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EMBRYOLOGY
The small intestine is derived entirely from the midgut of
the embryo except for the segment of the duodenum proximal to the ampulla of Vater, which is of foregut origin.
The ascending colon and the right half of the transverse
colon are also derived from the midgut, while the rest of
the colon develops from the hindgut (Figure 8.1). During
early fetal life, the small intestine is located in the yolk sac
of the embryo. In the tenth week of fetal life, the intestine
returns into the abdominal cavity of the embryo and, in
so doing, it undergoes a 180° rotation counterclockwise.
Because of this rotation, the C-loop of the duodenum faces
to the left, and the small intestine fixes obliquely on a
mesentery that extends from the right side of L-3 to the
left side of L-1.
A number of congenital abnormalities related to malrotation are seen in pediatric surgical practice. On rare occasions, however, intestinal malrotation may not cause
problems until adult life and may even be an incidental
finding during abdominal operations for other reasons.
Other congenital abnormalities of the intestine include
strictures, diaphragm formation, duplications, and rem-
8
Small and Large Intestine
ANATOMY
nants of the vitelline duct manifesting as a Meckel’s diverticulum. Finally, rests of pancreatic tissue in the duodenum and
gastric mucosa in a Meckel’s diverticulum may occur, and
again, these may not be symptomatic until after childhood.
SMALL INTESTINE
The small intestine is about 20 feet long and extends from
the pylorus to the ileocecal valve. The absorptive surface,
however, is greatly increased by rugal folds and mucosal
villus formation. Although it is fixed on a mesentery, as
described above, the mesentery is broad based and the
small intestine can move freely within the abdominal
cavity without becoming obstructed. The mesentery contains one or two vascular arcades in the jejunum, but in
the ileum it may contain as many as four or five vascular
arcades. The small intestine wall consists of a well-formed
serosa as well as a muscular coat made up of an outer longitudinal and inner circular layers, a submucosa, muscularis mucosa, and a mucosal layer.
As a derivative of the midgut, the small intestine
receives its blood supply from the superior mesenteric
Digestion of food and absorption of nutrients occurs in the small intestine, the segment of
the gastrointestinal tract between the pylorus and the ileocecal valve. While life can be sustained by total parenteral nutrition in the absence of the small intestine, perfect growth and
health depend on normal function of this organ.
Although the major function of the colon is reservoir and transport, it also has absorptive and endocrine functions. Some of the most emergent surgical conditions arise from
obstruction, perforation, or vascular compromise of the small intestine and colon. The
surgeon must treat these conditions adequately, often by resection. In the case of the small
intestine, as much of the bowel as possible must be saved; and, in the case of both organs,
appropriate conditions for safe anastomosis must be ensured.Not all procedures on the small
intestine are performed to treat pathology within it. In some procedures, segments of it are
used to replace other organs (e.g., esophagus, ureter). The colon has served a similar function to replace the esophagus. Surgery of the small and large intestine is an important chapter
in abdominal surgery.
FIGURE 8.1. Embryologic chart and view of the gastrointestinal tract shows the origins and nerve and
arterial supply. Abbreviations: IMA, inferior mesenteric artery; SMA, superior mesenteric artery.
artery, except for the proximal half of the duodenum,
which receives its blood supply from the gastroduodenal
artery, a branch of the artery of the foregut, the celiac axis.
Venous drainage, which is corresponding, is through the
superior mesenteric vein and gastroduodenal veins. The
blood supply of the small intestine is depicted in Figure
8.2.
The parasympathetic innervation is derived from the
vagus mostly through the celiac branch. The sympathetic
innervation, distributed along the adventitia of the arterial supply, derives from the greater and lesser splanchnic
nerves. The small intestine is entirely intraperitoneal
except for the second, third and fourth portions of the
duodenum, which are retroperitoneal.
The absorptive surface of the mucosa is increased
many times, first by the formation of finger-like projections called villi and second by the presence of microvilli
on the luminal surface of epithelial cells. The villi are
covered with a single layer of columnar epithelial cells that
includes absorptive, goblet, and endocrine cells, sparsely
scattered throughout the layer. While the villi project into
the lumen, the crypts of Lieberkühn project into the
lamina propria. The crypts have a vital role in cell renewal
and secretion (Figure 8.3).
