Anal Canal

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Introduction

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.

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.

Anatomical Parts

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).

Anal Sphincters

Internal anal sphincter, an involuntary smooth muscle fibers under the control from the sympathetic, it is always contracted in normal, healthy person

External anal sphincter, a voluntary skeletal muscle fibers around the  anus, supplied by parasympathetic nervous system.

Innervation

Nerve supply

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

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

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, and hence relaxation of the rectum wall will happen.

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

Hemorrhoids/ piles is 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.