What is Large Intestine?
As the products we ingest near the end of their digestive journey, we come to the home stretch. The final touches in the journey of digestion are made in the large intestine. While the large intestine is only about one-fourth the length of the small intestine, its diameter is three times that of the small intestine.
The large intestine is the body’s last chance to soak up nutrients from what we ingest. It is in the large intestine that water is absorbed from the products introduced to it from the small intestine. The large intestine also contains “commensal bacteria”. Commensal bacteria are bacteria that aid in digestion in the large intestine. These bacteria are often called microflora.
Location and Structure of the Large Intestine
When the abdomen is divided into quadrants we see that the large intestine resides in every quadrant. The large intestine wraps around the abdomen in almost a question mark shape. It begins on the right side of an individual’s abdomen. It then travels up and across and then down the left side. It then comes back toward the right side and down the middle to the rectum.
There are five sections of the large intestine and a few landmark structures.
Cecum
The cecum is the first part of the large intestine. It is connected to the ileal portion of the small intestine at the appropriately named ileocecal junction. Partially digested material (chyme) makes it way through the small intestine and ends up at the ileocecal junction where the ileocecal valve opens and introduces the partially digested material into the cecum. Here, bacteria is mixed with chyme in a churning process caused by muscle contractions. It is here that the chyme begins to become feces. The cecum is in lower right quadrant of the abdomen and can be compared to a pouch in appearance.
Vermiform Appendix
Also just known as the appendix, many doctors and scientists hold differing views on what this four-inch long projection is for. Some think that it is a leftover organ from evolution. That it may have been an extension of the cecum that would have allowed us to digest plant cellulose. Others think that since it is filled with more lymphatic tissue than other parts of the large intestine that it serves in our immune system to defend our bodies against illness. What almost every physician does agree on is that the appendix is susceptible to infections and may be taken out to preemptively avoid appendicitis.
Ascending colon
This section of the colon runs up (ascending) the right side of the abdomen. This 8-inch long section of the large intestine receives the fecal matter formed in the cecum. Here, water is absorbed and bacteria creates vitamins from the fecal matter through the process of fermentation. The water and the vitamins are then absorbed into the bloodstream. Peristalsis, wave-like muscle contractions, move the fecal matter to the next section of the large intestine.
Hepatic Flexure
A flexure, in anatomic terms, is a complex way of saying “a bend”. In this case, the Hepatic flexure of the large intestine signifies that this bend occurs by the liver (hepatic). This is the transitory bend from the ascending colon to the transverse colon.
Transverse Colon
Running horizontally from the right upper quadrant to the left upper quadrant is the transverse colon. In this segment, the process of mixing bacteria with feces continues as well as absorption of nutrients and water. Muscle contractions called segmentation mix the bacteria. Once this is complete the wavelike peristalsis contractions move the fecal matter into the next segment.
Splenic Flexure
This bend is named for its location. It resides next to the spleen.
Descending colon
Just as the ascending colon moves fecal matter up, the descending colon moves it down. The fecal matter then sits here until it is ready to be pushed to the final section.
Sigmoid Colon
The last section marks the end of what many people regard as the large intestine. The large intestine’s last chance to absorb any nutrients or water is done in this section. Fecal matter is kept here until the waste product is expelled into the rectum and then out of the body through the anus.
Every section of the large intestine contains haustra. Haustra are small pockets that, because of their increased surface area, help the body absorb nutrients.
The large intestine is held in place and attached to the abdominal wall by a sac-like structure called the mesentery. The mesentery also supplies the large intestine with blood from the superior and inferior mesenteric arteries.
Histology
Like the rest of the digestive system, the large intestine is comprised of four layers. Each layer plays a different part in making sure the large intestine can function.
Listed from inside to the outside, these four layers are:
- Mucous membrane: Also known as the mucosa. This layer secretes substances that help keep the innermost parts of the intestine lubricated.
- Submucosa: This layer contains blood vessels and nerves.
- Muscularis: This is a layer of muscle cells that create peristalsis and segmentation.
- Serous: Lying on the outside of the large intestine, the serous layer expels substances to keep the intestine lubricated against surrounding organs.
The mucous membrane is made from columnar epithelial cells. Lying below the mucosa there are pockets called the crypts of Lieberkuhn. These pockets are glands that are lined with goblet cells. There are many more goblet cells in the large intestine compared to the small intestine. Goblet cells secrete glycoproteins, called mucin. It is this substance that lubricates the inside of the large intestine.
The large intestine also contains lymphocytes that aid in immunity. Lymphocytes are small leukocytes, white blood cells, that are unique to lymphatic tissues.
The large intestine does not have villi that the small intestine has. It is also much more muscular. Taenia Coli are three muscular strips that line the entire large intestine. The taenia coli are actually shorter than the large intestine. This is the reason why the large intestine has the haustra. The taenia coli literally bunch up the intestine and creates pockets.
The large intestine also contains many types of bacteria that aid in digestion and forming fecal matter. This process creates gas as a byproduct. Some of the bacteria found in the large intestine are:
- Bifidobacterium
- Lactobacillus
- B. Fragilis
- B. Vulgatus
- Desulfovibrio
- Enterococci
These are just a few of the many bacteria that live in the small intestine.
The small intestine also contains some types of fungus and plant-like organisms.
Disorders and Diseases involving Large Intestine
The large intestine can develop certain disorders. The most common being Inflammatory bowel disease or IBD. IBD is a term for a number of different disorders that cause inflammation of the large intestine. The large intestine may also be susceptible to cancer. Colonic cancer is often screened by examining the sigmoid colon.
Some common disorders of the large intestine are:
Ulcerative Colitis: A type of IBS, this chronic disease occurs when the colon becomes inflamed and as a result, suffers from open sores called ulcers. Ulcers often fill with pus and may cause loose stool and frequent trips to the bathroom.
Crohn’s Disease: Every aspect of the digestive system can be affected by Crohn’s disease. Another type of IBS, Crohn’s disease is an autoimmune disorder in which your bodies immune defenses attack the tissues of the digestive tract.
Diverticulitis: Not drinking enough water and having a low fiber diet will make it difficult for your large intestine to move fecal matter through. When this happens, tiny pockets called diverticula form in the intestinal walls. These pockets can collect bacteria and eventually become inflamed. Often a liquid diet can clear mild cases, but some may require surgery.
Colonic Polyps: Polyps are small formations of cells. Colonic polyps are often harmless but if not regularly checked during a sigmoidoscopy, they may develop into cancer without you knowing.
Colorectal Cancer: Obesity, alcohol, smoking, and lack of physical activity may leave you more susceptible to colorectal cancer. Colorectal cancer can easily spread to other parts of the body. Men over 50 are at an increased risk and should undergo regular screenings.
Good colon health usually comes from a good diet rich in fiber. Symptoms of intestinal disorders can be embarrassing for a person, but it is important to seek medical advice if you experience abdominal pain or any other change.