The cells lining the crypts are progenitor, goblet,
enterochromaffin (argentaffin), and Paneth. Paneth cells
secrete lysozyme and cytokines. The lamina propria, the
loose areolar layer between the mucosa and the muscularis
mucosa, contains connective tissue, numerous blood
vessels and nerves, and several types of cells including
lymphocytes, plasma cells, eosinophils, macrophages, and
mast cells. Large numbers of lymphatic cells are organized
in follicles, known as Peyer’s patches, which are found
throughout the small intestine and are even more numerous in the ileum. They are thought to play a crucial role in
the immune response and immune regulation of the gut.
The submucosa is a connective tissue layer containing
blood vessels, lymphocytes, neural plexuses, and the submucosal (Meissner’s) ganglia. Outside the submucosa is
the muscularis, consisting of an inner circular and outer
longitudinal smooth muscle layer. Between these two
muscle layers are the myenteric ganglia and the myenteric
plexus, an important component of the enteric nervous
system that controls motility. Outside the muscularis is a
well-developed, relatively less distensible serosal layer.
Blood vessels and extrinsic nerves enter and leave the gut
wall through the mesentery.
LARGE INTESTINE
The large intestine or colon extends from the ileocecal
valve proximally to the retrosigmoid junction distally and
is 3 to 5 feet in length. The colon occupies the periphery
of the abdominal cavity and is made up of the cecum and
ascending colon on the right, the transverse colon, and the
descending colon and sigmoid on the left. The splenic
flexure is closely related to the spleen and is attached to it
by the splenocolic ligament. The right colon has a larger
caliber than the left, and the cecum has the largest caliber
and is most distensible. The cecum is subject to rupture
when it reaches a diameter of 12 cm as a result of complete
distal obstruction in the presence of a competent ileocecal
valve.
The layers of the colon wall include the mucosa,
submucosa, inner circular and outer longitudinal muscu-
240 ................................................................................................................ Small and L arge Intestine
A
B
FIGURE 8.2. Arterial blood supply of the (A) small and (B) large intestine from the superior and inferior mesenteric arteries. (Adapted from Schwartz SI, ed: Principles of Surgery, 6th ed. New York:
McGraw Hill, 1994:1192.)
Absorptive
enterocytes
Intraepithelial
lymphocytes
Goblet cells
Undifferentiated cells
Enterochromaffin cells
FIGURE 8.3. Anatomy of the small intestinal mucosa, showing the structure of the villi and crypts.
Arteries and
veins
Paneth cells
Central
lymphatic
Enteric nerves
Brush border
Lamina propria
- Lymphocytes
- Plasma cells
- Eosinophils
Cell extrusion zone
Crypts
laris, and a serosa. The longitudinal muscle is arranged
into three separate bundles called tenia coli. The three
teniae are 120° apart around the circumference of the
colon. Haustra, or sacculations, are formed because
the teniae foreshorten the colon. The serosal surface
contains fatty appendages called appendices epiploicae
(Figure 8.4).
The cecum is a free intraperitoneal organ, but the
ascending colon is partially retroperitoneal up to the
hepatic flexure. The transverse colon is draped with
the greater omentum, which hangs down from it. The
descending colon is fixed to the lateral abdominal wall by
a fascia. Where this fascia meets the colon, an avascular
plane exists called the line of Tolt, which is incised when
the left colon is mobilized.
The superior mesenteric artery supplies the ascending
colon and the transverse colon through the ileocecal, right
colic, and middle colic branches. The inferior mesenteric
artery supplies the ascending colon and the splenic flexure
through the left colic branch, and the sigmoid through
sigmoid branches. The splenic flexure area is a vascular
watershed between the middle colic and left colic arteries
and has a more precarious blood supply. As a result, it is
susceptible to ischemic disease of the colon. The blood
supply of the colon is shown in Figure 8.2.
The arteries to the colon communicate on the mesenteric aspect to form a vascular arcade, which is called
the marginal artery of Drummond. The marginal artery
enlarges when the superior mesenteric artery becomes
occluded, and the entire colon must receive its blood
supply in retrograde fashion from the inferior mesenteric
artery. Venous drainage follows the arterial supply.
