Large Intestine

The large intestine is the terminal segment of the human gastrointestinal (GI) tract, which is a continuous, tube-like channel via which food passes along the human digestive system. It runs from the small intestine to the anal canal, wherein food waste is eliminated. In simpler terms, the large intestine is involved in faeces production. Prior to defecation, the large intestine absorbs water from the remaining indigestible food and compacts excrement. The large intestine begins at the right iliac region of the pelvis, just below or at the right waist, and connects to the small intestine at the bottom end.

What is Digestion?

Digestion refers to the person's ability to digest food. Under this process, food is treated with heat, enzymes, or a solvent to promote decomposition or to extract essential components from food. Under this process breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the bloodstream. During all this processing the digestive system/tract plays a very crucial role.

The gastrointestinal tract, often known as the GI tract or digestive tract, as well as the liver, pancreas, and gallbladder, constitute the digestive system. From the mouth to the anus, the GI tract is made up of a succession of hollow organs connected by a long, twisting tube. The mouth, oesophagus, stomach, small intestine, large intestine, and anus are the hollow organs that make up the GI tract. The solid organs of the digestive system are the liver, pancreas, and gallbladder. The complete digestion process can be broadly classified into mechanical and chemical digestion. Mechanical digestion includes the physical breakage of food into smaller parts like in the process of chewing. Chemical digestion refers to the action of enzymes and gastric juices on food particles that result in the breakage of the particles into their monomeric and dimeric forms.

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Introduction to the Large Intestine

The large intestine is the last part of the gastrointestinal tract that performs the important task of absorbing water and vitamins while converting digested food into feces. Large intestine is shorter than the small intestine in length, the large intestine is considerably thicker in diameter. The length of the large intestine is about 1.5 m in length and 2.5 inches in diameter in the living body. The large intestine wraps around the border of the abdominal body cavity from the right side of the body, across the top of the abdomen, and finally down the left side. Starting on the right side of the abdomen, the large intestine is connected to the ilium of the small intestine by ileocecal sphincter.

Anatomical Positioning of the Large Intestine

It begins in the right iliac region of the pelvis, directly beneath the right waist, where it connects to the small intestine's end, which is termed as cecum. It then continues along the ascending colon, then across the breadth of the abdominal cavity as the transverse colon. It then turns down to create the descending colon and continuing to the anus (sigmoid colon to rectum to anus).

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Image illustrating the schematic of the large intestine, with the colon marked as follows: 1) cecum; 2) ascending colon; 3) transverse colon; 4) descending colon; 5) sigmoid colon, rectum, and anus.

Histology of Large Intestine

Large intestine is made up of four tissue layer they are:

• The innermost layer of the large intestine is known as the mucosa and it is made up of simple epithelial tissue. Here mucosa lacks villi which is found in the small intestine. Many mucosal glands secrete mucus which helps in the lubrication of food.

• Above the mucosal layer, there is a layer of blood vessels, nerves and connective tissues known as submucosa, which provides support to other layers of the intestine.

• The further submucosal layer is surrounded by the muscularis layer which contains several visceral muscle cells, which help in the contraction and relaxation of the large intestine.

• Outermost layer of the large intestine is known as the serosa layer, they are made up of squamous epithelial tissue and their secretion helps in the lubrication process.

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Parts of Large Intestine

Large intestine is made up of three parts they are:

Cecum

The cecum is the first part of the large intestine. The cecum is intraperitoneal with various folds and pockets are commonly known as retrocecal peritoneal recess surrounding it. 

Colon

Colon is further divided into two parts, they are:

• Ascending Colon:  It travels through the right iliac fossa, right flank, and right hypochondriac region . It ends at the right colic flexure. 

• Descending Colon: The descending colon extends between the left colic flexure and sigmoid colon. It travels through the left hypochondriac region, left flank and left iliac fossa.

Rectum

The rectum stretches between the rectosigmoid junction and the anal canal.

Anal Canal

The ana canal, forms the terminal part of the gastrointestinal tract. It extends from the anorectal junction to the anus. 

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The Function of the Large Intestine

There are so many major function of digestion which are performed by the large intestine: 

• Formation and Elimination of Faeces: When food is consumed by the small intestine then undigested food moves to the large intestine where absorption of remaining water takes place. Under this process conversion of liquid chyme into faeces takes place. These faeces consists of food, bacteria, inorganic salt, unabsorbed substance, etc.

• Absorption of Vitamins: Large intestine absorbs various vitamins produced by its own bacteria. There are more than 700 types of bacteria present in our large intestine. The major function of these bacteria is to breakdown left over fibre and polysaccharides and further convert them into short-chain fatty acid, which are absorbed by the large intestine by diffusion process.

• Absorption of Water and Electrolytes: During this process, water is absorbed by the large intestine by osmosis process as during this process diffusion is happening as per concentration gradient. Sodium ion is absorbed by the sodium-potassium pump in the colon region.

• Antibody Production: The large intestine is a house for many lymphoid tissue, and these lymphoid tissue is crucial for immunity. They help in the creation of various antibodies, which are helpful against various harmful bacteria.

• Acid Reduction: bicarbonates which are present in the large intestine  neutralize acidity caused by the synthesis of fatty acids. 

Disorder of the Large Intestine

There are several disorders which are caused in large intestine, they are:

• Constipation is one of the major disorders caused by any kind of abnormality in the large intestine. During constipation there is painful and infrequent elimination of feces. This type of problem arises mainly due to dehydration.

• Coeliac disease is another disorder caused in the large intestine which leads to many problems like diarrhoea, abdominal pain, loss of appetite.

• Lactose intolerance is the most common type of disorder in the large intestine. Under this, the intestine is unable to digest lactose found in food and drink which cause gastric problems, bloating, cramps, etc.