By Dr Ananya Mandal, MD
What is the large intestine?
The large intestine comprises of the second part of the alimentary canal. The large intestine consists of the cecum and colon. It begins at the right iliac region of the pelvis (the region just at or below the right waist) where is continues from the small intestine and continues up the abdomen. Thereafter it traverses across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus.
Major function of the large intestine
The major function of the large intestine is to absorb water from the remaining indigestible food matter and transmit the useless waste material from the body.
Structure of the large intestine
The large intestine is made up of the colon and the rectum.
The colon is a tubular structure that is around 1.5 m, about one quarter of the length of the small bowel. Its largest diameter is at the cecum (7.5 cm) and narrowest in the sigmoid (2.5 cm). The colon is continuous with the small intestine from the ileocecal valve and ends finally at the anal verge. The parts of the colon include the ascending, transverse and descending colon.
The rectum is 10 cm in length in the adult. It starts at the peritoneal reflexion and follows the curve of the sacrum ending at the anal canal. The anal canal is 5 cm in length in the adult, has discrete upper and lower demarcations.
Over the surface of the large intestine are longitudinal muscle fibers called taeniae coli, each about 5 mm wide. There are three bands and they start at the base of the appendix and extend from the cecum to the rectum. There are sacculations called haustra that are characteristic features of the large intestine, and distinguish it from the rest of the intestines.
Microscopic structure of the large intestine
The large intestine is lined internally by a layer of mucosa. This mucosa contains tiny indentations called the crypts of Lieberkühn. These contain various glands and goblet cells that help in secretion and absorption of fluids.
There is also a large amount of lymphoid tissue that helps in maintaining an immune barrier. Macrophages or immune cells lie at the subepithelial layer.
Functions of the large intestine
The large intestine performs several important functions in digestion. Its main functions are:
Completing the process of digestion that largely takes place in the small intestine. It takes nearly 24 to 30 hours to complete the digestive process. Further digestion or breaking down of nutrients does not take place here but it helps by absorbing water and making the stools solid.
Absorption of vitamins
The large intestine also helps in absorption of vitamins made by bacteria that normally live in the large intestine. These are friendly bacteria called commensal bacteria. There are over 700 species of bacteria that perform a variety of functions.
These commensal bacteria breakdown the undigested polysaccharides or fibers in diet into short-chain fatty acids. These can be absorbed by the large intestine by passive diffusion. The bacteria also produce gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulphide. These result from the bacterial fermentation of undigested polysaccharides.
These bacteria also produce large amounts of vitamins. The most important of these is Vitamin K and Biotin (a B vitamin). When the vitamin intake in diet is low, this can be an important source of these vitamins. A person who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin deficient if treated with drugs like antibiotics that kill the commensal bacteria.
Reducing acidity and protecting from infections
The mucosa of the large intestine also secretes bicarbonates to neutralise the increased acidity resulting from the formation of these fatty acids and other digestive components at earlier parts of the intestines.
The mucosal layer of the large intestine also acts as a mucosal barrier and protects from microbial infections and invasions.
The large intestine, particularly the appendix, is a confluence of several lymphoid tissues. These play an important role in immunity. The lymphoid tissues of the large intestine also help in the production of antibodies and cross reactive antibodies. These antibodies are produced by the immune system against the normal commensal bacteria but may also be active against related harmful bacteria and thus prevent infections.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)