– Mucosal surface of colon is smooth i.e. there are no villi on the Mucosa.
– Simple tubular glands are abundant which secrete Mucus.
– Wall of the colon is usually folded into sacs by contraction.
– The longitudinal smooth muscle layer is collected into the three distinct bands called taenia coli.
Two main types of waves of contraction are seen in the large intestine
A. Segmental Contraction : –
They are of two types.
Type I contraction :
These are small amplitude waves which aid mixing of the contents of the colon.
Type II Contraction :
These are larger pressure waves which aid mixing of the contents of the colon and facilitate absorption by exposing more of the contents of the colon.
B. Peristaltic contractions :
They are also of two types :
1. Type III contraction : These are very small pressure wave of prolonged duration. They propel the contents towards the rectum.
2. Type IV : contraction : This is also called Mass Action contraction. These are simultaneous contraction over a large portion of the colon.
These contractions propel contents from caecal region towards the rectum. They are the predominant contraction force during defecation.
Both segmental & peristaltic contraction occur infrequently but are particularly evident following a meal. Thus, distension of the stomach by the food initiates contraction of the rectum and frequently a desire to defecate callled Gastrocolic Reflex.
Absorption & Secretion :
– The main function of the colon is absorption of water.
Main sites of water absorption are caecum and ascending colon.
– No digestive enzymes are secreted in the colon.
– At birth, the colon is sterile but the colonic bacterial flora becomes established early in life.
Some of these are beneficial and others harmful.
The Faecus :
1. The faecus are derived :-
(i) Mainly from the intestinal secretions
(ii) Partly from the ingested food.
2. If vegetables and ground cereals are excluded from the diet, the faces have fairly constant composition.
3. The brown colour of stools is due to pigment formed from the bile pigments by the intestinal bacteria.
4. On an average fat intake of 100 gm/day, 5-6 gm/day is normally post in faecus where as on an average protein intake of 100 gm/day 1.5 gm/day nitrogen is normally lost in faecus.
5. Dietary Fibres :
In humans there is no appreciable digestion of dietary fibres e.g. cellulose, hemcellulose and ligin etc. due to absence of certain micro-organism in GIT which break down these substances. Therefore, ingested cellulose passes out unchanged and substances which are enclosed in cellulose wall escape digestion and absorption.