The small intestine joins with the large intestine at the ileal orifice. This opening is closed most of the time but opens to allow chyme to enter the large intestine.
The large intestine gets its name because its diameter (6.5 cm; 2.5 in) is larger than that of the small intestine, although its length (1.5 m; 5 ft) is much shorter. The large intestine consists of four segments: cecum, colon, rectum, and anal canal.
The first portion of the large intestine is the pouchlike cecum, which bulges inferior to the ileal orifice. The slender, wormlike appendix extends from the cecum and, although it has no digestive function, it contributes to the immune defense of the body.
The colon forms most of the large intestine and is subdivided into four segments. The ascending colon extends superiorly from the cecum along the right side of the abdominopelvic cavity. As it nears the liver, it turns left to become the transverse colon. Near the spleen, the transverse colon turns inferiorly to become the descending colon along the left side of the abdominopelvic cavity. Near the pelvis, the descending colon becomes the sigmoid colon, which is characterized by an S-shaped curvature leading to the rectum.
The rectum is the straight portion of the large intestine that continues inferiorly from the sigmoid colon through the pelvic cavity and ends at the anal canal. The anal canal is the last 3 cm of the large intestine and its external opening is the anus. The mucosa of the anal canal is folded to form the anal columns, which contain networks of arteries and veins. The anus is kept closed except during defecation by the involuntarily controlled internal anal sphincter and the voluntarily controlled external anal sphincter.
The colon has a puckered appearance when viewed externally. This results because the longitudinal muscles are not uniformly layered but are reduced to three longitudinal bands, the taeniae coli, that run the length of the colon. Contraction of the taeniae coli gathers the colon into a series of pouches called haustra. Like the small intestine, the large intestine is supported by a mesentery.
The mucosa of the large intestine is also different from that of the small intestine. Villi are absent, and the simple columnar epithelium contains numerous mucus- producing goblet cells.
Chyme residue entering the large intestine contains water, minerals, bacteria, and other substances that were not digested or absorbed while in the small intestine. There are no digestive enzymes secreted by the large intestine.
Instead, intestinal bacteria decompose the undigested food molecules. This action yields certain B vitamins and vitamin K, in addition to gas (flatus). The mucosa of the large intestine secretes large quantities of mucus that lubricate the intestinal lining and reduce abrasion as materials are moved along.
A major function of the large intestine is the absorption of water, some minerals, and vitamins as the contents slowly move through the colon. Much of this absorption occurs before the chyme reaches the descending colon, where it is congealed to form the feces (fe-sez). Feces contain large amounts of bacteria, mucus, and water as well as undigested food molecules.
Segmentation and peristalsis within the large intestine are more sluggish than those of the small intestine. Vigorous peristalsis occurs only two to four times a day, usually following a meal. These peristaltic contractions are called mass movements because they move the contents of the descending and sigmoid colons toward the rectum. The defecation reflex is activated when the rectum fills with feces and its wall is stretched. Parasympathetic nerve impulses stimulate muscular contractions that increase pressure within the rectum and relax the internal anal sphincter. Defecation, or expulsion of feces, occurs if the external anal sphincter is voluntarily relaxed. If its contraction is voluntarily maintained, defecation is postponed.