Parts of Large Intestine

Table of Contents

• Large Intestine
• Large Intestine Location
• Large Intestine Structure
• Parts of Large Intestine and its Function
• Functions of the Large Intestine
• How is faeces formed?
• Ailments Associated with Intestines

Large Intestine

Forming the terminating part of the gastrointestinal tract, the large intestine or the large bowel is hence named as the lumen is large in diameter and not as it extends in length compared to the small intestine. For a fact, the small intestine is longer than the large intestine. It partially develops from the large intestine from the proctodeum (under the dentate line), hindgut (from the distal transverse colon to the dentate line in the anorectum), and the midgut (from the cecum to the distal transverse colon).

Approximately 1.5 m long (5 feet long), the large intestine comprises the caecum, the colon, rectum, the anal canal and the anus. Structurally, the large intestine is very much like that of the small intestine, the difference being the mucosa is fully devoid of the villi.

Where can the Large Intestine be seen?

The large intestine forms the terminating part of the gastrointestinal tract in the vertebrates. In this structure, water gets absorbed while the remaining wastes are gathered in the rectum in the form of faeces before the body eliminates it in defecation.

Out of the different parts of the large intestine – cecum, colon, rectum, anal canal and anus, the colon forms the longest part. The average length of the colon in human adult males is about 166 cm, and that of adult females is about 155 cm. The large intestine in humans starts in the right iliac part of the pelvis, found just at or below the waist. Here, it unites to the terminating part of the small intestine at the cecum through the ileocecal valve. Then it extends as the colon moves upwards the abdomen through the broadness of the abdominal cavity as the transverse colon. Following this, it descends to the rectum, ending at the anal canal.

Large Intestine – Structure

The colon appears segmented as a result of a haustra – a series of saccules. It extracts salts and water from the solid wastes before it gets removed from the body; it is the region wherein fermentation of the unabsorbed particles occurs by the gut microbes. On a daily basis, the amount of water arriving at the colon is about 1.5 litres and unlike the small intestine, the colon is not mainly involved in absorbing food and nutrients.

Parts of Large Intestine and its Function

Different sections of the large intestine are –

• Cecum
• Colon
• Rectum
• Anal canal
• Anus

The large intestine comprises the cecum (appendix included), the colon, the rectum and the anal canal. The colon in turn can be sectioned into 4 parts which turn at the colic flexures, they are – the ascending colon, then the transverse colon next is the descending colon, and the sigmoid colon.

The sections of the colon either are intraperitoneal or after it in the retroperitoneum. Generally, the retroperitoneal structures lack a complete covering of the peritoneum, hence they are intact. The intraperitoneal structures are surrounded completely by the peritoneum, and hence can move. The appendix, cecum, sigmoid and transverse colon are intraperitoneal while the rectum, ascending and descending colon are retroperitoneal.

Cecum and Appendix – Parts of Large Intestine and its Function

The cecum possesses the widest diameter, ideally, the average diameter is about 9 cm. It is the first part of the large intestine, about 6 cm long, and participates in digestion. The caecum stretches downwards into the appendix, which is a winding tube-like sac that has lymphoid tissue. The Appendix, which embryologically develops from it, does not participate in digestion and is not deemed to be a section of the gut-associated lymphoid tissue.

The significance of the appendix is not clear as yet but as per some, it seems to harbour some gut microbiota. It is capable of helping the repopulation of the colon with the microbiota if it is depleted at the time of an immune reaction. It is considered to be a vestige of a redundant structure. This structure is depicted to have a high level of lymphatic cells.

The unabsorbed chyme from the small intestine moves through the ileocaecal valve to enter into the caecum of the large intestine. On receiving the matter from the ileum, the caecum absorbs salts and water from it.

Colon – Parts of Large Intestine and its Function

The caecum unites with the colon. The food residues begin to travel upwards through the ascending colon present on the right side of the abdomen. The ascending colon curves near the liver at the right colic flexure and turns into the transverse colon as it passes through the left side of the abdomen.

At the left colic flexure above the spleen, the transverse colon turns into the descending colon, running down the left side of the abdomen. The descending colon changes into the sigmoid colon before it bends next. The colon appears segmented due to sacculation, referred to as haustra.

While the descending, ascending colon and the rectum are situated in the retroperitoneum. The sigmoid and transverse colon is attached to the posterior abdominal wall by the mesocolon.

Ascending colon – Parts of Large Intestine and its Function

It forms the first of the 4 main parts of the large intestine. It is associated with the small intestine by a part of the bowel referred to as the cecum. This ascending colon extends up through the abdominal cavity towards the transverse colon for about 8 inches.

