normal rectum and anal canal
TRANSCRIPT
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Rectum and Anal Canal
General Anatomy
The rectal wall consists of mucosa, submucosa, and two complete muscular
layers: inner circular and outer longitudinal (Fig. 26-3). The rectum is
approximately 12 to 15 cm in length and extends from the sigmoid colon
to the anal canal following the curve of the sacrum (Fig. 26-4). The anterior
peritoneal reflection is about 5 to 7.5 cm above the anus in females and 7
to 9cm above the anus in males. The posterior peritoneal reflection is
usually 12 to 15 cm above the anus. The upper third of the rectum is
covered by peritoneum on its anterior and lateral surfaces. The middle third
of the rectum is covered by peritoneum only on its anterior surface, and the
lower third of the rectum is below the peritoneal reflection. The proximal
rectum is identified as the level at which the teniae coli of the colon coalesce
to form a complete layer of longitudinal muscle at approximately the level of
the sacral promontory.
The rectum contains three distinct curves: The proximal and distal curves
are convex to the right, whereas the middle curve is convex to the left.
These folds project into the lumen as the valves of Houston. These
mucosal infoldings present some difficulty for proctoscopic examination,
but they are excellent targets for mucosal biopsy because they do not
contain all layers of the muscular rectal wall and the risk of perforation is
therefore diminished. The middle valve of Houston roughly correlates with
the level of the anterior peritoneal reflection.
Waldeyer's fascia is a dense rectosacral fascia that begins at the level of
the fourth sacral body and extends anteriorly to the rectum, covering the
sacrum and overlying the vessels and nerves (see Fig. 26-86 E). Anterior
to the extraperitoneal rectum is Denonvilliers' fascia, which is the
rectovesical septum in men and the rectovaginal septum in women. The
lateral ligaments of endopelvic fascia support the lower rectum but do not
usually contain major blood vessels, as previously believed. Division of the
lateral ligaments is thus possible without impairing the blood supply to the
rectum or encountering a significant bleeding. Accessory middle
hemorrhoidal arteries can be located in the lateral ligaments but are not
critical to the blood supply of the rectum.
Thepelvic flooris a musculotendinous sheet formed by the levator ani
muscle and is innervated by the fourth sacral nerve (Fig. 26-5). The
pubococcygeus, iliococcygeus, and puborectalis muscles make up the
levator ani muscle. These are paired muscles that intertwine and act as a
single unit. The line of decussation is called the anococcygeal raphe. The
rectum, vagina, urethra, and the dorsal vein of the penis pass through the
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levator hiatus in the pubococcygeal portion of the levator ani. During
defecation, the puborectalis relaxes and the levator ani contracts, widening
the levator hiatus.
The anal canalstarts at the pelvic diaphragm and ends at the anal verge
(see Fig. 26-4). It is approximately 4 cm long and normally exists as acollapsed anteroposterior slit. The anatomic anal canalextends from the
anal verge to the dentate line. For practical purposes, however, surgeons
usually define the surgical anal canalas extending from the anal verge to
the anorectal ring, which is the circular lower (see Fig. 26-3) border of the
puborectalis that is palpable by digital rectal examination. The anorectal ring
is 1 to 1.5 cm above the dentate line.
The anal verge is the junction between anoderm and perianal skin. The
anoderm is a specialized epithelium rich in nerves but devoid of secondary
skin appendages (hair follicles, sebaceous glands, or sweat glands). The
dentate line is a true mucocutaneous junction located 1 to 1.5 cm above
the anal verge (see Fig. 26-3). A 6- to 12-mm transitional zone exists
above the dentate line over which the squamous epithelium of the anoderm
becomes cuboidal and then columnar epithelium.
The anal canal is surrounded by an internal and external sphincter, which
together constitute the anal sphincter mechanism (see Fig. 26-5). The
internal sphincteris a specialized continuation of the inner circular smooth
muscle of the rectum. It is an involuntary muscle and is normally contracted
at rest. The intersphincteric plane represents the fibrous continuation of the
longitudinal smooth muscle layer of the rectum.
The external sphincteris a voluntary, striated muscle divided into three U-
shaped loops (subcutaneous, superficial, and deep) acting as a single
functional unit. It is a specialized continuation of the levator muscles of the
pelvic floor, specifically of the puborectalis muscle. Thepuborectalis
originates at the pubis and joins posterior to the rectum. It is normally
contracted, causing an 80 angulation of the anorectal junction.
The columns of Morgagniconsist of 8 to 14 longitudinal mucosal folds
located just above the dentate line and forming the anal crypts at theirdistal end (see Fig. 26-3). Small rudimentary glands open into some of
these crypts. The ducts of these glands penetrate the internal sphincter,
and the body of the gland resides in the intersphincteric plane.
Arterial Supply
The terminal branch of the inferior mesenteric artery becomes the superior
rectal arteryas it crosses the left common iliac artery (Fig. 26-6 A). It
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descends in the sigmoid mesocolon and bifurcates at the level of the third
sacral body. The left and right branches of the superior rectal artery supply
the upper and middle rectum.
The middle and inferior rectal arteries supply the lower third of the rectum.
