HUMAN BIOLOGY

206 Chapter 11

anal canal Short passage

through which feces move

from the rectum to the anus.

anus Terminal opening of

the GI tract.

appendix Small, slender

pouch off the cecum that

contains lymphocytes.

It doesn’t function in

digestion.

colon Portion of the large

intestine that connects at

its upper end to the small

intestine and at its lower

end to the rectum.

rectum Region of the large

intestine that stores unelimi-

nated feces.

What is the role of the large intestine?

• In the large intestine, water, salts, certain vitamins, and other
  useful substances are reabsorbed from food residues.


• The remaining material is eliminated as feces.

taKe-Home message

Anus

Transverse

colon

External anal sphincter

Internal anal sphincter

Sigmoid colon

Descending

colon

Ileum of small intestine

Cecum

Rectum

Anal

canal

Appendix

Sphincter between small

and large intestines

Ascending
colon

the Large intestine

n Anything not absorbed in the small intestine moves into the

large intestine.

n Link to Osmosis 3.10

The large intestine is about

1.2 meters (5 feet) long. It begins as

a blind pouch called the cecum (see-

cum). The cecum merges with the

colon, which is divided into four

regions in an inverted U-shape. The

ascending colon travels up the right

side of the abdomen, the transverse

colon continues across to the left

side, and the descending colon then

turns downward. The sigmoid colon

makes an S-curve and connects with

the rectum (Figure 11.12A).

Cells in the colon’s lining actively

transport sodium ions out of the

tube. When the ion concentration

there falls, water moves out by

osmosis and returns to the blood-

stream. As water leaves, material left in the colon is gradu-

ally concentrated into feces, a mixture of the remaining

water, undigested and unabsorbed matter, and bacteria. It

is stored and then eliminated. The typical brown color of

feces comes mainly from bile pigments.

Bacteria make up almost a third of the dry weight of

feces. In fact, at least fifty-seven species of bacteria, includ-

ing Escherichia coli, normally inhabit our intestines and

are nourished by the food residues there. Their metabo-

lism produces useful fatty acids and some vitamins (such

as vitamins K and B 12 ). These substances are absorbed

across the colon lining as waves of peristalsis push mate-

rial against its absorptive surface. Feces of humans and

other animals can contain a variety of disease-causing

organisms, too. Health officials use evidence of “coliform

bacteria,” including E. coli, in water and food supplies as a

measure of fecal contamination in general (Figure 11.12B).

You r appendix projects from the cecum like the little

finger of a glove. It doesn’t function in digestion but does

contain patches of lymphoid tissue where B and T cells are

present. These lymphocytes may attack parasites or harm-

ful bacteria consumed in food.

Shortly after you eat, signals from the nervous system

and hormones direct large portions of the ascending and

transverse colon to contract at the same time. Within a

few seconds, residues in the colon may move as much

as three-fourths of the colon’s length and make way for

incoming food. When feces distend the wall of the rectum,

the stretching triggers defecation—elimination of feces

from the body. From the rectum feces move into the anal

canal. The nervous system also controls defecation. It can

stimulate or inhibit contractions of sphincter muscles at the

anus, the terminal opening of the GI tract.

F i g u r e 11.12 in the large intestine feces form and

some substances are reabsorbed. The photograph

shows a scientist obtaining a water sample for testing.

(© Cengage Learning)

a

11.7

B

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