Lymphatics from the colonic wall drain into pericolic
mesenteric lymph nodes.
ANORECTUM
The rectum and anal canal differ in their embryologic
development, both in their type of mucosa and the derivations of their blood and nerve supplies. The rectum, deriving from the terminal portion of the hindgut, is lined with
colonic-type mucosa; its principal blood supply comes
from the inferior mesenteric vessels. The anal canal derives
from the cloaca, and its blood supply is derived from the
pudendal vessels. The anal mucosa consists of stratified
squamous cells in its distal half and transitional epithelium in its proximal half.
Rectum
The rectum begins at the anorectal junction 3cm from the
anal verge and ends at the retrosigmoid junction just in
front of the sacral promontory. The distance from the anal
verge to the rectosigmoid junction is 20cm. The supporting structures of the rectum include the mesorectum posteriorly, the lateral ligaments laterally, and the fascia of
Waldeyer and the puborectalis muscle distally (Figure 8.5).
The mesorectum fixes the rectum to the anterior surface
of the sacrum, which is covered with the presacral plexus
of veins.
Blood Supply
Three arterial systems supply the rectum:
1. The superior hemorrhoidal artery, a continuation of
the inferior mesenteric artery.
2. The middle hemorrhoidal arteries, branches of the
internal iliac arteries running in the lateral ligaments.
3. The inferior hemorrhoidal arteries.
Appendix epiploica
Tenia
Serosa
FIGURE 8.4. Cross-sectional anatomy of the colon, showing the location of the three tenia coli.
(Adapted from Schwartz SI, ed: Principles of Surgery, 6th ed. New York: McGraw Hill, 1994:1193.)
Mucosa
Submucosa
Mesocolon with
arterial arcade
Longitudinal muscle
Circular muscle
242 ................................................................................................................ Small and L arge Intestine
FIGURE 8.5. Anorectal anatomy shows the external and internal anal sphincters. (Adapted from Fry
RD, Koduer IJ. Anorectal disorders. Ciba Clinical Symposia 37:6, 1985. Copyright 1985 Ciba-Geigy Corp.
Reprinted with permission from Clinical Symposia illustrated by John A. Craig, MD. All rights reserved.)
All three vessels anastomose over the surface of the
rectum. Venous drainage is provided via the superior
hemorrhoidal veins to the inferior mesenteric veins. The
middle and inferior hemorrhoidal veins drain into the
internal iliac veins. Several important clinical implications
arise from this vascular arrangement:
1. Increased portal pressure is reflected to the hemorrhoidal vessels via the superior hemorrhoidal vein,
which lacks valves. This anatomical fact explains the
development of hemorrhoids in portal hypertension.
2. If, during rectal mobilization in surgery for rectal
cancer, the inferior mesenteric vein is ligated early, a
higher number of cancer calls can be recovered from
the internal iliac vein.
3. Severely infected hemorrhoids can cause pyelophlebitis, due to septic emboli going up the mesenteric
vein.
4. Hematogenous spread to the liver from rectal cancer
occurs via the inferior mesenteric vein.
Lymphatic Drainage
Lymphatic drainage of the rectum occurs through the
perirectal nodes and then into inferior mesenteric nodes.
Nerve Supply
The nerve supply to the rectum comes from both sympathetic and parasympathetic nerves. The sympathetic
innervation is from the thoracolumbar chain and is
distributed surrounding the inferior mesenteric artery
as a network plexus to the superior hypogastric plexus,
just below the aortic bifurcation. The hypogastric
plexus gives off the hypogastric nerves that supply the
lower rectum, the bladder, and the genitals. Parasympa-
thetic fibers originate from S-2, S-3, and S-4 roots as the
nervi erigentes and descend to form the pelvic plexus anterior and lateral to the rectum. These nerves supply the
rectum, the internal anal sphincter, prostate, bladder and
penis.
Penile erection is mediated by VIP-containing neurons
in the parasympathetic nerves, causing vasodilatation,
while the sympathetic nerves cause venoconstriction in the
corpus callosum, trapping the blood to sustain erection.
Thus, damage to either the sympathetic nerves (most often
incurred during high ligation of the inferior mesenteric
artery) or the parasympathetic nerves (incurred during
lateral and periprostatic mobilization of the rectum) can
lead to impotence and bladder dysfunction.
