The large intestine is the final section of the gastrointestinal tract that performs the vital task of absorbing water and vitamins while converting digested food into feces. Although shorter than the small intestine in length, the large intestine is considerably thicker in diameter, thus giving it its name. The large intestine is about 5 feet (1.5 m) in length and 2.5 inches (6-7 cm) in diameter in the living body, but becomes much larger postmortem as the smooth muscle tissue of the intestinal wall relaxes.
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. Beginning on the right side of the abdomen, the large intestine is connected to the ilium of the small intestine via the ileocecal sphincter. From the ileocecal sphincter, the large intestine forms a sideways “T,” extending both superiorly and inferiorly. The inferior region of the large intestine forms a short dead-end segment known as the cecum that terminates in the vermiform appendix. The superior region forms a hollow tube known as the ascending colon that climbs along the right side of the abdomen. Just inferior to the diaphragm, the ascending colon turns about 90 degrees toward the middle of the body at the hepatic flexure and continues across the abdomen as the transverse colon. At the left side of the abdomen, the transverse colon turns about 90 degrees at the splenic flexure and runs down the left side of the abdomen as the descending colon. At the end of the descending colon, the large intestine bends slightly medially at the sigmoid flexure to form the S-shaped sigmoid colon before straightening into the rectum. The rectum is the enlarged final segment of the large intestine that terminates at the anus.
Like the rest of the gastrointestinal canal, the large intestine is made of four tissue layers:
- The innermost layer, known as the mucosa, is made of simple columnar epithelial tissue. The mucosa of the large intestine is smooth, lacking the villi found in the small intestine. Many mucous glands secrete mucus into the hollow lumen of the large intestine to lubricate its surface and protect it from rough food particles.
- Surrounding the mucosa is a layer of blood vessels, nerves and connective tissue known as the submucosa, which supports the other layers of the large intestine.
- The muscularis layer surrounds the submucosa and contains many layers of visceral muscle cells that contract and move the large intestine. Continuous contraction of smooth muscle bands in the muscularis produces lumpy, pouch-like structures known as haustra in the large intestine.
- Finally, the serosa forms the outermost layer. The serosa is a thin layer of simple squamous epithelial tissue that secretes watery serous fluid to lubricate the surface of the large intestine, protecting it from friction between abdominal organs and the surrounding muscles and bones of the lower torso.
The large intestine performs the vital functions of converting food into feces, absorbing essential vitamins produced by gut bacteria, and reclaiming water from feces. A slurry of digested food, known as chyme, enters the large intestine from the small intestine via the ileocecal sphincter. Chyme passes through the cecum where it is mixed with beneficial bacteria that have colonized the large intestine throughout a person’s lifetime. The chyme is then slowly moved from one haustra to the next through the four regions of the colon. Most of the movement of chyme is achieved by slow waves of peristalsis over a period of several hours, but the colon can also be emptied quickly by stronger waves of mass peristalsis following a large meal.
While chyme moves through the large intestine, bacteria digest substances in the chyme that are not digestible by the human digestive system. Bacterial fermentation converts the chyme into feces and releases vitamins including vitamins K, B1, B2, B6, B12, and biotin. Vitamin K is almost exclusively produced by the gut bacteria and is essential in the proper clotting of blood. Gases such as carbon dioxide and methane are also produced as a byproduct of bacterial fermentation and lead to flatulence, or gas passed through the anus.
The absorption of water by the large intestine not only helps to condense and solidify feces, but also allows the body to retain water to be used in other metabolic processes. Ions and nutrients released by gut bacteria and dissolved in water are also absorbed in the large intestine and used by the body for metabolism. The dried, condensed fecal matter is finally stored in the rectum and sigmoid colon until it can be eliminated from the body through the process of defecation.