Ano-rectal anatomy and physiology

| The rectum and the anal canal are the places where the feces reach after digestion, accumulate and where they are evacuated to the outside.

The rectum and anal canal are the final parts of the digestive tract, between the large intestine (colon) and the outside. The rectum and the anal canal are the places where the feces reach after the digestion of the food bolus, they accumulate and where they are evacuated to the outside.

The rectum is the continuation of the colon and has a length about 12 - 15 centimeters. Upon entering the so-called pelvic diaphragm becomes the anal canal, surrounded by the strong. This anal canal is about 3-4 centimeters long and contains the internal anal sphincter and external anal sphincter, the anal glands, and the hemorrhoidal plexuses.

Both the rectum and the internal anal sphincter are innervated by the sympathetic and parasympathetic nervous systems (involuntary), while the external anal sphincter has somatic (voluntary) innervation. The entire area has a very fine sensory innervation and is richly vascularized.

Continence

The continence of stool, in addition to being influenced by the characteristics of the fecal matter, it depends largely on the elements that act as a reservoir (sigmoid colon and rectum), rectal adaptability and the structures that exercise a barrier function (internal anal sphincters and external).

The anal continence and the authorization for defecation also depend on a specific sensitivity by which the passage of gas and stool is recognized, interpreting rectal distention as a desire to defecate.

The reservoir function of the large intestine allows solid stool to be stored for a long time but not liquid stool. The rectum is normally empty but if it becomes full, its adaptability allows it to delay the emptying of the stool it contains.

This is important, since continence implies a sufficient margin between filling at which the presence of feces in the rectum becomes aware and the threshold for distension, after which the two sphincters reflexively relax and causes the escape of feces.

The defecation mechanism has several stages

• Segmental, non-propulsive contractions of intestinal contents.
• Propulsion peristaltic contractions, to transfer the fecal bolus to the left colon.
• Segmental, non-propulsive contractions that facilitate the reabsorption of intestinal contents.

When the intestinal content reaches the rectal ampulla and is of sufficient volume, a stimulus is produced on the wall of the rectum, transformed into a desire to defecate, which is accompanied by a reflex relaxation of the internal anal sphincter and a contraction of the abdominal press , the levator ani muscle and a relaxation of the external sphincter (voluntary phase).

The internal anal sphincter represents a permanent pressure barrier, essential in “basal” continence. Whenever intra-abdominal pressure increases, its tone does, except when it is the consequence of a Valsalva maneuver (in which case it relaxes). The external anal sphincter ensures "urgency" continence, through its contraction (up to 60 seconds).

The two sphincters are normally contracted. The internal anal sphincter (smooth muscle) it is subject to non-conscious, stimulating (sympathetic) and inhibitory (parasympathetic) influences. The external anal sphincter (striated muscle) is innervated by branches of the pudendal nerves, so it voluntarily contracts or relaxes its tone.

Fecal continence is predominantly maintained by proper function of the anorectal neuromuscular apparatus. Continence is also affected by the consistency and arrival of stool to the anorectal area.

In addition, the individual must be motivated to remain continent, an important element in the elderly in whom cognitive dysfunction is more frequent. Fecal incontinence is usually associated with one or more abnormalities of the continence mechanisms, which are divided into rectal and colonic storage elements, and sensory and motor abnormalities that affect the pelvic floor and anal sphincter muscles.

In general there are 3 categories of fecal incontinence in the elderly, including overflow incontinence, reservoir syndrome, and recto-sphincteric syndrome.

For a person to have fecal continence, the sensitive capacity of the need to defecate must be preserved, to be able to distinguish between solid, liquid, pasty and gas stools, and also have the mechanisms to slow down defecation until reaching the bathroom.

After childhood, control of stool evacuation has to be voluntary and any loss of control of fecal evacuation will be considered pathological.