12.7: Anatomy of the Large Intestine

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The large intestine is the final region of the digestive tract, where the body reclaims water, houses beneficial bacteria, and transforms the remaining material from the small intestine into feces for elimination.

Master this section and you'll be able to:

• Identify the anatomical regions of the large intestine and explain the role of the ileocecal valve in regulating movement of chyme.
• Compare the histological features of the large intestine to those of the small intestine.
• Trace the pathway of food from its point of entry into the body through its exit from as feces.
• Summarize key risk factors, early signs, and prevention strategies for colorectal cancer.

The large intestine is the final stretch of the digestive tract. While some absorption is finished off, the large intestine is mainly responsible for reclaiming water, compacting indigestible material, and preparing waste for elimination. After leaving the ileum, the remaining chyme enters a wider, slower-moving tube where helpful bacteria break down some of what is left, producing vitamins and gases. As water and electrolytes are absorbed, the material gradually transforms into feces, which is stored until defecation. Although its job seems simple, the large intestine plays an essential role in maintaining fluid balance, supporting a healthy gut microbiome, and completing the digestive process.

Anatomy of the Large Intestine

The large intestine runs from the appendix to the anus (see figure below). It frames the small intestine on three sides. Despite its being about one-half as long as the small intestine, it is called large because it has a diameter that is more than twice the diameter of the small intestine, about 3 inches.

The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve (or sphincter), located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine.

Figure \(\PageIndex{1}\): Regions of the Large Intestine. The large intestine frames the coils of the small intestine and includes the cecum (with its attached appendix), the ascending, transverse, descending, and sigmoid colon, and finally the rectum and anal canal. (Image credit: "Regions of the Large Intestine" by Jennifer Lange is licensed under CC BY-NC-SA, modification of original “Drawing of Abdominal Organs”  by KnowledgeWorks Global Ltd. and Open Learning Initiative.)

Cecum

The first part of the large intestine is the cecum, a sac-like structure that is suspended distal to the ileocecal valve. It is about 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding, dead-end tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial (a structure present in our ancestors that has over time lost its original function). However, at least one recent report postulates a survival advantage conferred by the appendix: in diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric bacteria for those surviving the initial phases of the illness. Moreover, its twisted anatomy provides a haven for the accumulation and multiplication of enteric bacteria. 

Rectum

Food residue leaving the sigmoid colon enters the rectum in the pelvis near the third sacral vertebra. Although the word rectum comes from a Latin term meaning “straight,” it is not  straight. Instead, it follows the curved contour of the sacrum and contains three lateral bends that form internal transverse folds called rectal valves. These valves help separate feces from gas so that each can pass at appropriate times.

Anal Canal

The final segment of the large intestine is the anal canal, located in the perineum, completely outside the abdominopelvic cavity. This short canal, measuring about 4–5 cm (1.5–2 in), opens to the exterior at the anus. The anal canal contains two sphincters. The internal anal sphincter is composed of smooth muscle and contracts involuntarily. The external anal sphincter is composed of skeletal muscle and is under voluntary control. Except during defecation, both sphincters normally remain closed.

Figure \(\PageIndex{2}\): Rectum, Anal Canal, and Anal Sphincters. The rectum forms the final segment of the large intestine and stores feces before elimination. The anal canal extends from the rectum to the exterior of the body at the anus. Two sphincters control the passage of feces: the internal anal sphincter, made of smooth muscle and under involuntary control, and the external anal sphincter, made of skeletal muscle and under voluntary control. (Image credit: "Anal Canal" by Jennifer Lange is licensed under CC BY-NC-SA, modification of original “Drawing of Abdominal Organs”  by KnowledgeWorks Global Ltd. and Open Learning Initiative.)

 

DISORDER OF THE...

Digestive System: Colorectal Cancer (CRC)

Colorectal cancer is still one of the most common cancers worldwide, but the pattern of who gets it is changing. The American Cancer Society estimates that

Right now in the United States, the American Cancer Society estimates that:

• in 2025 about 154,270 people will be diagnosed with colorectal cancer (around 107,320 colon + 46,950 rectal cancers). Cancer.org
• colorectal cancer will cause about 52,900 deaths in 2025, , making it one of the most deadly malignancies.  Cancer.org

In people over 50, rates and deaths have been dropping thanks to better screening and treatment. At the same time, cases in younger adults are rising: about 1 in 5 new CRC cases now occur in people under 55, and it has become a leading cause of cancer death in men under 50 and a top cause in women under 50.

