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Defecation

What is Defecation:

Definition of Defecation:

Defecation is the final action of the digestive system and the means by which organisms eliminate undigested (mostly indigestible) solid, semi-solid or liquid waste material in the form of faeces from the digestive tract via the rectum, anal canal and finally the anus.

In the simplest terms, defecation is the elimation (= expulsion from the body) of faeces.


Anatomical Structures involved in the process of defecation:

  • Faeces are formed in the rectum.
  • During defecation faeces pass via the anal canal to the anus:

The anal canal is the final part (approx. 2-3 cm or 1 inch long in the average adult) of the rectum, the inferior aspect of which is normally closed by two sphincter muscles:

  • The internal sphincter muscle, a smooth muscle - under involuntary control,
    is a thickened continuation of the circular muscle of the colon. It relaxes as a reflex response to distension of the distal rectum.
  • The external sphincter muscle, a skeletal muscle - under voluntary control.

These sphincters are closed except during defecation, i.e. elimation of faeces from the body.

  • The anus is the opening of the anal canal to the exterior of the body.


Short Notes
Re. Formation & Expulsion of Faeces:

Faeces expelled through anus.


Formation of Faeces:

  • Mass peristaltic movements push faecal material from the sigmoid colon into the rectum.


Expulsion of Faeces
:

Conscious vs unconscious control:
Defecation is under conscious (voluntary) control in adults but unconscious (involuntary) control in babies and infants until voluntary control of the external anal sphincter muscle has developed. Defecation is also under unconscious (involuntary) control in some adults who have certain medical conditions - which may be due to congenital conditions, illnesses, accidents or as a result of age-related conditions. The medical term for inability to control bowel movements is faecal incontinence, "incontinence" being the inappropriate involuntary passing of urine.

  • Accumulation of faecal material in the rectum distends the rectal wall - stimulating stretch receptors.
  • Stretch receptors in the wall of the rectum initiate a "defecation reflex", which leads to emptying of the rectum via several stages:
  • Nerve impulses from the spinal cord travel via parasympathetic nerves to the descending colon, sigmoid colon, rectum and anus.
  • Contraction of the longitudinal rectal muscles shortens the rectum, increasing the pressure within it.
  • Increased pressure within the rectum together with stimulation from the parasympathetic nervous system relaxes the internal sphincter surrounding the anal canal, opening the internal sphincter*.
  • Defecation follows (if and) when the external sphincter muscle surrounding the anal canal is voluntarily relaxed. However, if the external sphincter muscle surrounding the anal canal is voluntarily constricted (i.e. held closed), defecation is postponed.
  • *This relaxation of the internal sphincter muscle is, however, only temporary. If (voluntary relaxation of the external sphincter muscle and, therefore,) defecation does not follow, then internal sphincter muscle contracts again. The urge to defecate then subsides - until the internal sphincter muscle relaxes again some time later.
Sigmoid Colon Rectum File and licensing information

Contribution to defecation from other parts of the body and the systems of the body:

The process (also called the "cycle") of defecation can be described in terms of actions from the digestive system, musclar system/actions, cardiovascular aspects and the nervous system .

  • When the external sphincter muscle is voluntarily relaxed for defecation an increase in intra-abdominal pressure helps to expel faeces from the body:
  • With an intake of breath the epiglottis closes over the trachea. The respiratory muscles contract on air-filled lungs, increasing both intra-thoracic and intra-abdominal pressure.
  • Voluntary contractions of the diaphragm and abdominal muscles also aid defecation by increasing the pressure within the abdomen and, in turn, increasing pressure on the walls of the sigmoid colon and the rectum.

Finally:

  • Pelvic floor muscles relax.
    The rectum is straightened.
  • Faeces are expelled from the anus.


Common Defecation Disorders:

  • Diarrhea
    (which is also spelt "diarrhoea") is an increase in the volume and fluidity of faeces (also spelt "feces"). It is also defined as frequent bowel evacuation involving passage of abnormally soft or liquid faeces. See also causes of diarrhea.

  • Constipation
    is infrequent and/or difficult defecation. It can involve difficulty and pain in passing faeces that may be due to the faeces having become hard and dry due to dehydration, e.g. because a defecation has been delayed and so a greater proportion of their water content absorbed into the final parts of the large intestine.

Less common - but important :

  • Tenesmus
    is the excessively frequent (or continuous) sensation of the need to defecate despite the lack of expulsion of significant amounts of faecal material. It is also described as the unpleasant sensation of "something still being there" after defecation. This is cause for concern. Although the urge to defecate normally results from distension of the wall of the rectum due to amassed volume of faecal material, when this is not the cause the rectum may be distended by some other mass, such as a neoplasm - i.e. a new and abnormal growth, which may be either benign or malignant.

See also disorders of the digestive system and causes of diarrhea.

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