Rectum and Anal Canal

• The rectum is continuous with the sigmoid colon at the level of S3 and ends at the anorectal junction. It is about 12cm. The rectum is differentiated from the colon by the disappearance of the mesentery which surrounds the sigmoid colon. There is also disappearance of haustrae, taeniae coli and appendices epiploicae, therefore, it has a very different appearance to the sigmoid colon. The rectum regains a full layer of outer longitudinal smooth muscle.
• The rectum descends along the sacrococcygeal concavity as the sacral flexure, and eventually joins the anal canal, 2-3cm in front of the coccygeal tip. This bend is known as the perineal flexure of the rectum.
• The lower part of the rectum is dilated as the rectal ampulla. The upper part has 3 transverse rectal folds: the upper and lower folds are on the left while the middle fold is on the right. Hence, the rectum bends from side to side.

  The upper part of the rectum above the middle fold may contain faeces, but the lower part only contains faeces in chronic constipation or during the call to defaecate.
• In the female rectal pouch, the upper third of the rectum is covered in front and at the sides by peritoneum. Its upper third has peritoneum anteriorly and laterally. The middle third has only peritoneum in front, and the lower third is devoid of peritoneum, because this has moved forwards to form the rectouterine pouch (of Douglas). This is a fold of peritoneum that dips down between the rectum and uterus (and upper vagina). Below the level of the pouch lies the lower part of the vagina.
• In the male rectal pouch, the peritoneal relations are:

  • peritoneum in front and to the sides of the upper third of the rectum.
  • peritoneum at the front of the middle third of the rectum.
  • no peritoneum over the lower third of the rectum.

  The pouch of peritoneum in the male is the rectovesical pouch that dips down between the rectum and bladder. In front of the rectum, below the level of the pouch, lies the prostate gland, the ends of the ureter and ductus deferens, and rectovesical fascia.
• Looking, at the pelvic outlet from below, the boundary of the perineum is diamond-shaped, made up of a urogenital triangle in front and an anal triangle behind. The roof of this triangle is the pelvic floor, which is made up of levator ani muscles.
• In the middle of the anal triangle there is the anal canal, surrounded by the anal sphincters in the midline. The levator ani muscles are angled downwards, and its lateral attachment is to the fascia covering obturator internus. Medially, the levator ani muscles become part of the external anal sphincter.
• Surrounding the anal canal is a fat-filled space and this is known as the ischioanal fossa. This fossa allows room for expansion of the anal canal during defaecation.

  On the lateral wall of the ischioanal fossa there is a neurovascular bundle contained within a canal. The vasculature is from the internal pudendal artery and vein, and their innervation is via the pudendal nerve (S2-4).
• The pudendal nerve supplies the entire region of the perineum, and hence supplies the structures of the external genitalia in the urogenital triangle.

  The lower half of the anal canal receives the inferior rectal branch.
• The anal canal is approximately 4cm long, so it's possible to perform a digital rectal examination.
• During a digital rectal examination in both sexes, it is possible to test the resting tone of the external anal sphincter. The patient can be asked to contract the muscle voluntarily, but it can also be tested via a reflex as it contracts during coughing.

  Obvious pathologies can be palpated but they may also bleed when the digit is removed. A painful examination may be associated with abscesses, fissures or fistulae.
• The contents of the rectum can be assessed in a digital rectal examination. The faeces may be hard or soft, the rectum may be empty or collapsed and sometimes ballooned out. This allows assessment of faecal impaction. Any pathology can be assessed, for example for the presence of a rectocele or enterocele.
• In the female, the perineal body can be assessed as this is an important structure to support the pelvic floor and external anal sphincter. It is often damaged during childbirth. The reproductive organs are best accessed via a digital vaginal examination rather than a digital rectal rectal examination.
• During a digital rectal examination, the prostate gland is the most important palpable structure in the male. The prostate gland is normally the size of a walnut and has a prominent median groove. It is frequently enlarged in middle-aged men so that its median sulcus is no longer palpable.

