Parts of Large Intestine

The large intestine is the terminal portion of the GI tract. Its functions are to complete absorption, produce certain vitamins, form faeces, and expel faeces from the body.

1.0Anatomy of large intestine

• The large intestine is about 1.5 m long and extends from the ileum to the anus. 
• The joining of the small and large intestines occurs at the ileocecal sphincter, which controls the movement of material from the small intestine to the large intestine. 
• The large intestine consists of 4 major regions: the cecum, colon, rectum and anal canal. 
• The cecum is a small pouch-like organ next to the ileocecal sphincter. 
• Attached to the cecum is a coiled and twisted tube called an appendix or vermiform appendix. 
• The colon, a long tube connected to the cecum, is divided into four sections: ascending, transverse, descending, and sigmoid colon. The rectum forms the final 20 cm of the gastrointestinal tract, with the last 2-3 cm referred to as the anal canal. The anus, which opens to the exterior, is controlled by an internal smooth muscle sphincter and an external skeletal muscle sphincter.

2.0Histology of large intestine

Walls of large intestine consists of same basic 4 layers: 

Mucosa:

• It mainly consists of absorptive and goblet cells. 
• The absorptive cells are responsible for absorbing water, while the goblet cells secrete mucus to lubricate the movement of contents through the colon.
• Villi and circular folds are absent in the large intestine. 

Submucosa:

• Submucosa is similar to the rest of GIT. 

Muscularis:

• Muscularis consists of circular and longitudinal muscles. 
• Tonic contraction of circular muscles divides the colon into pouches called huastra. 

Serosa:

• It consist of a visceral peritoneum. 

3.0Mechanical digestion in large intestine

• As food moves through the ileocecal sphincter, it enters the cecum and then accumulates in the ascending colon. In the colon, haustral churning occurs, where the haustra (pouches in the colon) stay relaxed and gradually expand as they fill. 
• Once the haustra are sufficiently distended, the colon walls contract, pushing the contents into the next haustrum. Peristalsis, the wave-like muscle contractions, occurs slowly in this region. 
• However, a more powerful movement called mass peristalsis happens 3-4 times a day, beginning in the middle of the transverse colon and propelling the contents into the rectum.

4.0Chemical digestion in large intestine

• Bacteria chemically digest the large intestine, and no enzyme is secreted. They  ferment any remaining carbohydrates and release hydrogen, CO2, and methane gas. 
• If excessive, these glasses cause flatulence.  
• Bacteria also convert the remaining protein to amino acids, which are then converted into simple substances like indole and hydrogen sulfide, which are further converted to less toxic substances by the liver.  
• Bacteria also decompose bilirubin into simple pigments like stercobilin, which gives a brown color to fecal material.  
• Certain vitamins like Vitamin B and K are produced by bacteria, which are absorbed in the colon. 

5.0Absorption and feces formation in large intestine

• After spending 3 to 10 hours in the large intestine, the chyme gradually becomes solid or semisolid due to water absorption, forming what is known as feces. 
• Feces are composed of water, inorganic salts, shed epithelial cells from the gastrointestinal tract, bacteria, byproducts of bacterial breakdown, undigested food materials, and indigestible food components. 
• While 90% of water absorption takes place in the small intestine, the large intestine plays a crucial role in maintaining the body's water balance by absorbing additional water, as well as ions like sodium and chloride, and certain vitamins.