The rectum has three spirally arranged mucosal folds
called the valves of Houston, two on the left and one on
the right, although not every individual has all three
valves. These valves are clinically significant because
a small tumor can hide behind one and escape detection during sigmoidoscopy unless the area is examined
carefully.
Anal Canal
The anal canal is 3 cm long and ends at the anorectal junction, the mucocutaneous junction also referred to as the
dentate or pectinate line. Anal crypts and the openings of
the anal glands are located at the dentate line. The anal
crypts and the openings of the anal glands are located at
the distal end of the Columns of Morgagni. The anal canal
is pulled anteriorly by the puborectalis, causing it to point
towards the umbilicus and form an angle with the rectum
(Figure 8.6). The anorectal sphincteric ring is formed by
the fusion of the puborectalis and the internal sphincter,
the longitudinal muscle, and the muscles of the external
sphincter.
A natomy .............................................................................................................................................. 243
FIGURE 8.6. The puborectalis sling creates an angle between
the rectum and the anus that plays an important role in continence. (Adapted from Fleshman JW, et al. In Shackelford’s
Surgery of the Alimentary Tract, 3rd edition. Philadelphia: WB
Saunders, 1991.)
PHYSIOLOGY
Blood Supply
The anal canal is supplied by the inferior and middle hemorrhoidal arteries and veins.
Lymphatic Drainage
The anal canal mucosa drains into inguinal lymph nodes
and then to the external iliac and common iliac lymph
nodes.
Nerve Supply
The nerve supply is derived from the inferior rectal and
perineal branches of the pudendal nerve. Sensation of
heat, cold, pain, and touch is appreciated in the mucosa
distal to the dentate line.
SMALL INTESTINE
The four key functions of the small intestine are: digestion, absorption, secretion, and motility (transportation).
Digestion
The small intestine is the primary site of digestion of carbohydrate, proteins, and amino acids.
Digestion of Carbohydrate
Carbohydrate digestion begins with the action of salivary
amylase, but the complete digestion into monosaccharides
occurs in the small intestine by the action of pancreatic
amylase and brush border enzymes as shown in Figure 8.7.
Digestion of Fat
The average fat intake in the U.K. and United States is 100
to 150g/day, although both populations are slowly reducing their fat consumption. Dietary triglycerides contain
largely oleate and palmitate as their fatty acids. In addition, 2 to 8g of phospholipids are ingested daily. The most
common phospholipid ingested is lecithin, and the predominant fatty acids are linoleate and arachidonate. Most
dietary fat is digested and absorbed in the first half of the
jejunum.
FIGURE 8.7. Digestion of carbohydrate.
244 ................................................................................................................ Small and L arge Intestine
FIGURE 8.8. Action of lipase and colipase in yielding fatty acids and monoglycerides.
Because fat is not water soluble, an important first step
in its digestion is emulsification.
MULSIFICATION The action of gastric lipase yields
E
fatty acids and diglycerides, which enhance emulsification.
The process is further enhanced in the duodenum by bile
salts and phsopholipids. The emulsion so produced is then
presented to pancreatic lipase.
IPOLYSIS In the duodenum, the action of lipase and
L
colipase yields fatty acids and monoglycerides as shown in
Figure 8.8. Fatty acids and monoglycerides enter bile acid
micelles for absorption (see below).
Digestion of Proteins
On the average, 10% to 15% of the energy provided in the
Western diet derives from proteins. An average daily consumption is about 70 g. Protein digestion begins in the
stomach and is completed in the small intestine.
ASTRIC DIGESTION Gastric digestion is due to the
G
action of pepsin, which is active in acid pH. Proteins are
acted upon to produce a mixture of peptides and a small
amount of amino acids.
NTESTINAL DIGESTION The main site of protein diges-
I
tion is the duodenum as a result of the action of pancreatic proteases, which are active in alkaline pH. Trypsinogen
and chymotryprisinogen are secreted by the exocrine
pancreas and, when they enter the duodenum, the
brush-border enzyme enterokinase converts them into the
active enzymes trypsin and chymotrypsin. Once trypsin is
formed, it promotes this process by autocatalytic action. It
also activates other proteases. The active proteases are
either endopeptidases (i.e., acting within the structure of
the protein) or exopeptidases (i.e., split amino acids off the
carboxyl terminus of the protein). Trypsin, chymotrypsin,
and lipase are endopeptidases.