Mainly, the colon is involved in eliminating water and other nutrients from the residues and recycling it. As there is an exit of wastes from the small intestine via the ileocecal valve, it proceeds towards the caecum and into the ascending colon, where the extraction process begins.

The wastes are pumped towards the transverse colon by peristaltic motion. At times, the ascending colon is attached to the appendix through the Gerlach’s valve. The ascending colon in the ruminants is referred to as the spiral colon.

Transverse colon – Parts of Large Intestine and its Function

This colon establishes a section of the colon from the hepatic flexure referred to as the right colic, to the splenic flexure also called the left colic. This colon suspends the stomach, secured to it by the great momentum. Posteriorly, the colon is associated with the posterior abdominal wall by the mesentery referred to as the transverse mesocolon.

The colon is enclosed in the peritoneum and hence is mobile. The majority of the colon is imbued by the middle colic artery, while the other part is provided with branches of the inferior mesenteric artery.

Descending colon – Parts of Large Intestine and its Function

Also referred to as the distal gut (since it is further along the GI tract compared to the proximal gut). It forms the part of the colon from the splenic flexure to the start of the sigmoid colon. One of its roles is it stores the faeces, which is led to the rectum. Its arterial supply is through the left colic artery.

Sigmoid colon – Parts Of Large Intestine And Its Function

As the name suggests, it is S-shaped and forms the section of the large intestine before the rectum, after the descending colon. Its walls are muscular in nature and undergo contraction to increase the pressure in the colon, thereby causing the stool to be pushed into the rectum.

Rectum, Anal Canal and Anus – Parts Of Large Intestine And Its Function

About 12 cm in length, the rectum forms the last part of the large intestine and has the formed faeces, which is to be eliminated through the process of defecation.

The large intestine distally leads to the rectum that directs to the anal canal. The rectum goes on to constitute the last 20 cm of the gastrointestinal tract. It is continuous with that of the sigmoid colon and links with the anal canal and the anus. The rectum terminates in an expanded region referred to as the rectal ampulla. It is here where the faeces is collected before it is eliminated. Usually, the rectum is empty, as the faeces is not normally reserved for long there.

The anal canal situated in the perineum is 3.8 – 5 cm long and opens into the outside at the anus. It has two sphincters –

• External anal sphincter – composed of skeletal muscles and voluntary in nature
• Internal anal sphincter – regulated by the involuntary muscles

Both the anal sphincters stay closed except for the time of defecation.

Functions of the Large Intestine

Once food enters the small intestine, after absorption, it exits into the large intestine. The time duration from ingestion of food up till this stage is about 8-9 hours. About 90% of the absorption of the ingested water is carried out by the small intestine by this time. Most of the remaining quantity of water is absorbed by the large intestine, which converts the liquid chyme remains into faeces or semi-solid stools. Mainly, the large intestine is involved in the following functions –

• faeces formation
• Electrolyte and water absorption
• Chemical digestion conducted by the gut microbiota

How is faeces Formed?

About 30-35% of your daily food residue entering into the caecum turns into faeces. Its content mostly comprises some amount of proteins, fats, older epithelial cells, bacteria from the inorganic residues, intestinal mucosa, fibre and the undigested food content in addition to the water to aid the smooth passage of it through the gastrointestinal tract.

Its distinct brownish appearance is a result of the urobilin and stercobilin, disintegrating products of the haemoglobin from the older red blood cells. As the chyme residue stays in the large intestine for a long time, about 12-24 hours, most part of the liquid entering into the large intestine on a daily basis gets absorbed, leaving merely some quantity to be passed in the faeces. This is the quantity of water that renders the faeces its semi-solid consistency. Additionally, what also softens it is the dietary fibre.

The mucus produced by the goblet cells which line the colon aids in binding the dehydrated chyme and in lubricating the passage of the faeces. Also, the transit is slow in the colon consuming anywhere about 72 hours to pass 70% of the ingested food, while the full expulsion of all the remains can take about a week.

Ailments associated with the Intestines

Some other associated ailments are Inflammatory bowel disease (IBD), Colorectal cancer, Crohn’s disease, Bowel obstruction, Diverticulitis, Malabsorption syndrome, Ulcerative colitis, and Coeliac disease.

You just read about the large intestine, location and structure of the large intestine, different parts and sections of it, and also the ailments associated with intestines.

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Also see:

• Important Notes For NEET Biology – Digestion and Absorption
• Flashcards For NEET Biology – Digestion and Absorption
• MCQs on Gut For NEET