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The large intestine or large bowel is approximately 1.5 m long and 7.5 cm wide in the adult. It consists of the:
- Caecum
- Colon
- Rectum
The mucosa of the large intestine has a
smooth absorptive surface composed of epithelial and mucus-secreting cells. The wall is
puckered forming pouches called haustra.
Functions of the large intestine
- The absorption of water and salt.
- Mucus secretion for the lubrication of faeces.
- Movement (haustral contractions and mass movements which tend to be slow and non propulsive) and storage of faeces.
- Defaecation caused by the stimulation of stretch receptors in the rectal wall.
- The synthesis of vitamins K and B12 by bacterial activity.
Figure 2.1 - Anatomy of the large intestine
Figure 2.2 -
Basic structure of the large intestine
Constipation
Constipation refers to the difficult passage of stools. This may be due to an abnormality of stool bulk, hardness, or frequency, causing difficulty in expulsion. The reasons for constipation are multifaceted and focus on the following:
- Faecal volume - An inadequate intake of fibre leads to smaller, less bulky stools, which can predispose to constipation.
- Transit time -If transit time is increased i.e. it takes longer for stools to travel through the colon, greater quantities of water are reabsorbed which can give rise to constipation. Certain disorders and medication can effect transit time.
- Anatomical integrity - Diseases threatening anatomical integrity can be life threatening and must be detected. A mass in the lumen of the colon may completely or partially obstruct the passage of stools.
- Defaecation - A disruption of the process
of defaecation at any stage can result in constipation e.g. multiple sclerosis or spinal
cord injury will disrupt the sensation of a distended rectum.
