Pelvis and Perineum

Defecation

Natisha Busick, B.S., Eric Brandser, M.D., Alexander Sandra, Ph.D.
Peer Review Status: Internally Peer Reviewed

Defecation is a complex action requiring coordination and sequential activation of a large number of muscles. It is controlled by the autonomic nervous system, but is also under voluntary control. Defecation is initiated by distension of the rectum by feces arriving from the sigmoid colon. This sensation leads to a chain of events which ends in expulsion of feces from the anus. The act of defecation is voluntarily controlled in healthy, normally functioning people.

Defecography

Defecography, or evacuation proctography, is a radiologic study which produces an anatomic depiction of the changes that occur in the rectum and anal canal during defecation. It also provides information about the movement of the pelvic floor, such as movement of the puborectalis sling during evacuation, squeezing and rest. Defecography is a technique in which a barium contrast medium is introduced into the rectum. The barium blocks X rays and is visible within the rectum. Radiographic images are taken in the lateral position by fluoroscopy and videotape while the person is at rest, straining, squeezing, and during defecation. Therefore, it is a dynamic procedure.

The Act of Defecation

At the beginning of defecation, the subject voluntarily raises intra-abdominal pressure by muscular contraction of the quadratus lumborum, rectus abdominis, external and internal obliques, transversus abdominis and diaphragm. The puborectalis muscle surrounding the anorectal junction then relaxes so as to allow the straightening of the anorectal angle. Remember that the puborectal sling usually produces an angle of about 90° between the rectal ampulla and the anal canal, so that it is closed off. However, as it straightens, the angle increases to about 130° to 140° so the canal straightens and the feces, or in this case barium, can be evacuated. The external anal sphincter then constricts and lengthens the anal canal. It accentuates the action of the puborectalis by also pulling the anorectal junction forward anteriorly to further increase the anorectal angle. The fatty tissue of the ischiorectal fossa allows for changes in position and size of the anal canal and anus during defecation. During evacuation, the anorectal junction moves down and backwards and the pelvic floor usually descends slightly.

The internal anal sphincter, which usually closes off the upper and middle portion of the canal, relaxes to allow passage. The circular muscles of the rectum then stimulate a wave of contraction to push feces towards the anus. As feces emerges from the anus, the longitudinal muscles of the rectum and levator ani bring the anal canal back up, the feces is expelled, and the anus and rectum rise back up and forward to return to their normal position. The anal canal is closed tightly once again.

In the video, when the patient performs the squeeze maneuver, the anal canal can be seen tightly closed between the anorectal junction and the external anal orifice. Then after contraction, towards the end of defecation, the rectum is emptied. Notice how the pelvic floor has slightly descended and the anal canal is wide open. Also, notice that the anorectal angle is greater than at rest or squeezing.

These defecograms have been an overview of the action produced by muscles of the pelvic girdle during defecation.

Previous Page | Title Page

See related Provider Textbooks about Anatomy and Cell Biology or Radiology.

See related Provider Topics Anatomy, Anatomy and Cell Biology, Bones, Joints and Muscles, Brain and Nervous System, Diagnostic Imaging, Musculoskeletal, Procedures and Therapies or Radiology.

See related Patient Textbooks about or Radiology.

See related Patient Topics Bones, Joints and Muscles, Brain and Nervous System, Diagnostic Imaging, Procedures and Therapies or Radiology.

All contents copyright © 1992-2004 the Author(s) and The University of Iowa. All rights reserved.

http://www.vh.org/adult/provider/radiology/pelvis/defecation.html