Large intestine

The large intestine, or large bowel, is the final segment of the digestive tract in tetrapods. Its main jobs are to absorb water and salts from the leftover material, then store that waste as feces in the rectum until it is expelled through defecation. Even though it is shorter in length than the small intestine, it gets its name because its diameter is larger.

In humans, the large intestine starts in the right iliac region of the pelvis, near the waist, where it connects to the small intestine at the cecum via the ileocecal valve. From there, it moves upward as the ascending colon, crosses the abdomen as the transverse colon, then goes down to the rectum and ends at the anal canal. The human large intestine is roughly 1.5 meters (5 feet) long, which is about one-fifth of the entire gastrointestinal tract.

In medical practice, the terms "large intestine" and "colon" are often used as synonyms, but most sources define the large intestine as including the cecum, colon, rectum, and anal canal. Some sources leave out the anal canal.

**Structure**

The colon is the last part of the digestive system. It has a segmented look because of pouch-like sacs called haustra. It extracts water and salt from solid waste before elimination and is where gut microbiota ferment unabsorbed material. Unlike the small intestine, the colon does not play a major role in absorbing food and nutrients. About 1.5 liters of water enter the colon each day. The colon is the longest part of the large intestine, averaging 166 cm (range 80–313 cm) in adult males and 155 cm (range 80–214 cm) in adult females.

**Sections**

In mammals, the large intestine is made up of the cecum (including the appendix), the colon (the longest part), the rectum, and the anal canal. The colon itself has four sections: ascending, transverse, descending, and sigmoid. These sections turn at the colic flexures. Some parts of the colon are intraperitoneal (fully wrapped in peritoneum and mobile), while others are retroperitoneal (behind the peritoneum and fixed in place). Specifically, the ascending colon, descending colon, and rectum are retroperitoneal, while the cecum, appendix, transverse colon, and sigmoid colon are intraperitoneal. This distinction matters for surgery, such as during a laparotomy. In terms of diameter, the cecum is the widest, averaging under 9 cm in healthy people, while the transverse colon averages under 6 cm. The descending and sigmoid colon are slightly smaller, with the sigmoid averaging 4–5 cm. Diameters above certain thresholds for each section can indicate megacolon.

**Cecum and Appendix**

The cecum is the first part of the large intestine and is involved in digestion. The appendix, which develops from the cecum during embryonic growth, is not involved in digestion and is considered part of the gut-associated lymphoid tissue. Its function is uncertain, but some sources think it houses a sample of gut microbiota and can help repopulate the colon if microbiota are depleted during an immune response. The appendix also has a high concentration of lymphatic cells.

**Ascending Colon**

The ascending colon is the first of the four main colon sections. It connects to the small intestine through the cecum and runs upward through the abdominal cavity toward the transverse colon for about 8 inches (20 cm). One of the colon's main jobs is to remove water and key nutrients from waste and recycle them. As waste leaves the small intestine through the ileocecal valve, it moves into the cecum and then the ascending colon, where this extraction process begins. Peristalsis pumps the waste upward toward the transverse colon. The ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants, it is called the spiral colon.

**Transverse Colon**

The transverse colon runs from the hepatic flexure (the turn near the liver) to the splenic flexure (the turn near the spleen). It hangs off the stomach, attached by a large peritoneal fold called the greater omentum. On its posterior side, it is connected to the posterior abdominal wall by the transverse mesocolon. Because it is encased in peritoneum, the transverse colon is mobile, unlike the sections immediately before and after it. The proximal two-thirds of the transverse colon get blood from the middle colic artery (a branch of the superior mesenteric artery), while the last third is supplied by branches of the inferior mesenteric artery. The "watershed" area between these two blood supplies, which marks the embryologic division between midgut and hindgut, is vulnerable to ischemia.

**Descending Colon**

The descending colon runs from the splenic flexure to the start of the sigmoid colon. One of its functions is to store feces that will later be emptied into the rectum. In two-thirds of humans, it is retroperitoneal; in the other third, it has a usually short mesentery. Its arterial supply comes via the inferior mesenteric artery.