The luminal digestion of dietary peptides is summarized in Figure 8.9.
Amino acids are absorbed either as monomers or as
dipeptides and tripeptides. Approximately 70% of the
products of protein digestions are absorbed as dipeptides
or tripeptides. By the time absorbed protein-derived nutrients reach the portal circulation, they are all amino acids,
suggesting the importance of peptidase activity in the gut
epithelium.
Absorption
Fluid Absorption
On average, approximately 9 L of fluid enter the small
intestine daily, either through the pylorus or the sphincter
of Oddi. Approximately 2 L consist of ingested fluid; 1L of
saliva; another 2L of gastric juice; and 4L of bile, pancreatic juice, and succus entericus. Of this, 4 to 5L are reabsorbed in the jejunum and 3 to 5 L in the ileum. Thus,
about 1 L of fluid enters the colon in a 24-h period. Of this
amount, approximately 800 ml are reabsorbed in the colon
and 200 ml excreted in the feces.
Absorption of Electrolytes
SODIUM ABSORPTION Sodium is absorbed by both
active and passive mechanisms. Sodium is actively cotransported with chloride and nutrients such as glucose in the
jejunum and bile salts in the terminal ileum. Cotransport
depends on the sodium gradient across the apical mem-
FIGURE 8.9. Luminal digestion.
P hysiology .......................................................................................................................................... 245
brane created by the Na+K+ATPase pump located in the
basolateral membrane. Water absorption follows passively
to maintain isoosmolality. The sodium-glucose cotransport carrier is the most important clinically. Because this
cotransport carrier mechanism is unaffected in most
diarrheal states, administration of glucose-salt solution
is an important therapeutic strategy to control diarrhea.
Another mechanism for sodium absorption is the Na
+-H+
exchange carrier that permits entry of sodium and chloride
into the cell in exchange for hydrogen.
C
HLORIDE ABSORPTION The sodium-glucose cotrans-
port mechanism promotes absorption of chloride through
a paracellular pathway. In addition, the Na
+-H+
exchange
mechanism permits entry of chloride into the cell in
exchange for hydrogen. A third mechanism for chloride
absorption is the Cl
-
-HCO
3
-
exchange carrier, in which
chloride is absorbed in exchange for HCO
3
-
.
P
OTASSIUM ABSORPTION Potassium is absorbed in
exchange for hydrogen.
C
ALCIUM ABSORPTION Absorption of calcium is
regulated by vitamin D and 1,2,5-dihydroxyvitamin D,
parathyroid hormone, calcitonin, and a number of
calcium-binding proteins.
Absorption of Nutrients
Four types of processes are used for absorption: active
transport, passive diffusion, facilitated diffusion, and
endocytosis. Active absorption requires transport against
an electric or chemical gradient and, therefore, requires
energy. Passive diffusion, on the other hand is downhill
transport with electric and chemical gradient and requires
no energy. Facilitated transport is similar to passive diffusion but requires a carrier-mediated system. Endocytosis,
the reverse of exocytosis, is akin to phagocytosis of soluble
or particulate substance. This mechanism is used in uptake
of antigens.
Various nutrients are absorbed by these different
mechanisms as follows:
1. Monosaccharide. The monosaccharide products of
carbohydrate digestion are absorbed by active
transport.
2. Amino acids. Different mechanisms, both active and
passive, are used in the absorption of amino acids.
3. Fatty acids and monoglycerides. The products of
triglyceride lipolysis, fatty acids and monoglycerides
are absorbed into the cell through micelle formation
with bile salts.
Secretion
The intestine secretes water and electrolytes by neurally
and humorally controlled mechanisms that are integrated
with the mechanisms of absorption. Some secretory mechanisms are active while others are passive. Channels, carriers, and pumps located in the epithelial membrane
participate in the process of absorption. Water secretion is
inextricably linked to the movement of solutes, but a
number of mechanisms are known to stimulate the secretion of water and electrolytes. These include:
Intestinal Distention
Rapid increase in intraluminal pressure stimulates water
and chloride secretion into the lumen. This mechanism
further contributes to the contraction of the extracellular
fluid volume in intestinal obstruction.
