The structure of large intestine and its role in digestio
The large intestine is shorter than the small intestine and it is called large because it has a wide lumen. It is arranged around the small intestine in the form of a question mark. The large intestine is about 5 feet long and has the following parts Caecum, Colon, Rectum and Anus.
The caecum is a blind dilated sac into which is connected the ileum. The mucous membrane at the point of connection of the ileum and Caecum is so arranged that it allows the contents of the ileum to pass into the caecum but prevents their return into the ileum. The caecum has a short slender worm like projection called the vermiform appendix.
The appendix is usually about 9 cms long. It is lined with mucous membrane and contains lymphoid tissue in its walls. In human beings the appendix has no specific function to perform and is regarded as a vestigial organ having been continued as an organ which was present in the ancestors of human beings.
It is believed that in the non human primate ancestors appendix helped in the digestion of fibrous plant matter. Inflammation of appendix is quite harmful. Known as appendicitis it causes acute abdominal pain. Under this condition surgical removal of appendix is the only remedy. The colon has four parts namely the ascending colon, the transverse colon, the descending colon and the pelvic or sigmoid colon.
The ascending colon passes upwards from the caecum through the right side of the abdominal cavity. When it reaches the under surface of the liver it becomes transverse colon. This passes across the abdominal cavity as a loop. The descending colon passes downwards in the lumbar region. It is anchored to the abdominal wall by the peritoneum.
The pelvic or the sigmoid colon is the last portion of the colon and it makes an S shaped or sigmoid curve. The rectum lies in the pelvic cavity and follows the colon. It is about 13 cms long and has longitudinal folds made up of stratified squammous epithelium. The last part of the rectum which is about 2.5 cms long is called the anal cannal. Finally the anal cannal opens to the exterior by anus.
The anus is guarded by two anal sphincter muscles namely the internal smooth involuntary muscles and external straited voluntary muscles. The functions of the large intestine are the absorbtion of water and electrolytes from the chyme as it comes from the small intestine.
Another function of large intestine is the storage of faecal matter until it is thrown out as faeces. By the time the contents of the small intestine reach the caecum, the digestion and absorption would have already been over and only the unwanted residue in a liquid form will reach the large intestine. Most of the water and electrolytes are reabsorbed by the first half of the large intestine leaving only a faecal residue between 100 to 200 ml (out of a total of 800 ml) per day.
The faeces are normally a semisolid paste like matter coloured brown by stercobilin, a pigment derived from the bile pigments. Water forms (65 to 70%) the bulk of the faecal matter. The remainder consists of undercomposed cellulose, fatty acids, protein residues etc. In addition to the above organs two other organs play an important role in the digestion of food. These are the liver and the pancreas.