The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossae. As the final functional segment of the bowel, it functions to regulate release of feces by two muscular anal sphincter complexes. The anus is the aperture at the terminal portion of the anal canal.
The pelvic floor or pelvic diaphragm is an anatomical location in the human body which has an important role in urinary and anal continence, sexual function, and support of the pelvic organs. The pelvic floor includes muscles, both skeletal and smooth, ligaments, and fascia and separates between the pelvic cavity from above, and the perineum from below. It is formed by the levator ani muscle and coccygeus muscle, and associated connective tissue.
The pelvic floor has two hiatuses (gaps): (anteriorly) the urogenital hiatus through which the urethra and vagina pass, and (posteriorly) the rectal hiatus through which the anal canal passes.
The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation. The colon (progressing from the ascending colon to the transverse, the descending and finally the sigmoid colon) is the longest portion of the large intestine, and the terms "large intestine" and "colon" are often used interchangeably, but most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Some other sources exclude the anal canal.
In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the human gastrointestinal tract.
The rectum (pl.: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its diameter is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal, which is about 4 centimetres (1.6 in) long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin rēctum intestīnum, meaning straight intestine.
Hemorrhoids (or haemorrhoids), also known as piles, are vascular structures in the anal canal. In their normal state, they are cushions that help with stool control. They become a disease when swollen or inflamed; the unqualified term hemorrhoid is often used to refer to the disease. The signs and symptoms of hemorrhoids depend on the type present. Internal hemorrhoids often result in painless, bright red rectal bleeding when defecating. External hemorrhoids often result in pain and swelling in the area of the anus. If bleeding occurs, it is usually darker. Symptoms frequently get better after a few days. A skin tag may remain after the healing of an external hemorrhoid.
While the exact cause of hemorrhoids remains unknown, a number of factors that increase pressure in the abdomen are believed to be involved. This may include constipation, diarrhea, and sitting on the toilet for long periods. Hemorrhoids are also more common during pregnancy. Diagnosis is made by looking at the area. Many people incorrectly refer to any symptom occurring around the anal area as hemorrhoids, and serious causes of the symptoms should be ruled out. Colonoscopy or sigmoidoscopy is reasonable to confirm the diagnosis and rule out more serious causes.
The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The internal anal sphincter aids the sphincter ani externus to occlude the anal aperture and aids in the expulsion of the feces. Its action is entirely involuntary. It is normally in a state of continuous maximal contraction to prevent leakage of faeces or gases. Sympathetic stimulation stimulates and maintains the sphincter's contraction, and parasympathetic stimulation inhibits it. It becomes relaxed in response to distention of the rectal ampulla, requiring voluntary contraction of the puborectalis and external anal sphincter to maintain continence, and also contracts during the bulbospongiosus reflex.
The pectinate line (also known as the dentate line) is a line which divides the upper two-thirds and lower third of the anal canal. Developmentally, this line represents the hindgut-proctodeum junction.
It is an important anatomical landmark in humans, and forms the boundary between the anal canal and the rectum according to the anatomic definition. Colorectal surgeons instead define the anal canal as the zone from the anal verge to the anorectal ring (palpable structure formed by the external anal sphincter and the puborectalis muscle). Several distinctions can be made based upon the location of a structure relative to the pectinate line: