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Large intestine

life science Maturity 9-11 Next article →

The large intestine is the final segment of the gastrointestinal tract in tetrapods.

Blausen 0604 LargeIntestine2
Blausen 0604 LargeIntestine2

It serves as a critical processing center for the body's digestive system. Its primary roles include absorbing water and salts from waste material. It also serves as a site for the fermentation of unabsorbed material by gut microbiota. While the small intestine focuses on absorbing nutrients from food, the large intestine manages the remaining solids. In humans, this organ makes up about one-fifth of the total length of the gastrointestinal tract.

3DPX-002736 Large intestine Nevit Dilmen
3DPX-002736 Large intestine Nevit Dilmen

The movement of waste through the large intestine follows a specific, step-by-step sequence. First, waste exits the small intestine through the ileocecal valve. It enters the cecum, which is the first section of the large intestine. From there, the material moves into the ascending colon. The body uses a process called peristalsis to pump the waste upward. Peristalsis consists of muscular contractions that move the material along the tract. The waste then travels through the transverse colon, which crosses the width of the abdominal cavity. Next, it moves down the descending colon and through the S-shaped sigmoid colon. Finally, the waste is held in the rectum before being removed through defecation.

The large intestine is composed of several distinct anatomical parts. The cecum is the widest section, averaging slightly less than 9 cm in diameter. Attached to the cecum is the appendix, which contains a high concentration of lymphatic cells. Some scientists believe the appendix helps house a sample of gut microbiota to repopulate the colon after an immune reaction. The colon is the longest portion and is divided into four specific sections. These are the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. These sections are separated by turns known as colic flexures. The rectum is the final section, measuring approximately 12 cm in length.

Each section of the colon has unique physical characteristics and locations. The ascending and descending colons are retroperitoneal, meaning they are fixed in place behind the peritoneum. In contrast, the cecum, appendix, transverse colon, and sigmoid colon are intraperitoneal. This means they are completely surrounded by the peritoneum and are more mobile. The transverse colon is attached to the stomach by a large fold of peritoneum called the greater omentum. It is also connected to the posterior abdominal wall by the transverse mesocolon. The sigmoid colon is named for its S-shape. Its muscular walls contract to increase internal pressure, which pushes stool into the rectum.

Blood supply and microscopic structures are essential to how these parts function. The colon receives blood from the superior mesenteric artery and the inferior mesenteric artery. A vessel called the marginal artery runs parallel to the entire length of the colon to connect these supplies. On a microscopic level, the walls are lined with simple columnar epithelium. This lining contains tiny invaginations called colonic crypts.

Colonic crypts within four tissue sections
Colonic crypts within four tissue sections

These crypts are shaped like microscopic, thick-walled test tubes with a central hole called a lumen. The appearance of the colon is also characterized by haustra. These are shelf-like bulges created because the taeniae coli, or muscle bands, are shorter than the bowel itself.

There is significant variation in the size and shape of the large intestine among humans. In adult males, the average length of the colon ranges from 80 to 313 cm. In adult females, the average length is approximately 155 cm, with a range of 80 to 214 cm. The diameter also changes depending on the section. The transverse colon averages less than 6 cm in diameter, while the sigmoid colon is slightly smaller. Some individuals have a condition called a redundant colon. This means the colon has extra loops and can be up to five meters longer than normal. While often harmless, this can make medical procedures like colonoscopies more difficult.

Understanding the large intestine connects us to broader biological systems like microbiology and immunology. The gut microbiota living within the colon play a major role in fermentation. This process helps break down materials that the body could not digest on its own. The presence of lymphatic tissue in the appendix also links the digestive system to the body's immune defenses. By managing water recycling and waste storage, the large intestine ensures the body maintains a stable internal environment.

Large intestine histology
Large intestine histology

This complex system of tubes, muscles, and microbes works constantly to complete the process of digestion.

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🖼️ Images & Media (6)
File:Blausen 0604 LargeIntestine2.png
Blausen 0604 LargeIntestine2.png
File:Diameters of the large intestine.svg
Diameters of the large intestine.svg
3DPX-002736 Large intestine Nevit Dilmen.stl
File:Colonic crypts within four tissue sections.jpg
Colonic crypts within four tissue sections.jpg
File:Large intestine histology.jpg
Large intestine histology.jpg
File:Colonoscopy splenic flexure.jpg
Colonoscopy splenic flexure.jpg
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