Published May 16, 2023 by

Anatomy, Histology and Function of Large Intestine

General Discussion  

A component of the digestive tract is the intestine, a long, winding tube.  The intestine aids in the extraction and absorption of nutrients and water from meals as well as the removal of waste.

The intestine has two main parts:
Food enters the small intestine from the lower portion of the stomach and is further digested there before moving on to the big intestine. The small intestine is about 20 feet (6.1 m) long.
The large intestine absorbs water, nutrients, and compounds (such salt and chloride) from digested food.  Additionally, it gets the body ready to expel waste (feces).  The term "colon" also refers to the big intestine.  It is roughly 5 feet (1.5 meters) long.

The terminal portion of the intestine extends from the ileocecal valve to the anus, although this valve does not properly correspond to the extremity of the large intestine.  Along this path, the large intestine measures 1.5 m in length, and its diameter is greatest at its origin, that is, in the ascending colon, where it measures 7 cm;  It narrows in the transverse and descending colon, where it reaches 4 cm, to dilate again when it reaches the rectum, giving rise to the rectal ampulla. The right colon begins at the cecum, continues with the ascending colon, continues at the hepatic flexure, and ends at the proximal transverse colon.  The splenic flexure, descending colon, sigmoid, rectosigmoid, and anal canal are all located in the left colon. The left colon also contains the distal portion of the transverse colon. The transverse and sigmoid colons are suspended in the peritoneal cavity by their mesenteries, while the ascending and descending colons are attached to the retroperitoneum.
The anal canal, which is located between the rectum above and the anal verge (anal orifice) in the perineum below, is the most terminal portion of the lower GI tract/large intestine.

Histological structure of the large intestine
The colon, rectum, and cecum make up the three sections of the large intestine.  Similar to the rest of the digestive system, the three sections of the wall have a similar structure.
It is made up of the four characteristic layers: the mucosa, submucosa, muscularis, and serosa.  The mucosa is supported by a band of fine bundles of smooth muscle (muscularis mucosa).
The morphology is quite similar between the three sections of the large intestine, so histological differentiation is not always possible.  In the large intestine there are no villi, the height of the mucosa is uniform.  This mucosa contains numerous deep and narrow crypts that span the full thickness of the mucosa.  Goblet cells predominate on the surface and entire length of the crypts.
There are muscular mucosa, and the submucosa is similar to the small intestine.  There may be lymphoid nodules developed in the submucosa and Meissner's nerve plexus.
Auerbach's nerve plexus is found between the circular and longitudinal muscle layer.

The serosa is composed of connective tissue and mesothelium.  In the final portion of the rectum the serosa becomes adventitious (lacks mesothelium)

Main functions of the large intestine
The large intestine performs the following functions, all of which are essential for the proper functioning of the body:
* Water absorption and stool formation
* Vitamins, short-chain fatty acids and nutrient     recycling
* pH regulation
*immune function
* function of the appendix

Water absorption and stool formation
A very important function of the large intestine is the absorption of water and the transformation of food material into feces, a process that takes place mainly in the colon.
By the time the chyme reaches the large intestine, most of the nutrients and 80-90% of the water have been absorbed.  At this point, electrolytes such as sodium, magnesium, and chlorine remain along with dietary fiber and other indigestible parts of food.
The ascending colon receives fecal matter still with a large amount of fluids.  The muscles of the colon are making peristaltic movements and move the fecal material slowly, the water is absorbed and the feces are acquiring more and more consistency until they reach the descending colon.
As it passes through the colon, the fecal material also compacts and mixes with mucus and bacteria from the natural flora of the intestine.  The mucus exerts a lubricating action to facilitate the advance of the feces, while the bacteria ferment the dietary fiber.

Vitamins, short-chain fatty acids and nutrient recycling
Bacterial metabolism ferments various types of dietary fiber and produces short-chain fatty acids, such as acetate, propionate, or butyrate.  These fatty acids are nutrients used by the cells of the intestinal mucosa itself (enterocytes), with which a very important symbiotic relationship is established.
Bacterial metabolism also produces some vitamins that are also absorbed in the colon, such as vitamin K, biotin, and several B vitamins, such as B1, B2, B6, and B12.  In the colon, various substances previously secreted into the intestinal lumen are also recycled, for example bile acids, cholesterol and part of the vitamin B12 that are secreted in the small intestine.
Bacterial fermentation also produces gaseous substances, such as carbon dioxide and methane, which are responsible for farting and flatulence.

pH regulation
Digestive enzymes are not secreted in the large intestine.  The chemical digestion of food is considered to be completed in the small intestine, so it is not necessary to maintain an acidic pH.  The pH in the colon varies between 5.5 and 7, and is regulated by the production of alkaline substances that counteract the gastric acids and the acids produced by the bacteria of the intestinal flora during fermentation.

Immune function
The large intestine is home to significant populations of various microorganisms, primarily bacteria, which live in a symbiotic relationship with our bodies.  In addition to producing some vitamins and nutrients for the enterocytes, as we saw before, the bacterial flora has other important effects on our health, including its protective action against infections.
In addition, the contact between the GALT (gut-associated lymphatic tissue) and the intestinal flora serves as training for the immune system that involves the production of cross-reactive antibodies. They are effective against related pathogens, preventing infections even at first contact with those pathogens in the future.

Function of the Appendix
The vermiform appendix, a small appendage attached to the cecum, contains a small amount of lymphatic tissue and may have some role in the immune system, but its true function is not well understood and it is often considered a vestigial organ.
During fetal development, the appendix appears to secrete several hormone-acting peptides and amines important during early fetal development. During postnatal life, and especially in adult life, the appendix could serve as a reservoir for beneficial bacteria that allow the reestablishment of a healthy intestinal flora if it is lost for any reason, for example after episodes of diarrhea.