The middle rectal arteries arise from the internal iliac arteries, run throughDenonvilliers' fascia, and enter the anterolateral aspect of the rectal wall at
the level of the anorectal ring. Collaterals exist between the middle and
superior rectal arteries. Preservation of the middle rectal arteries is
necessary to maintain viability of the remaining rectum after proximal
ligation of the inferior mesenteric artery.
The inferior rectal arteries are branches of the internal pudendal arteries.
They traverse Alcock's canal and enter the posterolateral aspect of the
ischiorectal fossa. They supply the internal and external sphincters and the
lining of the anal canal and do not form collaterals with the other rectal
arteries. The middle sacral arteryarises just proximal to the aortic
bifurcation and provides very little blood supply to the rectum.
Venous Drainage
The venous drainage of the rectum parallels the arterial supply and empties
into both the portal and the systemic (caval) systems. The upper and
middle rectum are drained by the superior rectal vein, which enters the
portal system via the inferior mesenteric vein (see Fig. 26-6 B). The lower
rectum and upper anal canal are drained by the middle rectal veins, which
empty into the internal iliac veins and then into the caval system. The
inferior rectal veins drain the lower anal canal and empty into the pudendal
veins, which drain into the caval system via the internal iliac veins. Low rectal
tumors can thus metastasize through venous channels into both the portal
and systemic venous systems.
There are three submucosal internal hemorrhoidal complexes located
above the dentate line (see Fig. 26-3). The left lateral, right posterolateral,
and right anterolateral internal hemorrhoidal veins drain into the superior
rectal vein. Below the dentate line the external hemorrhoidal veins drain
into the pudendal veins. There is communication between the internal and
external plexi.
Lymphatic Drainage
The rectal lymphatic flow is segmental and circumferential and follows the
same distribution as the arterial blood supply (Fig. 26-7). Lymph from the
upper and middle rectum drains into the inferior mesenteric nodes. The
lower rectum is drained primarily by lymphatics that follow the superior
rectal artery and enter the inferior mesenteric nodes. Lymph from the lower
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rectum also can flow laterally along the middle and inferior rectal arteries,
posteriorly along the middle sacral artery, or anteriorly through channels in
the rectovesical or rectovaginal septum. These channels drain to the iliac
nodes and subsequently to periaortic lymph nodes.
Lymphatics from the anal canal above the dentate line drain via thesuperior rectal lymphatics to the inferior mesenteric lymph nodes or
laterally to the internal iliac lymph nodes. Below the dentate line, the
lymphatics drain primarily to the inguinal nodes but can drain to the inferior
or superior rectal lymph nodes as well.
Nerve Supply
The innervation of the rectum is shared with the urogenital organs of the
pelvis and consists of both sympathetic and parasympathetic nerves (Fig.
26-8). Sympatheticnerves from thoracolumnar segments unite below the
inferior mesenteric artery to form the inferior mesenteric plexus. Thesepurely sympathetic nerves descend to the superior hypogastric plexus
located below the aortic bifurcation. They then bifurcate and descend in the
pelvis as the hypogastric nerves. The lower rectum, bladder, and sexual
organs in both men and women receive sympathetic innervation via the
hypogastric nerve. Injury to the inferior mesenteric plexus can result
during ligation of the inferior mesenteric artery at its origin.
Parasympatheticfibers from the second, third, and fourth sacral roots (the
nervi erigentes) unite with the hypogastric nerves anterior and lateral to the
rectum forming thepelvic plexus, which runs laterally in the pelvis. The
periprostatic plexus arises from the pelvic plexus. Mixed fibers from these
plexi innervate the rectum, internal anal sphincter, prostate, bladder, and
penis. Thepudendal nerve (S2, S3, S4) mediates sensory stimuli from the
penis and clitoris via the dorsal nerve.
Both sympathetic and parasympathetic fibers are essential for penile
erection. The parasympathetic fibers cause vasodilation and increased
blood flow in the corpus cavernosum, resulting in an erection. The
sympathetic fibers cause vasoconstriction of the penile veins and thus
sustain the erection. Sympathetic nerves cause contraction of the
ejaculatory ducts, seminal vesicles, and prostate and are necessary forejaculation. Damage to the periprostatic plexus might occur during surgical
dissection of the rectum. Injury to the pelvic autonomic nerves may result in
bladder dysfunction, impotence, or both.
The internal anal sphincteris innervated by both sympathetic and
parasympathetic nerves, and both are inhibitory to the sphincter. The
internal sphincter has a continuous tone that decreases as rectal pressure
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increases. Once the rectum empties, the internal sphincter tone rises again.
The external anal sphincterand levator ani muscles are innervated by the
inferior rectal branch of the internal pudendal nerve (S2, S3, S4) and the
perineal branch of the fourth sacral nerve. Any distention of the rectum
results in relaxation of the internal sphincter. The external sphincter can be
contracted voluntarily and kept in that state for approximately 1 minute.
Below the dentate line, cutaneous sensations of heat, cold, pain, and touch
are conveyed by afferent fibers of the inferior rectal and perineal branches of
the pudendal nerve. Above the dentate line, a poorly defined dull sensation,
experienced when the mucosa is pinched or when internal hemorrhoids are
ligated, is probably mediated by parasympathetic fibers.
