Anus – Tissue and Systems

This article describes the basic anatomy and functions of the human anus in order to better understand symptoms that might arise from anal cancer.  The anus is not a region that most people consider often however when there is disease in this area it can be very troubling.The primary function of the anus is to assist in continence.  Continence to both stool and gas is very important for hygiene and social reasons.  If this function is impaired, the consequences to one’s life can be devastating.

The anus is composed of several parts.  The anal canal is about 3 inches and extends from the hairy skin around the anal sphincter and “upstream” to the rectum.  The anus is surrounded by circular and oval muscles that form a sphincter or valve-like structure.  The anal canal is lined with glands that secrete mucus that help lubricate to assist with bowel movements.  Occasionally these glands get infected and can be quite bothersome.  The lining of the anus changes from skin like cells (i.e. squamous epithelium) to lining that is similar to the colon and rectum (i.e. columnar epithelium).  The area of change is termed the transitional zone.  The anal canal also has three large collections of veins nearby that help fill the canal assisting with continence.  When these vein collections swell or cause problems such as bleeding or rash this is what most people refer to as “hemorrhoids.”  Really we all have the same hemorrhoid tissue but only when they cause a problem does someone notice they are there.

The change in the lining of the anus is extremely important for several reasons.  Cancer that occurs in the region of the anus can arise from the cell layers from the adjacent skin, the lining of the distal anal canal, transitional zone, or the part that is similar to the rectum.  Thus there is actually a wide variety of cancers that can occur in this area.  The other major problems that can occur in this area include dysfunction of the muscles/sphincter mechanism, the hemorrhoid veins, the glands and associated infections, and the skin area with rash and infection also.  This seemingly simple area is actually quite a complex region and not just a sphincter.

The nerves that supply sensation to the skin also allow sensation to the skin around the anus and the first inch or so.  At the transitional area, the nerve supply changes to be similar to that of the colon and rectum and thus unable to feel.  It is possible to feel distention or stretching of the colon, rectum or anus and this feeling is what contributes to continence.  Distention of the rectum and upper anus is what causes the urge or sensation to have a bowel movement.  Without feedback from these important nerves, continence would be severely impaired.  Similarly, the muscles that line the upper anus are not under conscious control as are the muscles that line the colon.  You cannot “flex” your colon, no matter how hard you think about it.  The muscles of the lower and outside anus are under conscious (i.e. voluntary) control just as a skeletal muscle is such as the bicep or pectorals.  The ability to control these muscles contributes to continence as well.

Unfortunately, this complex control mechanism is not well protected and can be injured or made dysfunctional in many ways.  Injury to the muscles or nerves can occur during childbirth.  The anus has a poor healing function in that scar tissue in this region will not function like a healthy anus.  This may cause partial blockage of stool evacuation (or stricture) or incontinence.  Any type of injury to the nerves that supply the pelvis such as in someone with a spinal cord injury will cause similar problems.

The anus also has a very rich blood supply.  This is why many disorders of the anus including cancers can cause bleeding in this area.  The average person (including many physicians) will describe almost any symptom of the anal region as “hemorrhoids.”  It should be obvious now considering the above discussion that there are many types of disease of the anus and that just calling an anal problem “hemorrhoids” is inaccurate and possibly dangerous.  If an anal cancer is just chalked up to “hemorrhoids” and ignored, a window of cure may be missed and a potentially deadly disease could be overlooked.

A common reason for ignoring problems of the anal region is embarrassment.  It is uncomfortable to discuss this area, undergo examination and often embarrassing.  Knowledge of the basic anatomy and function of this area may ease some of this anxiety and make discussion with your doctor easier.  Having some knowledge of certain “red flag” symptoms (i.e. things that cannot or should not be ignored) will also prompt investigation when needed.


  1. Townsend Jr, CM; Beauchamp RD; Evers BM; Mattox KL. (2008) Townsend: Sabiston Textbook of Surgery, 18th ed.  Chapter 51.  New York, NY: Saunders.
  4. Cameron JL; Cameron AM.  (2010) Cameron: Current Surgical Therapy, 10th ed. Chapter 196. Philadelphia, PA: Elsevier Saunders
  5. This article was originally published on September 3, 2012 and last revision and update was 9/4/2015.