Anal Canal

Original Editor - Khloud Shreif

Top Contributors - Khloud Shreif  

Introduction[edit | edit source]

Rectum is the end part of the large intestine, starts from rectosigmoid colon and end at anorectal junction, it is 12-14 cm in length and its diameter is 4cm. It contains vertical and lateral flexures (bends) they help to control and prevent urgent need to defecate they support and keep feces in rectum[1].

The anal canal is an extraperitoneal part, it is the final part of the gastrointestinal, starts from anorectal junction, extends posteroinferiorly as a termination of the rectum with 3- 4cm in length, and ends at the anus. Its main function is to maintain fecal continence and evacuate during defecation. In normal conditions it is always collapsed to prevent leakage and just open during defecation.  It is surrounded by internal and external anal sphincter. The rectum along with the anal canal are lined with a columnar epithelium and ends at the pectinate line where it becomes a squamous epithelium[2][1].

Anatomical Parts[edit | edit source]

Columnar zone: lined with columnar epithelium, the mucous membrane organized in longitudinal folds (anal columns), these longitudinal folds join at their ends and form the anal valves at their end they form the pectinate line, behind anal valves there are anal glands they are at different depths on the anal canal wall, and they are about 4-8 anal gland.

Intermediate zone: lined with non-keratinized columnar epithelium, below the pectinate line and extends to be 1 cm in length.

Cutaneous zone: lined with keratinized columnar epithelium, at the end of the anal canal (anal verge)[3].

Anal Sphincters[edit | edit source]

Internal anal sphincter, an involuntary smooth muscle fibers under the control from the sympathetic, it is always contracted in normal, healthy person, innervated by sympathetic system from inferior pelvic plexus, also receives a parasympathetic signals as an inhibitory innervation[1][4].

External anal sphincter, a voluntary skeletal muscle fibers around the  anus, supplied by parasympathetic nervous system. It is formed by two parts (deep sphincter and puborectalis), superficial sphincter and subcutaneous as a superficial compartment of the sphincter[4].

Innervation[edit | edit source]

Nerve supply[edit | edit source]

Parasympathetic innervation (s2-s4) by inferior hypogastric plexus that is sensitive to stretch and pelvic splanchnic nerves for the area above the pectinate line, while pudendal nerve (sensory innervation for pain, temperature, touch and pressure) to the area below the line.

Sympathetic innervation (t12-l2) by inferior mesenteric plexus to the upper part above the line.

Arterial supply[edit | edit source]

Above the pectinate line supplied with a branch from the inferior mesenteric artery, superior rectal artery. Below the pectinate line branches from the internal iliac artery, inferior and middle rectal arteries.

Mechanism of defecation[edit | edit source]

The digested food moves in the gastrointestinal tract till it reaches to the large intestine, the colon muscles contract (peristaltic movement) to move the undigested food to the rectum in a movement called (mass movement). When there is an enough large mass reached to the rectum, the defecation reflex will be triggered, the stretch receptors of the rectum wall will stretch in response to the mass, this stretch signals will transmit to the brain (afferent nerve fibers) to the defecation center. The defection center will send back motor efferent impulses to the descending, sigmoid colon, and the rectum the impulses will cause contraction of the parts mentioned above  and relaxation of internal and external anal sphincter. If it is not the appropriate time to defecate, voluntary contraction of external anal sphincter will delay defection, this will followed by contraction of internal anal sphincter and relaxation of the rectum wall will happen (rectoanal inhibitory reflex RAIR)[5][1].

First sensation we feel the urge to defecate associated with the first distention of rectum when filled with mass it is usually happens when the pressure in the rectum reaches to 20-25 cm h2o.


Clinical Relevance[edit | edit source]

Hemorrhoids/ piles, a swollen or enlargement of blood vessels in lower rectum and anus, chronic constipation, straining during defection are the most common causes.

Constipation, it is an infrequent bowel movement, the person is considered a constipated patient if he has less than 3 bowel movement/ week, bad eating habits or prolonged repetitive delaying the need to defecate will make your problem get worse as the feces will be more dryer and harder to empty.