Resection of the sacrum with sacrifice ofsacral nerves occasionally may be
required for total resection of pelvic tumors. Sacrifice of the lower sacral
nerves will lead to saddle anesthesia and possible motor weakness in the
lower extremities. Preservation of at least one of the third sacral nerves is
required to maintain acceptable anal continence. Near-normal continence
will be maintained if the upper three sacral roots on one side are preserved
along with the upper two sacral roots on the contralateral side. If all the
sacral roots are destroyed unilaterally but the contralateral nerves are
preserved, the patient should maintain continence. If both S3 roots are
destroyed, the patient will be incontinent. The upper half of S1 is needed
for stability of the spine and pelvis.
Normal Function of AnorectumThe rectum functions mostly as a storage capacitance vessel. The rectum
has very little peristaltic function of its own and relies on external pressure
to empty. The outer longitudinal muscle is thick and has some contractility
but has lost the organization of the teniae found on the colon. The rectum
has a normal manometric resting pressure of approximately 10 mmHg,
mostly due to intraperitoneal pressure and resting muscle tone. Conditions
such as Crohn's disease or radiation injury cause the rectum to lose its
natural compliance. This loss of compliance and capacitance is occasionally
incapacitating to the patient.
The normal rectum can hold 650 to 1200 mL of liquid. A rectum that holds
more than 1500 mL can be classified as a megarectum. The normal daily
volume of stool eliminated by the rectum ranges from 250 to 750 mL of
formed feces.
The anal sphincter mechanism is the other component of defecation and
continence. Its anatomy and innervation have been described previously
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(see Fig. 26-5). The external sphincter fibers are responsible for 20
percent of the resting pressure and 100 percent of generated squeeze
pressure. The internal sphincter provides 80 percent of anal resting
pressure. Both the internal and external sphincter muscles are contracted
at rest.
Defecation and continence are coordinated mechanisms. Continence can be
described as controlled elimination of the rectal contents at a socially
acceptable time and place. The coordination of rectal emptying and
sphincter contraction and relaxation is very complex, and there are
numerous abnormalities that occur.
Defecation can be divided into four components. The first is movement of
feces into the rectal vault or capacitance organ. A mass peristaltic wave in
the proximal colon and sigmoid colon occurs two or three times per day to
pass solid substance into the rectum. The gastrocolic reflex is a well-known
phenomenon that results in colonic mass peristaltic movement after
distention of the stomach, probably hormonally mediated.
The second component of defecation is the rectal-anal inhibitory reflexor
sampling reflex. Distention of the rectum results in involuntary relaxation
of the internal anal sphincter and allows sensation of the rectal contents at
the transitional zone. The sampling reflex has been shown by ambulatory
manometry to occur frequently throughout the day and night.
The third component of defecation is voluntary relaxation of the external
sphincter mechanism. Voluntary relaxation of the pelvic floor, puborectalismuscle, and external sphincter allows the rectal contents to be pushed
farther into the anal canal and expelled. The relaxation of the sphincter
mechanism is actually a failure to contract rather than an active relaxation,
because the mechanism is paradoxically contracted when the rest of the
individual is relaxed, yet continent (Fig. 26-21).
The fourth component of defecation is the voluntary increase of
intraabdominal pressure, using the diaphragm and abdominal wall muscles.
This increase in pressure serves to propel the rectal contents through the
anal canal and accomplish defecation.
The passage of flatus also requires coordination of multiple factors. The
sensation of gas at the transitional zone and in the anoderm informs the
individual that gas is present to be eliminated. If the situation is such that
full evacuation of the rectum is not possible but elimination of gas is
desired, a voluntary contraction of the pelvic floor including the
puborectalis and external sphincter muscles occurs to prevent loss of solid
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rectal contents. With an increase in abdominal pressure and a coordinated
relaxation of some of the external sphincter, selective passage of flatus
may be accomplished. During defecation, the gaseous contents of the
rectum will be expelled with the solid contents without discrimination. This
mechanism of discrimination appears to be learned. Patients eventually can
pass flatus selectively even after the rectum has been removed andreplaced by a reservoir of ileum for diseases such as ulcerative colitis and
familial adenomatous polyposis.
Continence, or the control of rectal contents, requires an adequate rectal
capacity and normal compliance. It may be difficult to retain rectal contents
in conditions such as Crohn's disease, in which the rectum becomes a rigid
tube rather than a soft distensible bag, even if the external and internal
sphincter mechanisms work properly. Adequate sensation at the
transitional cell zone is required to coordinate pelvic pressure and sphincter
tone during the sampling reflex. The external sphincter is most responsible
for the fine control of solid, liquid, and gas. The puborectalis muscle has
been proposed as the mainstay of the sphincter mechanism and is
probably responsible for the control of solid stool. The internal sphincter
may be responsible for fine control of gas on the basis of the sampling
reflex and constantly provides resting pressure to prevent release of
flatus. The pudendal nerves provide both the sensory afferents of the anal
canal and the motor efferents to the voluntary muscles of the anal canal.