Human Physiology, 14th edition (2016)

The Digestive System 633

Intestinal Microbiota

Microorganisms, primarily bacteria, are present in relatively

small numbers in the stomach and proximal portion of the small

intestine. Their numbers increase in the distal ileum and are

greatest in the colon, where an estimated 10^14 reside. This enor-

mous number is about 10 times more than the number of human

cells in the body and represents several thousand different spe-

cies with perhaps 100 times more genes than in human cells.

These microorganisms are known collectively as the intestinal

microbiota or microflora. In the colon, the intestinal micro-

biota are comprised mostly of anaerobic bacterial species.

The large intestine, or colon, extends from the ileoce-
cal valve to the anus, framing the small intestine on three
sides. Chyme from the ileum passes into the cecum, which is
a blind pouch (open only at one end) at the beginning of the
large intestine. Waste material then passes in sequence through
the ascending colon, transverse colon, descending colon,
sigmoid colon, rectum, and anal canal ( fig.  18.16 ). Waste
material (feces) is excreted through the anus, the external open-
ing of the anal canal.
The mucosa of the large intestine, like that of the small
intestine, contains many scattered lymphocytes and lymphatic
nodules and is covered by columnar epithelial cells and mucus-
secreting goblet cells. Although this epithelium does form crypts
( fig. 18.17 ), there are no villi in the large intestine— the intes-
tinal mucosa therefore appears flat. The outer surface of the
colon bulges outward to form pouches, or haustra ( figs. 18.16
and 18.18 ). Occasionally, the muscularis externa of the haustra
may become so weakened that the wall forms a more elongated
outpouching, or diverticulum ( divert 5 turned aside). Inflam-
mation of one or more of these structures is called diverticulitis.
The large intestine has little or no digestive function, but it does
absorb water and electrolytes from the remaining chyme, as well
as several B complex vitamins and vitamin K.

CLINICAL APPLICATION

Appendicitis is inflammation of the vermiform (Latin for

worm-shaped) appendix, a thin, blind-ended outpouching

from the cecum. The appendix does not function in digestion,

but contains numerous lymphatic nodules (see fig. 8.17) and

may house a population of commensal bacteria that helps to

replenish a normal intestinal microbiota (discussed next) when

needed. Appendicitis is a medical emergency that produces

pain in the lower right abdominal quadrant, nausea, and other

symptoms that sometimes confuse the diagnosis. A burst

appendix spreads the inflammation to the surrounding perito-

neal membranes, a condition called peritonitis that can pro-

duce circulatory shock and death. This can be prevented by

an appendectomy, the surgical removal of the appendix.

Transverse colon

Ascending
colon

Ileocecal
valve

Cecum
Appendix

Ileum

Haustrum

Sigmoid colon

Descending

colon

Anal canal

Rectum

Figure 18.16 The large intestine. The different
regions of the large intestine (colon) are illustrated.

Figure 18.17 A photomicrograph of the human

appendix. This cross section reveals numerous lymphatic

nodules, which function in immunity.

Lymphatic

nodules

Muscularis

externa

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Describe the structure and functions of the large
   intestine.


9. Explain the nature and significance of the intestinal
   microbiota.