normal rectum and anal canal

7/31/2019 Normal Rectum and Anal Canal

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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.

normal rectum and anal canal

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