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United Ostomy Association IncGrand Rapids Ostomy AssociationHelping People with Ostomies and Alternate Surgeries in Southwest Michigan |
Anal Canal The anal canal, which is 2.5 to 3 cm in length, is the terminal portion of the colon, extending from the anorectal junction to the anal verge. The muscles surrounding the anal canal are tonically contracted, thus completely collapsing the anal canal. An angulation of approximately 60 to 105 degrees is created by the presence of the puborectalis muscle that loops around the anal canal at the anorectal junction. Midpoint in the anal canal is the dentate line (also referred to as the pectinate line). This is an important reference point because of the differences that exist in the tissues above and below this line. Proximal to this line, the mucosa assumes a pleated appearance known as the columns of Morgagni. These longitudinal folds are created by the narrowing of the rectum into the anal canal. Above the dentate line, the anal canal is lined with columnar epithelium, whereas squamous epithelium lines the anal canal distal to the dentate line. This transition from columnar to squamous epithelium is gradual, occurring in an area 6 to 12 mm proximal to the dentate line, known as the transition, or cloacogenic, zone.6 Other important differences are observed in the tissue surrounding the dentate line. The color of the epithelium changes; whereas the rectal mucosa is pink, the area proximal to the dentate line is deep purple or plum color as a result of the intemal hemorrhoidal plexus.Proximal to the dentate line, the epithelium is innervated with autonomic nerves; on the other hand, the submucosa distal to the dentate line contains numerous encapsulated and free sensory nerve endings. Thus, whereas the rectum is insensitive to ordinary tactile and painful stimuli, the mucosa in the distal anal canal is highly sensitive. The nerve endings in the anal canal provide differentiation among gas, liquids, and solids. The area below the dentate line, referred to as the anoderm,resembles skin except for the absence of accessory skin structures (i.e., hair, sebaceous glands and sweat glands.)
Ileocecal Valve
The ileocecal valve is a one-way valve located at the junction between the ileum and the large intestine. The ileocecal valve works in conjunction with the ileocecal sphincter, a ring of smooth muscle, to regulate emptying into the large intestine and to prevent reflux of contents back into the small intestine. Normally the sphincter is partially contracted; peristaltic waves in the distal ileum cause the sphincter to relax, permitting passage of chyme into the colon. In contrast, distention of the cecum causes increased contraction of the sphincter, which protects the small bowel from reflux.
The ileocecal valve and sphincter provide some delay in the passage of chyme from the small bowel into the colon; this delay factor may be critical in the patient with a short-bowel syndrome or compromised absorptive capacity becauseit increases the exposure of nutrients to the absorptive surface of the small bowel.