Obstructed defection syndrome (ODS), it is a condition of inability of a person to evacuate the rectum properly and more related to constipation, tenesmus, anismus, or pelvic heaviness are common symptoms associated with ODS.

Fecal Incontinence inability to control the bowel movement, there will be an involuntary stool leakage or a sudden desire to defecate that can not be controlled voluntary.

Slow transit constipation, a rare condition of chronic constipation defined as an ineffective colonic propulsion due to deficit and abnormalities of the enteric system that has a control on the motility of the large intestine[7].

Physical Therapy Intervention[edit | edit source]

Exercise therapy is a promising and may be an effective intervention for whom suffer from constipation[8], physical activity in in addition to diet control and change in life style are effective to reduce symptoms of constipation in middle-aged obese women suffer from constipation [9]. Other interventions like acupuncture are an effective treatment option for chronic and functional constipation[10][11]. Pelvic floor muscle training PFMT, biofeedback, exercise programs, and electrotherapy are other options to treat other pelvic and bowel disorders an expert pelvic floor physical therapist can deal with.

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 Siegfried WB, Rao SS. Anorectal physiology and pathophysiology in the elderly. Clinics in geriatric medicine. 2014 Feb 1;30(1):95-106.
  2. Mawer S, Alhawaj AF. Physiology, defecation. InStatPearls [Internet] 2021 Sep 14. StatPearls Publishing.
  3. Muranaka F, Nakajima T, Iwaya M, Ishii K, Higuchi K, Ogiwara N, Miyagawa S, Ota H. AC comparative I mmunohistochemical S tudy of A nal C anal E pithelium in H umans and S wine, F ocusing on the A nal T ransitional Z one E pithelium and the A nal G lands. The Anatomical Record. 2018 May;301(5):796-805.
  4. 4.0 4.1 Lee JM, Kim NK. Essential anatomy of the anorectum for colorectal surgeons focused on the gross anatomy and histologic findings. Annals of Coloproctology. 2018 Apr;34(2):59.
  5. Desprez C, Turmel N, Chesnel C, Ismael SS, Tamiatto M, Tan E, Haddad R, Le Breton F, Leroi AM, Hentzen C, Amarenco G. Fecal incontinence subtype assessment (FI-SA): A new tool to distinguish among subtypes of fecal incontinence in a neurogenic population. Clinics and Research in Hepatology and Gastroenterology. 2022 May 1;46(5):101900.
  6. Dr.G Bhanu. Physiology of Defecation Animation / Defecation reflex / Constipation . Available from:[last accessed 27/2/2023]
  7. Heitmann PT, Vollebregt PF, Knowles CH, Lunniss PJ, Dinning PG, Scott SM. Understanding the physiology of human defaecation and disorders of continence and evacuation. Nature Reviews Gastroenterology & Hepatology. 2021 Nov;18(11):751-69.
  8. Gao R, Tao Y, Zhou C, Li J, Wang X, Chen L, Li F, Guo L. Exercise therapy in patients with constipation: a systematic review and meta-analysis of randomized controlled trials. Scandinavian journal of gastroenterology. 2019 Feb 1;54(2):169-77.
  9. Tantawy SA, Kamel DM, Abdelbasset WK, Elgohary HM. Effects of a proposed physical activity and diet control to manage constipation in middle-aged obese women. Diabetes, metabolic syndrome and obesity: targets and therapy. 2017 Dec 14:513-9.
  10. Yao JP, Chen LP, Xiao XJ, Hou TH, Zhou SY, Xu MM, Wang K, Hou YJ, Zhang L, Li Y. Effectiveness and safety of acupuncture for treating functional constipation: An overview of systematic reviews. Journal of Integrative Medicine. 2022 Jan 1;20(1):13-25.
  11. Zhang T, Chon TY, Liu B, Do A, Li G, Bauer B, Wang L, Liu Z. Efficacy of acupuncture for chronic constipation: a systematic review. The American journal of Chinese medicine. 2013;41(04):717-42.