Large Intestine
Objective 6
Indicate the location of the following regions of the large intestine: cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus. Identify and describe the function of the ileocecal sphincter. Describe the histology and function of the internal and external sphincters of the anus. Explain how rectal distension triggers the defecation reflex. Describe the function of the sphincters. Explain the conscious control of defecation. Define and describe the Valsalva maneuver, including its effects on the GI and cardiovascular systems.
The large intestine is composed of four parts: cecum, colon, rectum, and anus/anal canal.
The large intestine begins where the ileum (the final portion of the small intestine) has an outlet through the ileocecal sphincter into the cecum (Latin cæcus, “blind pouch”). The cecum is a common site for GI problems. Not only does it tend to catch objects that have passed into the large intestine, but it ends in the vermiform appendix (or just appendix), a small wormlike tube that often becomes inflamed (appendicitis). It may even burst, dumping the bacteria-laden intestinal contents into the peritoneal cavity causing peritonitis, a medical emergency. It has recently been proposed that the appendix acts as a reservoir for “friendly” gut bacteria so that their colonies can be replaced after antibiotic treatment (Bollinger RR. J Theoret Biol 249: 826-831, 2007).
The large intestine (colon) continues upward as the ascending colon; crosses from right to left as the transverse colon; and drops down the left flank as the descending colon. At this point, it makes an S-shaped curve (sigmoid colon, from the Greek letter Σ or Roman S). Finally, it attaches to the anus through a straight segment, the rectum (Latin rectus, “straight”).
The taeniae coli are 3 longitudinal ribbons of smooth muscle that run along the ascending, transverse, descending, and sigmoid colon. The tonic contraction of these muscles form haustra which are small pouches that give the colon its segmented appearance.
Like the small intestine, the histology of the large intestine follows the “standard” GI tract pattern very closely. In this case, the absorptive surface is limited to absorption of (mostly) water.
Also like the small intestine, the colon has a velvety appearance with numerous microvilli. There are openings for intestinal glands (crypts of Lieberkühn).
While the stomach and small intestine had a large number of cell types, in the colon, only two cell types are present: mucus-secreting goblet cells and water-absorbing absorptive cells. It is the absorptive cells that have microvilli on their surface.
Rather than secrete digestive enzymes, the large intestine mostly relies on bacteria to do its work. It is estimated that the human GI tract harbors 1014 living bacteria; of these, Escherichia coli (E. coli) is the best-known and best-studied. Bacteriodes fragilis, B. melaninogenicus, B. oralis, and