  Although the seminal vesicles are an anterior relation of the rectum, they are not normally palpable unless they are enlarged. Tenderness in the urinary bladder or rectovesical pouch may also trigger pain in a digital rectal exam.
• The pelvic floor is formed by two sheets of muscle which meet in the midline but leaving a gap in front for the urogenital structures. The midline of the pelvic floor is strengthened by fibrous bodies. Posteriorly there is the anococcygeal body and anteriorly there is the perineal body.

  The pelvic floor is formed by the levator ani and the coccygeus muscle. The levator ani has three parts, and the most medial of these three parts is the puborectalis muscle, which forms a sling around the anorectal junction.
• Each levator ani muscle arises from the pubis, passes posteriorly, and joins with its partner to form a U-shaped muscular sling around the anorectal junction. This serves to pull the anorectal junction forwards. It is under constant tone and helps to maintain rectal continence.
• Most of the voluntary control of defaecation is provided by the puborectalis muscle. The contraction of this muscle produces the rectoanal angle formed by the axis of the rectum and the axis of the anal canal.
• Defaecation normally occurs only when the puborectalis muscle relaxes, which results in the straightening of the rectoanal angle. This allows the passage of solid faeces from the rectum into the anal canal.

  The internal and external anal sphincters are important for controlling the passage of liquid stool and gas that readily traverse the rectoanal angle when the puborectalis muscle is contracted.
• The anal canal has two parts. The upper half is derived from endoderm, making it part of the hindgut. Its mucosa is arranged in vertical columns with sinuses between the columns. The sinuses have glands that secrete mucous. At the bottom of these sinuses are folds of membrane called anal valves. A line drawn across all of the valves defines the end of the upper half. This is called the pectinate line.

  The pectinate or dentate line marks the end of the hindgut. Above this line the neurovasculature is that of the hindgut. The mucosa has a columnar epithelium typical of the hindgut.
• Below the pectinate line the lower half of the anal canal begins as the pecten, which is bluish-pink in colour. This colour is due to the presence of a dense venous plexus under the mucosa. The pecten ends inferiorly at the white line of Hilton. This line can be felt upon digital examination of the anal canal as there is a groove here between the anal sphincters. The lower half of the anal canal continues below the white line to the anal verge.
• The lower half of the anal canal has a non-keratinising stratified squamous epithelium, and it lacks glands. It is derived embryologically from ectoderm from a region called the proctodeum. At the anal verge, there is a transition to keratinised squamous epithelium that characterises skin.
• The exit of faeces and flatus is controlled by sphincters around the anal canal. There are two anal sphincters: an internal anal sphincter and an external anal sphincter.
• The internal anal sphincter is composed on smooth muscle. This smooth muscle is a thickening of the circular layer of the muscularis externa, continuous above with that of the rectum. It ends at the anocutaneous white line (of Hilton).

  This sphincter is under the control of autonomic nerves. Generally, sympathetic nerves stimulate contraction of the muscle and parasympathetics inhibit contraction. Although, some sympathetic nerves have inhibitory effects, and only alpha-adrenergic nerves are excitatory.
• The external anal sphincter is composed of striated muscle and consists of three parts: subcutaneous, superficial and deep. Puborectalis is also generally considered to be part of the external anal sphincter, since this must also relax to permit defaecation.
• At the anorectal junction, the pubococcygeal fibres fuse with the longitudinal smooth muscle coat of the rectum to form a conjoint longitudinal coat for the anal canal, which lies between the internal and external anal sphincters.
• During defaecation the anal mucous membrane can evaginate. This is countered by the longitudinal smooth muscle, which is attached to the dermis of the anal verge and retracts the everted anal lining once a stool has passed. The fibres of the conjoined longitudinal muscle radiate down through the subcutaneous portion of the external anal sphincter. This causes these muscle fibres to form discrete bundles and are hence called the corrugator cutis ani muscles. Contraction of these muscles together with the pull of the conjoined longitudinal muscle is what causes the anus to be puckered.
• Above the pectinate line (to the upper part of the anal canal), the major arterial supply is from the superior rectal artery which is a branch of the inferior mesenteric artery. There is an additional supply from the middle rectal artery, which is a branch of the internal iliac artery.

