what is absorbed in the large intestine
Hello. Welcome to solsarin. This post is about “what is absorbed in the large intestine“.
Large intestine
The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation. The colon is the longest portion of the large intestine, and the terms are often used interchangeably but most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal. Some other sources exclude the anal canal.
In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve.
It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal. Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the human gastrointestinal tract.
Structure
The colon of the large intestine is the last part of the digestive system. It has a segmented appearance due to a series of saccules called haustra.[9] It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which the fermentation of unabsorbed material by the gut microbiota occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day.
The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166 cm (range of 80 to 313 cm) for males, and 61 inches or 155 cm (range of 80 to 214 cm) for females.
Sections
In mammals, the large intestine consists of the cecum (including the appendix), colon (the longest part), rectum, and anal canal.
The four sections of the colon are: the ascending colon, transverse colon, descending colon, and sigmoid colon. These sections turn at the colic flexures.
The parts of the colon are either intraperitoneal or behind it in the retroperitoneum. Retroperitoneal organs, in general, do not have a complete covering of peritoneum, so they fixed in location. Intraperitoneal organs completely surrounded by peritoneum and are therefore mobile.
Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal. This is important as it affects which organs can easily accessed during surgery, such as a laparotomy.
Absorption, Secretion and Formation of Feces in the Large Intestine
To a first approximation, absorption and secretion in the colon is straighforward:
- Absorption: water, sodium ions and chloride ions
- Secretion: bicarbonate ions and mucus
Water, as always, absorbed in response to an osmotic gradient. The mechanism responsible for generating this osmotic pressure essentially identical to what seen in the small intestine – sodium ions transported from the lumen across the epithelium by virtue of the epithelial cells having very active sodium pumps on their basolateral membranes and a means of absorbing sodium through their lumenal membranes.
The colonic epithelium actually more efficient at absorbing water than the small intestine and sodium absorption in the colon enhanced by the hormone aldosterone.
Chloride absorbed by exchange with bicarbonate. The resulting secretion of bicarbonate ions into the lumen aids in neutralization of the acids generated by microbial fermentation in the large gut.
Horses
In addition to water and electrolytes, the large intestine efficiently absorbs short-chain or volatile fatty acids. For humans and cats, this does not typically contribute substantially to body energy balance, but in herbivorous species like horses, colonic absorption of volatile fatty provides the major source for energy intake.
what is absorbed in the large intestinewhat is absorbed in the large intestine
Goblet cells are abundant in the colonic epithelium, and secrete mucus in response to tactile stimuli from lumenal contents, as well as parasympathetic stimuli from pelvic nerves. Mucus is an important lubricant that protects the epithelium, and also serves to bind the dehydrated ingesta to form feces.
Normal feces are roughly 75% water and 25% solids. The bulk of fecal solids are bacteria and undigested organic matter and fiber. The characteristic brown color of feces due to stercobilin and urobinin, both of which are produced by bacterial degradation of bilirubin. Fecal odor results from gases produced by bacterial metabolism, including skatole, mercaptans, and hydrogen sulfide.
Motility
The intestinal wall made up of multiple layers. The 4 layers of the large intestine from the lumen outward are the mucosa, submucosa, muscular layer, and serosa. The muscular layer made up of 2 layers of smooth muscle, the inner, circular layer, and the outer, longitudinal layer. These layers contribute to the motility of the large intestine.
There are 2 types of motility present in the colon, haustral contraction and mass movement. Haustra are saccules in the colon that give it its segmented appearance. Haustral contraction activated by the presence of chyme and serves to move food slowly to the next haustra, along with mixing the chyme to help with water absorption. Mass movements are stronger and serve to move the chyme to the rectum quickly.
Absorption of Water and Electrolytes
Absorption of water occurs by osmosis. Water diffuses in response to an osmotic gradient established by the absorption of electrolytes. Sodium actively absorbed in the colon by sodium channels. Potassium either absorbed or secreted depending on the concentration in the lumen. The electrochemical gradient created by the active absorption of sodium allows for this. Chloride ions exchanged for bicarbonate ions across an electrochemical gradient.
what is absorbed in the large intestine
Production/Absorption of Vitamins
The colon also plays a role in providing required vitamins through an environment that is conducive for bacterial cultivation. The colon houses trillions of bacteria that protect our gut and produce vitamins. The bacteria in the colon produce substantial amounts of vitamins by fermentation. Vitamin K and B vitamins, including biotin, produced by the colonic bacteria. These vitamins then absorbed into the blood. When dietary intake of these vitamins is low in an individual, the colon plays a significant role in minimizing vitamin disparity.
Clinical Significance
Disturbance or dysfunction of the large intestine’s normal physiology can result in poor quality of life and significant medical issues. Pathology of the large intestine is common. One out of every 10 Americans over the age of 40 have diverticular disease, and around 3 million people in the United States have inflammatory bowel disease.
It is important to incorporate a healthy diet and lifestyle to maintain a properly functioning colon. Eating a diet high in fiber and drinking plenty of water allows food to easily move through the colon, keeping the colon relatively clean, which can decrease the risk of diverticular disease. It is also important to maintain healthy colonic flora. Maintaining healthy colonic flora will decrease the risk of abdominal bloating, gas, diarrhea, constipation, and infectious colitis.
Caecum and appendix
Chyme that not absorbed by the time it leaves the small intestine passes through the ileocaecal valve and enters the large intestine at the caecum. On receipt of the contents of the ileum, the caecum continues the absorption of water and salts.
The caecum is about 6cm long and extends downwards into the appendix, a winding tubular sac containing lymphoid tissue. The appendix thought to be the vestige of a redundant organ; its narrow and twisted shape makes it an attractive site for the accumulation and multiplication of intestinal bacteria.
Colon
At its other end, the caecum seamlessly joins up with the colon, this is the longest portion of the large intestine (Fig 1). Food residue starts by travelling upwards through the ascending colon, located on the right side of the abdomen.
The ascending colon bends near the liver at the right colic flexure (or hepatic flexure) and becomes the transverse colon, passing across to the left side of the abdomen. Just above the spleen at the left colic flexure (or splenic flexure), the transverse colon becomes the descending colon, which runs down the left side of the abdomen. Before the next bend, the descending colon transforms into the sigmoid colon.
Rectum, anal canal and anus
Distally, the large intestine opens into the rectum, which continued by the anal canal. The rectum forms the final 20cm of the GI tract. It is continuous with the sigmoid colon and connects with the anal canal and anus (Fig 2, page 52). The rectum ends in an expanded section called the rectal ampulla, where faeces stored before released; the rectum usually empty since faeces not normally stored there for long.
Formation and transport of faeces
Of every 500ml of food residue that enters the caecum each day, about 150ml become faeces. These contain mostly bacteria, old epithelial cells from the intestinal mucosa, inorganic waste, undigested food matter and fibre, as well as water to help it pass smoothly through the GI tract. They also contain small quantities of fats and proteins. Their characteristic brown colour is due to the presence of stercobilin and urobilin, breakdown products of haemoglobin from old red blood cells.
Thank you for staying with this post “what is absorbed in the large intestine” until the end.
