40 Rectum

STUDYAIMS

At the end of your study, you should be able to:

Describe the gross structure of the rectum, including its musculature

List the relations of the rectum

Outline the blood and vascular supplyof the rectum

Describe structures that maybe palpated on digital rectal examination

Describe the anatomyof the anus

Describe the blood and vasculature supplyto the different regions of the anus

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GUIDE

Pelvis and Perineum: Rectum and Anal Canal

[Plate 372, Ischioanal Fossae]

Rectum

Gross structure

Approximately12 cm in length

Is a part of the large bowel

Commences at level S3 vertebral bodyand follows the curve of the sacrum and coccyx

Is a continuation of the sigmoid colon at rectosigmoid junction

Extends from rectosigmoid junction to anal canal (anorectal junction)

Anorectal junction

Lies at level of pelvic floor

Puborectalis muscle forms a U-shaped sling at this point

Perineal flexure annulates anorectal junction

Is palpated as anorectal ring on digital rectal examination

Three lateral curves/flexures in the rectum (upper and lower curves deviate to the right; the middle to the left)

Rectal ampulla

In region of middle and lower curves

Somewhat dilated

Verydistensible

Internal structure

Smooth mucosa, changes from rugose mucosa of sigmoid colon

Transverse rectal folds

Three infoldings of mucosa and submucosa project into lumen

Superior, middle, and inferior

Called rectal valves of Houston

Result from three lateral flexures

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Relations

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Peritoneum

Upper third of rectum covered byperitoneum on anterior aspect and sides

Middle third of rectum covered byperitoneum on anterior surface only

Lower third of rectum below peritoneal reflection

Pararectal fossae on lateral sides of rectum in peritoneal cavity

Rectovesical septum of pelvic fascia separates rectum from prostate

Rectovaginal septum of pelvic fascia separates rectum from vagina

Lateral ligaments of rectum-condensations of pelvic fascia around middle rectal vessels

Relations of Rectum

Arterial

Mainlybysuperior rectal artery(continuation of inferior mesenteric artery)

Also supplied bythe middle rectal arteries (from internal iliac arteries) to the middle and inferior parts

Inferior rectal arteries supplythe anorectal junctions and anal canal

Superior, middle, and inferior rectal arteries anastomose with each other

Venous

Chief drainage via a rectal venous plexus to the superior rectal vein

Middle and inferior rectal veins also drain the rectum

Superior rectal vein drains to the portal system

Middle and inferior rectal veins drain to the inferior vena cava via the internal iliac vein

Anastomoses between these veins link the portal and system systems

Lymphatic drainage

Lymphatic vessels from superior half of rectum drain to pararectal nodes and from there to inferior mesenteric and lumbar nodes

Lymphatic vessels from the inferior half of the rectum travel with the middle rectal vessels to the internal iliac nodes and anastomose with the lymphatics of the anal canal

Nerve supply

Sympathetic innervation

From lumbar sympathetic fibers via the inferior mesenteric arteries and the superior rectal arteries to the superior rectal plexus to blood vessels of rectum

Parasympathetic innervation

Are the main motor fibers to muscles of the rectal wall

From the pelvic splanchnic nerves (S2-S4) via the inferior hypogastric (pelvic) plexus to middle rectal plexus Visceral afferent (sensory) fibers travel via the inferior hypogastric (pelvic)

plexus and pelvic splanchnics back to spinal cord

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Anal Canal

Gross structure

Terminal part of gastrointestinal tract Is approximately3 cm long

Commences at anorectal junction and ends at anus Encircled byinternal and external sphincter muscles Descends between perineal bodyand anococcygeal ligament

Mucosal lining of superior has longitudinal ridges-anal columns

Inferior ends of columns joined byanal valves

Behind valves are small sinuses: anal sinuses

Anal glands (mucus) emptyinto anal sinuses

Inferior end of anal valves forms an irregular line: pectinate (or dentate) line

Pectinate line divides the superior portion of the anal canal, derived from embryonic endoderm (hindgut), from the inferior portion derived

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from ectoderm (proctodeum)

Inferior half lined bynonkeratinized squamous epithelium (skin)

Vascular, nerve and lymphatic supplyof these two regions different as a result of different embryologic origin

Vascular, Lymphatic, and Nerve Supply to the Anal Canal

Has two sphincters: external (voluntary) and internal (involuntary)

External sphincter

Described as having superficial, subcutaneous, and deep parts Parts are not readilydistinguishable

Fibers attach to the central perineal tendon (body) anteriorlyand anococcygeal ligament posteriorly

Internal sphincter: internal circular muscular layer continuous from rectum around upper two thirds of anal canal

Anorectal Musculature

Ischiorectal (Ischioanal) fossae

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Fat filled, wedge-shaped recesses either side of anal canal Communicate with each other posteriorlyover the anococcygeal ligament Bounded by

Laterally: ischium and obturator internus muscle and fascia

Medially: anal canal surrounded byexternal anal sphincter

Anteriorlybyexternal sphincter urethrae and deep transverse perineal muscles

Superiorlybypelvic diaphragm

Inferiorlybysuperficial perineal fascia and skin

Filled with fat spanned byfibrous bands to support anal canal but compressible during defecation

On lateral walls are found internal pudendal vessels and pudendal nerve within pudendal (Alcock's) canal in the fascia of the internal surfaces of obturator muscles

Extends anteriorlysuperior to perineal membrane as anterior recesses of ischiorectal (ischioanal) fossae Important site abscess formation

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