Large intestine | Physiology

                                                      Large intestine

This article discusses different  parts of large intestine. Here, learn about the structure ,secretion, function, and more.

Keywords: Hausrta, Taenia coli, ileocecal junction, Extraperitoneal, Intraperitoneal structure, Transit time, Gastro colic reflex, Colonocolonic reflex, Redundant

Table of contents

1.	Introduction
2.	Structure
3.	Secretion
4.	Mechanism of secretion
5.	Regulation of secretion
6.	Functions

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 Definition: Large intestine or large bowel is the last part of G.I.T. It begins in the right iliac region. Above it is connected with the ileum -3^rd part of the small intestine and below ends at the anus. Its connection with the ileum is marked by an ileocecal junction and valve. It is broader and shorter( about 1.5 meters) than the small intestine. Due to the presence of a series of saccules known as haustra, it appears segmented.

Structure: Its structure is like the general structure of the gastrointestinal tract, i.e., from outside to inside:

1. Serous layer

2. Muscular layer—outer longitudinal and inner circular layer, at the lower end of the rectum, the circular and longitudinal muscle layers terminate in the internal and external anal sphincter.

3. Subserous layer and

4. Mucous layer –lined simple columnar epithelium with invagination they form intestinal glands or colonic crypts.

Characteristic features of the large bowel  are :

     The longitudinal muscle layer is arranged into three bands known as Taenia coli.

1.     These bands are shorter than the length of the large intestine, so the wall of the colon forms haustra –or outpouching. Hausrta forms a shelf-like intraluminal structure.

2.     There are no villi on the mucosa, but numerous goblet cells are present.

3.     Colonic crypts are small and numerous with goblet cells. The goblet cells secrete mucin.

4.     Epiploic appendages are small fat accumulation on the visceral site.

Parts of the large intestine  :

1. Caecum with the appendix

2. Ascending colon

3. Transverse colon

4. Descending colon

5. Sigmoid colon (Sigmoid means S-shaped.)

6. Rectum and

7. Anal canal.

The appendix can be removed without apparent effect on health.

Intraperitoneal structure: The organ/structure of the body inside the peritoneum-completely surrounded by the peritoneum are Intraperitoneal structures. They are mobile, for example- Caecum with an appendix, transverse colon, and sigmoid colon.

Extraperitoneal structure: The organ/structures of the body behind the peritoneum – not wholly covered by the peritoneum are the extraperitoneal structures. They are almost fixed, for example, ascending colon, descending colon, sigmoid colon, and rectum.At the level of S3 sigmoid colon becomes extraperitonal.

Nerve supply of large intestine:

Parasympathetic nerve supply –stimulation increases colonic movements.

The caecum with the appendix, ascending colon, transverse colon, and part of descending colon is supplied from the vagus.

The sigmoid colon, rectum, and anal canal are supplied via pelvic nerves from the sacral spinal cord.

Sympathetic nerve supply –stimulation decreases colonic movements.

This is by superior and inferior mesenteric plexus and superior and inferior hypogastric plexus.

Secretion: Only mucus is secreted. No enzymes, etc., are secreted.

Functions  of the large intestine:

1. Storage of chyme. In the large intestine mixture of chymes from many meals of 1 to 3 days are present, in contrast to the small intestine, where no chime storage occurs.

Transit time: depends on many factors, i.e., type of diet and content of fiber in the food, but the average transit time is as follows:

Stomach                                    2-3 hours

Small intestine:                        4 hours.

Ascending colon                       6 hours

Transverse colon                    8 hours

Descending colon and                        12 hours

 From sigmoid colon to rectum 24-48 hours.

2. Absorption of water and electrolytes and any absorbable nutrients from the chyme. It receives 2 liters of chyme per day from the small intestine, but only 200ml/day output. It also absorbs vitamins such as thiamine, riboflavin, and vitamin K formed by colonic bacteria's gut flora. Recycling of various nutrients takes place in the colon.

3. Secretion of K⁺ and Cl^- ions. In cystic fibrosis, Cl^- ion secretion increases.

4. A mucus layer protects the large intestine from colonic commensal bacteria and other infections.

4. It also compacts feces and stores fecal matter in the rectum until it can be discharged via the anus in defecation.

5. Bacteria present in the large intestine are gut flora or colonic bacteria. They break down some of the fiber for their nourishment and create Acetate, Propionate, etc., as waste products, which are used by the cell lining of the colon for nutrition. 

Movements of the large intestine are also known as colonic movements. Types:

1. Haustral contractions are like segmental movements of the small intestine. Haustra becomes more prominent in this contraction and causes the mixing of chyme.

2. Propulsive movements are like intestinal peristalsis. The propulsive movements propel colonic contents toward the rectum.

3. Mass colonic movements or mass colonic peristalsis are strong peristaltic contractions that forcefully push the colon's contents into the rectum and finally lead to the defecation reflex. This is not affected by colostomy. Its frequency is 3-4 /day and usually after a meal.

The electrical activity of the colon muscles is more complex than that of the small intestine.

Reflexes

1.     Colonocolonic reflex is the relaxation of the entire colon in response to the distention of one part of the colon. This reflex is partly mediated by sympathetic nerve fibers.

2.     Gastro colic reflex –when food accumulates in the stomach causes motility of the proximal and distal colon, which stimulates the desire for defecation. This reflex is well developed in children and mediated by gastrin ? not by neural factors.

Feces –residual indigestible matter, sloughed off mucosal cells, dead bacteria food residues not digested by bacteria constitute the feces.

In the large intestine, over 700 species of bacteria with multiple functions and fungi and protozoa are present. The microbes in the distal gut are about 100 trillion. This mass is formed chiefly by symbiotic microbes. It has been called a 'forgotten organ.

The bacterial fermentation of undigested polysaccharides in the large intestine produces gas. This is also known as flatus. Flatus is a mixture of nitrogen, and carbon dioxide, with small amounts of hydrogen, methane, and hydrogen sulfide. Some fecal odor is due to indoles metabolized from the amino acid tryptophan.

Treatment with a broad-spectrum antibiotic may alter the gut flora, resulting in loss of vitamin production.

Redundant colon: In this case colon are longer than average (1.5 meters). The length may be up to 5 meters. It has no effect on health, though rarely does volvulus occur, resulting in intestinal obstruction.

Sigmoidscoy, colonoscopy

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Hashtags: Hausrta, Taenia coli, ileocecal junction, Extraperitoneal, Intraperitoneal structure, Transit time, Gastro colic reflex, Colonocolonic reflex, Redundant

Internal Link:

https://blog.totalphysiology.com/2021/06/movements-of-large-intestine.html

https://totalphysiology20.blogspot.com/2021/02/glands-we must know

https://blog.totalphysiology.com/2021/03/small-  .intestine-we-must know.

Large intestine and its parts

                                          

 

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