Because of these trends, major guidelines now recommend that average-risk adults start routine colorectal cancer screening at age 45 instead of 50. Screening can be done with stool tests (like FIT or stool DNA tests) or procedures that look directly at the colon (like colonoscopy), and any abnormal stool test needs a follow-up colonoscopy. People with higher risk, such as a strong family history or certain genetic syndromes, often need to start even earlier and be screened more often.

Researchers are also sharpening the picture of risk factors. Long-known risks include diets high in processed and red meats, low fiber intake, obesity, inactivity, smoking, alcohol, and chronic inflammation in the gut. Newer studies highlight ultra-processed foods as a concern and suggest that dietary patterns may harm the gut microbiome and increase polyp and cancer risk, especially in the colon. On the treatment side, there has been major progress in personalized medicine and immunotherapy, especially for tumors with specific genetic features, which is improving outcomes for many patients.

Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Bleeding from the rectum may be either obvious or occult (hidden in feces). Since most colon cancers arise from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy not only allows identification of precancerous polyps, the procedure also enables them to be removed before they become malignant.  

Figure \(\PageIndex{3}\): Polyps and Early Tumor Changes in the Large Intestine. Highlighted are two common findings seen during colonoscopy. The upper inset shows a small, raised polyp projecting from the lining of the large intestine. Polyps are growths of mucosal tissue that can be harmless at first but may develop into cancer over time if not removed. The lower inset shows an area where the tissue has become irregular and abnormal in appearance, representing early malignant change.  

Incidence and mortality are not equal across groups. In the US, CRC incidence is highest in American Indian/Alaska Native populations, followed by non-Hispanic Black individuals, and lowest in Asian American/Pacific Islander groups. Mortality is about 43% higher in men than women overall. Disparities reflect a mix of access to screening, differences in risk factors, and broader social determinants of health.  Cancer.org

Histology

The walls of the large intestine differ in several important ways from those of the small intestine. Unlike the small intestine, the large intestine has very few enzyme-secreting cells, and it lacks circular folds and villi.

The mucosa of the colon (except in the anal canal) is lined by simple columnar epithelium made mostly of absorptive enterocytes with microvilli and many goblet cells. Compared to the small intestine, the colon contains a much greater number of intestinal glands. These glands are packed with enterocytes and goblet cells.

Goblet cells secrete mucus that lubricates the passage of feces and protects the mucosa from acids and gases produced by resident bacteria. The enterocytes absorb water, salts, and vitamins that are synthesized by your intestinal microbiota.

Figure \(\PageIndex{4}\): Histology of the Large Intestine. (a) The mucosa of the large intestine contains many straight, tube-shaped intestinal glands lined mainly by absorptive cells and numerous mucus-secreting goblet cells. Absorptive cells remove water and electrolytes, while goblet cells produce the thick mucus that helps feces move smoothly through the colon. Beneath the mucosa, the muscularis mucosae and submucosa provide support, and lymphatic nodules offer immune protection. In contrast to the small intestine, the large intestine does not have villi.
(b) A histological section of the large intestine shows the densely packed intestinal glands and abundant goblet cells characteristic of this region. LM x 464. Micrograph provided by the Regents of University of Michigan Medical School © 2012. (Image credit: “Histology of Large Intestine” by OpenStax is licensed under CC BY 3.0)

Special Features of the Large Intestine

Teniae Coli and Haustra

The teniae coli (also spelled taenia) are three narrow bands of smooth muscle that form the longitudinal layer of the muscularis in the large intestine, except near its terminal end. These bands run along the length of the colon and are spaced roughly the same distance apart around the circumference of the tube. Tonic contractions of the teniae coli pull on the colon wall and pucker it into a series of pouch-like segments called haustra (singular = haustrum). These haustra give the colon its characteristic lumpy or segmented appearance.

The rectum and anal canal do not have teniae coli or haustra. Instead, they contain thick, continuous layers of smooth muscle within their muscularis, which produce the strong contractions required during defecation.

Figure \(\PageIndex{5}\): External Features of the Large Intestine. Compared are a schematic drawing (left) with a cadaver specimen (right) to highlight three key external structures of the large intestine. The taeniae coli are three narrow bands of longitudinal smooth muscle that run along the colon and create the segmented, pouch-like haustra seen on the surface. Together with the small, fat-filled peritoneal pouches called adipose appendages these features help identify the large intestine and distinguish it from the small intestine in both anatomical drawings and real specimens. (Image credit: "Large Intestine Structural Features by Jennifer Lange is licensed under CC BY-NC-SA 4.0 , modification of originals “Teniae Coli, Haustra, Epiploic Appendage” by OpenStax and "Leiden MOOC - Reader Anatomy on the Table - The Large Intestine" by O. Paul Gobée, LUMC.)

Epithelium of the Anal Canal

The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. Depressions within the canal's wall secrete mucus that facilitates defecation.