the gastrointestinal
tract is composed of five layers: an inner mucosal layer, a sub-
mucosal layer, a layer of circular smooth muscle fibers, a layer
of longitudinal smooth muscle fibers, and an outer serosal
layer that forms the peritoneum and is continuous with the
mesentery.
INNERVATION AND MOTILITY
The motility of the gastrointestinal tract propels food prod-
ucts and fluids along its length, from mouth to anus, in a
manner that facilitates digestion and absorption. Except in
the pharynx and upper third of the esophagus, smooth mus-
cle provides the contractile force for gastrointestinal motility
(the actions of smooth muscle are discussed in Chapter 1).
The rhythmic movements of the digestive tract are self-
perpetuating, much like the activity of the heart, and are in-
fluenced by local, humoral (
i.e.
, blood-borne), and neural in-
fluences. The ability to initiate impulses is a property of the
smooth muscle itself. Impulses are conducted from one mus-
cle fiber to another.
The smooth muscle movements of the gastrointestinal tract
are tonic and rhythmic. The
tonic movements
are continuous
movements that last for minutes or even hours. Tonic contrac-
tions occur at sphincters. The
rhythmic movements
consist of
intermittent contractions that are responsible for mixing and
moving food along the digestive tract.
Peristaltic movements
are
rhythmic propulsive movements that occur when the smooth
muscle layer constricts, forming a contractile band that forces
the intraluminal contents forward. During peristalsis, the seg-
ment that lies distal to, or ahead of, the contracted portion re-
laxes, and the contents move forward with ease. Normal peri-
stalsis always moves in the direction from the mouth toward
the anus.
Autonomic Nervous System
The gastrointestinal tract is innervated by both the sympa-
thetic and parasympathetic nervous systems. Parasympathetic
innervation to the stomach, small intestine, cecum, ascending
colon, and transverse colon occurs by way of the vagus nerve
(Fig. 26-7). The remainder of the colon is innervated by para-
sympathetic fibers that exit the sacral segments of the spinal
cord by way of the pelvic nerve. Preganglionic parasympa-
thetic fibers can synapse with intramural plexus neurons,
or they can act directly on intestinal smooth muscle. Most
parasympathetic fibers are excitatory. Numerous vagovagal
reflexes influence motility and secretions of the digestive
tract.
Sympathetic innervation of the gastrointestinal tract occurs
through the thoracic chain of sympathetic ganglia and the
celiac, superior mesenteric, and inferior mesenteric ganglia.
Sympathetic control of gastrointestinal function is largely me-
diated by altering the activity of neurons in the intramural
plexuses. The sympathetic nervous system exerts several effects
on gastrointestinal function. It controls mucus secretion by the
mucosal glands, reduces motility by inhibiting the activity of
intramural plexus neurons, enhances sphincter function, and
increases the vascular smooth muscle tone of the blood vessels
that supply the gastrointestinal tract. The sympathetic fibers
that supply the lower esophageal, pyloric, and internal and
external anal sphincters are largely excitatory, but their role in
controlling these sphincters is poorly understood.
Innervation
Gastrointestinal function is controlled by the
enteric nervous sys-
tem
, which lies entirely within the wall of the gastrointestinal
tract, and by the parasympathetic and sympathetic divisions of
the autonomic nervous system (ANS).
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