Rectum

Rectum

The rectum is the 12–15 cm pelvic segment of the large intestine between the sigmoid colon (at S3) and the anal canal. It follows the curve of the sacrum and stores faeces until defecation.

Overview

Length≈12–15 cm

BeginsS3 (rectosigmoid junction)

Anorectal angle≈90° at rest

Despite its name ("straight"), the rectum has an anteroposterior sacral flexure following the sacrum and coccyx. It also has three lateral flexures, which correspond internally to the transverse rectal folds (Houston valves): usually two on the left and a more constant middle fold on the right. The lower part dilates into the rectal ampulla, which holds faeces. Where it passes through the pelvic diaphragm, the puborectalis sling pulls the anorectal junction forward, creating the anorectal (perineal) flexure of about 90°. This angle is an important part of continence.

The rectum has no taeniae, haustra or omental appendices, because the longitudinal muscle forms a complete coat. Peritoneum covers its upper third in front and at the sides and its middle third in front only. Its lower third has no peritoneal covering. The peritoneum reflects off the rectum onto the bladder or uterus, forming the rectovesical pouch in males or the recto-uterine pouch (of Douglas) in females. This is the lowest point of the peritoneal cavity when upright and can be felt on digital rectal examination. The rectum is wrapped in the mesorectum, a fatty envelope of vessels and lymph nodes inside the mesorectal fascia.

Location

Pelvis, in front of the sacrum and coccyx, from S3 to the tip of the coccyx, where the anorectal junction passes through levator ani.

Parts & subdivisions

Transverse rectal folds

Usually three semicircular shelves of mucosa, submucosa and circular muscle (Houston valves). They support the faecal mass and must be negotiated during sigmoidoscopy.

Rectal ampulla

Distensible terminal part above the pelvic floor; a stretch-sensitive reservoir.

Mesorectum

Perirectal fat with superior rectal vessels and lymph nodes, enclosed by mesorectal fascia. Removed intact in total mesorectal excision.

Recto-uterine / rectovesical pouch

Peritoneal pouch in front of the rectum, where pelvic fluid, pus or metastases (Blumer shelf) collect.

Anatomy

Innervation

Sympathetic: lumbar splanchnics via the superior and inferior hypogastric plexuses. Parasympathetic: pelvic splanchnic nerves (S2–S4), which contract the rectal wall and relax the internal anal sphincter. Visceral afferents for stretch travel with the parasympathetic fibres, so the rectum senses fullness but not cutting.

Arterial supply

Superior rectal artery, the terminal branch of the IMA and the main supply. Middle rectal arteries (internal iliac). Inferior rectal arteries (internal pudendal) at the anorectal junction. Median sacral artery.

Venous drainage

Rectal venous plexus. The superior rectal vein drains to the IMV and portal system. The middle and inferior rectal veins drain to the internal iliac vein and IVC (systemic). This makes a portosystemic anastomosis.

Lymphatic drainage

Upper rectum: pararectal nodes → along the superior rectal vessels → inferior mesenteric nodes. Lower rectum: internal iliac nodes.

Relations

Posterior: sacrum, coccyx, median sacral vessels, sacral plexus and sympathetic trunks. Anterior in males: rectovesical pouch, bladder, seminal vesicles, ductus deferens and prostate, separated by the rectoprostatic (Denonvilliers) fascia. Anterior in females: recto-uterine pouch, cervix and posterior vaginal wall.

Clinical significance

Rectal cancer

Presents with bleeding, tenesmus and change in bowel habit. It is often palpable on digital rectal examination. Treatment is total mesorectal excision (anterior resection or abdominoperineal resection), often after neoadjuvant chemoradiotherapy. Low tumours can spread through systemic veins to the lungs.

Digital rectal examination

Allows palpation of the prostate (or cervix), the recto-uterine pouch (tenderness in pelvic peritonitis, Blumer shelf metastases), rectal masses and anal tone.

Rectal prolapse

Full-thickness protrusion of the rectum through the anus, usually in elderly women with a weak pelvic floor. It should be distinguished from prolapsing haemorrhoids, which show radial rather than concentric folds.