  Below the pectinate Iine, the arterial supply is from the inferior rectal arteries. These are branches of the paired internal pudendal arteries, which in turn arise from the internal iliac arteries of the pelvis.
• The venous drainage matches that of the arteries.

  Above the pectinate line venous blood drains to the hepatic portal system via the superior rectal vein.

  Below the pectinate line, the venous drainage is to the systemic internal iliac veins, via the inferior rectal and then internal pudendal veins.
• Above the pectinate line, lymph drains to the inferior mesenteric preaortic nodes. However, the presence of the paired middle rectal arteries means that some lymph may also drain directly to the internal iliac nodes. The latter drainage is minimal and only affects the lower part of the rectum.

  Below the pectinate line, lymph drains to the superficial inguinal nodes. From the superficial inguinal nodes, lymph drains to the external iliac nodes.
• Above the pectinate line the sensory nerve supply is via the inferior hypogastric plexus (autonomic (parasympathetic) nerve supply) signalling stretch and pain. The mucous membrane is relatively insensitive and can, for example, be pierced by a hypodermic needle with little or no discomfort.

  Below the pectinate line the nerve supply is somatic, via the pudendal nerves, and the mucosa and skin are very sensitive: jabbing here with a hypodermic needle is exquisitely painful. Below the pectinate line, the skin is a dermatome of S4 and S5.
• The rectum is not a chamber for prolonged storage: faeces accumulate in the sigmoid colon and eventually pass into the rectum in bulk. The rectum is best regarded as a sensory organ, since the resulting distension triggers sensory nerves that travel to the S2-S4 regions of the spinal cord via the parasympathetic nerves. These afferent signals are then transmitted to reach conscious awareness of the fullness of the rectum. Ignoring this signal can lead to more resorption of water from the faecal matter in the rectum, and faecal impaction or constipation may result.
• There is a second sensory stimulus that can trigger defaecation and that is stretch of the external anal sphincter. The ascending signals from this sensory feedback also alert the preganglionic sympathetics at the L1 and L2 regions of the spinal cord of defacation.
• In the 2nd step of defaecation, he lumbar sympathetic nerves are inhibited. These nerves descend to the lower part of the rectum and anal canal and exert a tonic contraction of the internal anal sphincter, to relax it. These nerves descend along the branches of the superior rectal artery and join the inferior hypogastric plexus in the pelvis, before reaching their target. The postganglionic nerves located there influence the myenteric plexus in the wall of the rectum and anal canal. The sympathetic nerves are often switched off by a reflex action by the ascending sacral nerves.
• The pudendal nerve also receives a signal to cease stimulation of the external anal sphincter and puborectalis muscle. Relaxation of the puborectalis muscle allows the ano-rectal angle to become more obtuse.
• The external anal sphincter is held under a state of tonic contraction, until the point of defaecation. When the moment is desirable, the brain sends a signal down the spinal cord, to switch off the tonic contraction of the external anal sphincter. With both external and internal sphincters relaxed, contractions of the rectum (and voluntary contraction of the abdominal wall muscles) can expel the faeces.
• Prior to toilet training, the baby/infant has not learnt to tonically contract the external sphincter at all, and hence relies purely on the internal sphincteric reflex. This means that there is no way to delay defaecation.
• The final stage of defaecation is to trigger the presynaptic sacral parasympathetic outflow. These nerves arise from S2-S4 and they travel via the inferior hypogastric plexus to reach the smooth muscle of the rectal wall and rest of hindgut causing them to contract. Through this means the rectum is emptied.