Large intestine consists of three segments namely caecum, colon and rectum. In human caecum is rudimentary structure and serves no function. The name of large intestine is so because of its wider diameter as compared to small intestine. Total length is shorter compared to small intestine. Intestinal contents from ileum are directed to colon through ileo-colic valve. Colon is comparatively larger and wider tubular organs. Main function of colon is to absorb water and mineral from the intestinal contents and convert undigested material into feces. Colon therefore, serves important function by returning important constituents like water and electrolytes back to blood thus maintains fluid and electrolyte balance. In colitis and diarrhoea this function is compromised and the patient suffers from dehydration and electrolyte deficit. A process of putrefaction also occurs in colon in which undigested food (especially amino acids) undergoes microbial putrefaction leading to production of colon gases (methane, H2S) and waste products such as indole, skatole, and mercaptans etc which are indicator of constipation.
Another important function of colon is synthesis of some water soluble (B-complex) vitamin and vitamin K with the help of microbes which can be absorbed and perform important functions. Deficiency of vitamin K (leading to bleeding disorder) is a common problem associated with prolonged diarrhoea.
Rectumis last segment of G1 tract which is associated with storage and expulsion of feces. Feces in animals composed of undigested dietary components, cellular fragments of intestinal wall, microbes and water.