Anatomy of the Rectum and Anal Canal

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The rectum is continuous with the sigmoid colon proximally at the rectosigmoid junction (S3) and with the anal canal distally1. The rectum has a sacral flexure which follows the curve of the sacrum and the coccyx1. The anorectal flexure is an S-shaped flexure which has a sharp postero-inferior bend antero-inferior to the tip of the coccyx and it penetrates the pelvic diaphragm1. It maintained by the puborectal sling through the puborectalis muscle1. Lastly, the rectum also has lateral flexures due to transverse rectal folds1. There is a superior, intermediate and inferior lateral flexure1.
Relationships of the rectum1:
• Inferior o Pelvic diaphragm (levator ani) o Anococcygeal ligament
• Anterolateral o Peritoneum of superior rectum
• Anterior o Peritoneum of middle rectum o Females
 Retrouterine pouch
 Vagina o Males
 Retrovesicular pouch
 Fundus of urinary bladder
 Terminal parts of ureters
 Vas deferens, seminal glands and prostate
• Lateral o Pararectal fossae
• Posterior o 3 inferior sacral vertebrae o Coccyx o Anococcygeal ligament o Median sacral vessels o Inferior ends of sympathetic trunk and sacral plexus

The arterial supply of the proximal rectum is by the superior rectal artery from the inferior mesenteric artery1. The middle and inferior rectum is supplied by the middle rectal arteries which are branches of the internal iliac arteries1. The anorectal junction and inferior anal canal are supplied by the inferior rectal artery from the internal pudendal artery1. The superior and inferior rectal arteries anastomose. The venous drainage flows via the superior, middle and inferior rectal veins1. The superior and middle rectal vein drains into the portal vein terminally, while the inferior rectal vein drai...

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...uperior ½ of anal canal drains deep into the internal iliac lymph nodes and then into the common iliac nodes to the lumbar lymph nodes1. The inferior ½ drains superficially into superficial inguinal lymph nodes1.
The superior ½ of the anal canal has visceral innervation1. It is involuntary, and it is supplied from the hypogastric plexus with sympathetic, parasympathetic and visceral components which travel to S2-S41. Inferior ½ has voluntary with somatic innervation from the inferior anal (rectal) nerves, which branch from the pudendal nerve1. This part of the canal is sensitive to pain, temperature, and touch1.

Works Cited

1. Moore KL, Dalley AF, Agur AMR. Clinically Orientated Anatomy. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2010. p.246-253; 411-414.
2. Netter, FH. Atlas of Human Anatomy. 5th ed. Philadelphia: Saunders Elsevier; 2009. Plate 373.

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