The large intestine is the final section of the gastrointestinal system before the rectum. In this section of the GI tract water is reabsorbed and any remaining waste material is stored as faeces to be removed. Further information on the anatomy of the large intestine can be found here.
This article shall consider the physiological process of defecation and clinical conditions that may occur during dysfunction.
The rectum is reponsible for temporary storage of faeces before defecation. As it becomes filled the rectal walls expand and stretch receptors stimulate the desire to defecate. The urge to defecate arises from contraction of rectal muscles, relaxation of the internal anal sphincter and an initial contraction of the external anal sphincter.
If the urge is not acted upon further water is absorbed and the faeces is stored until the next mass movement occurs. More information on the anatomy of the rectum can be found here.
There are two main anal sphincters; an internal and external sphincter. The internal anal sphincter is controlled by parasympathetic fibres which relax involuntarily. The external anal sphincter is skeletal muscle that is controlled by somatic nerve supply from the Inferior anal branch of the Pudendal nerve (S2,3,4), which allows conscious control of defecation.
When the rectum is distended the rectosphincteric reflex is initiated and relaxes the internal sphincter. If defecation is not desired, voluntary contraction of the external sphincter can delay it. If defecation is appropriate, then a series of reflexes take place that lead to:
- Relaxation of the external sphincter
- Contraction of abdominal wall muscles
- Relaxation of pelvic wall muscles
Peristaltic waves then facilitate the movement of faeces through the anal canal. Defecation can also be assisted by taking a deep breath and attempting to expel the air against a closed glottis, this is known as the Valsalva maneuver.
However, if a delay in defection is needed then voluntary contraction of the external sphincter is usually sufficient to override the reflexes that anal distension initiates.
Clinical Relevance – Faecal Incontinence
Defecation can either be voluntary or involuntary depending on the situation. Young children learn voluntary control of the process during toilet training and it typically remains under voluntary control throughout life. However in some situations loss of control may occur, particularly if defecation occurs too quickly, before excess water has been absorbed and diarrhoea occurs.
Other causes of faecal incontinence include:
- Physical injury
- Nerve injury
- Intense fright
- Inflammatory bowel disease
Treatments can vary depending on the cause of the incontinence and how easily it can be corrected.
Clinical Relevance – Laxatives
Laxatives are used in the treatment of constipation. There are two main types of laxative: bulk laxatives e.g: Isphagula husk and stimulant laxatives e.g. Senna.
A bulk laxative adds fibre to the contents of the intestine increasing the mass thus stimulating the walls of the intestine to cause peristalsis, thus increasing movement of the bowels, reducing constipation. Stimulant laxatives work on sensory receptors in the wall of the intestine to increase the amount of water and electrolytes being retained in the gut. This in turn causes peristalsis helping to empty the bowels.