Humoral Agents
A large number of endogenous secretagogues stimulate
intestinal secretion. These include eicosanoids from the
subepithelium, acetylcholine, VIP, and serotonin derived
from both extrinsic innervation and from the enteric
nervous system (ENS). Additionally, such GI hormones
as secretin and gastrin participate. The action of these
humoral agents is most pronounced in pathological
conditions in which they are produced in abnormally large quantitites, such as carcinoid, VIPoma, or
gastrinoma.
Luminal Secretagogues
Bile salts and large-chain fatty acids stimulate intestinal
secretion and can, under certain circumstances, cause
diarrhea. Bacterial enterotoxin (Vibrio cholera, Escherichia
coli, Salmonella, Campylocacter jejani, Yersina enterocolitica, Clostridium perfringens and C. difficile) cause diarrhea
by stimulating intestinal secretion under pathological
conditions.
Motility
Small intestinal motility is regulated through neuroluminal mechanisms. Peristalsis is a coordinated movement
that moves intestinal contents aborally. The peristaltic
reflex requires descending relaxation of the intestine ahead
of the bolus caused by VIP and proximal contraction
mediated by acetylcholine and substance P (see Figure
5.11). The small intestine also undergoes segmental contraction in which a portion contracts as a unit. These contractions may cause retropropulsion of intestinal contents
or retroperistalsis. Every 90 minutes a wave of contraction
starts in the duodenum and sweeps down the small intestine to the colon. This reflex has been called the “housekeeper potential” because it cleanses the small intestine of
its contents. It is also called the migrating motor complex
or MMC and the peptide motilin is associated with it
either as a triggering mechanism or as a secondary phe-
246 ................................................................................................................ Small and L arge Intestine
nomenon. Plasma motilin levels are elevated during the
MMC. Small intestinal contraction is stimulated by several
peptides including substance P, motilin, CCK, gastrin, and
gastrin-releasing peptide, but the significance of these
peptides in physiological motor function of the intestine
is unknown.
LARGE INTESTINE
The large intestine or colon has four functions. They are
motility (and reservoir), absorption, secretion, and
endocrine.
Motility
Three types of motor activity occur in large intestine
function, including segmentation, mass movement, and
retrograde peristalsis. Segmentation is the most common
motor activity and consists of segmental annular contractions that move intestinal contents short distances in both
directions. Mass movement is a strong contractile activity
that sweeps across the transverse and descending colon a
few times a day. It follows ingestion and may be a response
to the gastrocolic reflex. It is the main mechanism by
which feces is delivered to the rectum. Retrograde peristalsis begins in the transverse colon and moves proximally
into the right colon.
Absorption
Of the 800 ml of water delivered into the colon each day,
the colon absorbs 600ml. Sodium absorption is by electrogenic transport, unaccompanied by cation exchange or
anion cotransport. Sodium enters through channels in the
apical membrane and is pumped out across the basolateral membrane by Na
+K+
ATPase. Approximately 200 to
400 mEq of sodium can be absorbed each day. Chloride
is absorbed actively against a concentration gradient in
exchange for bicarbonate.
The colon also absorbs short-chain fatty acids, which
are formed by bacterial fermentation of carbohydrates and
cellulose and absorbed by passive transport. The major
short-chain fatty acids are butyrate, acetate, and propionate. It is estimated that daily short-chain fatty acid
absorption yields about 540 kcal/day.
Secretion
The colon secretes bicarbonate and potassium. Bicarbonate is secreted in exchange for chloride. Potassium secretion is active.
Endocrine Function
The colon contains L, K, and N cells, which release
enteroglucagon, peptide YY (PYY), and neurotensin,
respectively. Enteroglucagon is trophic to the small intestinal mucosa, and the colon might participate in regulation
of small intestinal mucosal growth. PYY is released from
the distal ileum and proximal colon in response to luminal
fat and is responsible for the so-called “ileal break,” which
serves to slow gastric emptying and transport across the
small intestine. Neurotensin also inhibits gastric emptying; converts fasting motor complex pattern into the
fed pattern; stimulates pancreatic secretion, release of
histamine from mast cells, and colon motility; and is a
powerful vasodilator. Which of these biological actions are
important in normal physiology is unknown.
ANORECTUM
The anorectum demonstrates a coordinated function to
both conserve continence and effect defecation.
Continence
Normal anorectal function maintains solid, liquid, and gas
continence. The structures responsible for maintaining
continence include: (1) the rectum; (2) the internal and
external anal sphincters; (3) the pelvic diaphragm, including the puborectalis; and (4) innervation of these structures. When the puborectalis contracts, it pulls the rectum
forward, creating a 90° angle between the rectum and anal
canal. The puborectalis muscle is believed to be the most
important factor in continence. If the puborectalis is
intact, division of the internal anal sphincter does not
produce severe incontinence.
When feces enter the rectum, the rectum relaxes in
accommodation. After a certain point, rectal distension
occurs and the internal sphincter relaxes reflexively, but
the puborectalis and external anal sphincter contract to
prevent defecation.
Defecation
Entrance of a sufficient volume of feces into the rectum
stimulates the urge to defecate. The response requires
relaxation of the internal sphincter. Defecation is
prevented until a squatting position is assumed and
the puborectalis muscle and external anal sphincter are
voluntarily inhibited. Subsequently, straining and performing a Valsalva maneuver contracts the abdominal
muscles and forces the feces out of the anus.
P hysiology .......................................................................................................................................... 247
PATHOPHYSIOLOGY
INTESTINAL OBSTRUCTION
Intestinal obstruction may be defined as failure of propulsion of intestinal contents aborally. The condition occurs
in many forms in both the small and large intestine (Table
8.1), due to either mechanical obstruction or a motility
problem caused by neuromuscular failure or ischemia.
Neuromuscular failure is frequently associated with
inflammation in the peritoneal cavity or in the retroperitoneum. This type of intestinal obstruction, where the
intestinal lumen is not compromised, is also known as
adynamic ileus. Pain originating from distension of the
intestine, as occurs in bowel obstruction, is initially
referred to the embryologic dermatome supplied by the
same somatic nerve (Figure 8.10).
TABLE 8.1. Classification of Intestinal Obstruction
Small Intestine
䊏
Mechanical obstruction
Simple bowel obstruction
Strangulated bowel obstruction
䊏
Adynamic ileus
䊏
Mesenteric ischemia
Colonic obstruction
䊏
Mechanical obstruction
Simple obstruction
Closed-loop obstruction
䊏
Adynamic ileus
䊏
Mesenteric ischemia
Pseudo-obstruction
Small Intestine
Mechanical Obstruction
Mechanical obstruction may be due to luminal causes,
intrinsic lesions in the bowel wall, or due to external compression (Table 8.2). Adhesions and hernias account for
75% to 80% of all mechanical small bowel obstruction,
adhesions being responsible for 60% to 65%. Obstruction
that is caused by an adhesion constricting the bowel at one
site is referred to as a simple obstruction, while obstruction at two points causes a closed-loop obstruction. The
classic features of small bowel obstruction are summarized
in Table 8.3 and the pathophysiology is shown in Figure
8.11.
IMPLE SMALL BOWEL OBSTRUCTION Simple bowel
S
obstruction usually occurs when an adhesive band compresses and obstructs the bowel at one point. Signs include
distention and pain.
Distension Fluid and air accumulate proximal to the
obstruction site, leading to progressive dilatation of the
proximal bowel and collapse of the distal bowel. Nearly all
the air in the dilated bowel is swallowed air, hence the
importance of nasogastric aspiration. While the fluid that
accumulates in the obstructed bowel (i.e., gastric, biliary,
pancreatic juice, succus entericus) is secreted primarily
higher up in the gastrointestinal tract, some is due to reflex
secretion from the distended bowel itself.
If the obstruction is high in the duodenum or
upper jejunum, little or no distension is perceived because
the patient will be vomiting and decompressing the
FIGURE 8.10. Sites (shaded areas) of referred pain. Sites of pain in the abdominal wall referred from
pathology of different parts of the gastrointestinal tract.
248 ................................................................................................................ Small and L arge Intestine
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