Figure 14.7 shows the liver, pancreas, and gallbladder.
Liver cells (hepatocytes) make bile. Bile salts break down
large lipid globules into small ones through emulsification,
the breakdown of large lipid globules into a suspension of
small lipid globules. Bile pigment consists mainly of bili-
rubin, which is derived from the destruction of hemoglobin
from worn-out red blood cells (see Chapter 10).
The gallbladder stores bile and releases it into the
duodenum of the small intestine through the common bile
duct when stimulated by CCK.
Pancreatic acinar cells secrete pancreatic juice,
which is clear, colorless, and consists of water, sodium bicar-
bonate, and various enzymes. The sodium bicarbonate neu-
tralizes stomach acid. The enzymes include the following:
Pancreatic amylase, which breaks down starches.
Trypsin, chymotrypsin, and carboxypeptidase, which
break proteins into amino acids. (Carboxypeptidase is
inactive in the pancreas but is activated by enterokinase
in the duodenum.)
Pancreatic lipase, which breaks triglycerideas into
fatty acids.
Ribonuclease and deoxyribonuclease, which break
down nucleic acids.
Pancreatic juice flows into the duodenum through the
pancreatic duct.
The actions of several digestive enzymes from the stom-
ach, pancreas, and small intestine completely break down
carbohydrates, fats, proteins, and nucleic acids into their
simplest monomers (small building-block molecules). Bile
salts from the liver and gallbladder break down fat droplets
into smaller ones called micelles, much as dishwashing de-
tergent breaks up grease in the dishwater.
As the chyme passes the absorptive cells on the villi,
sugars, amino acids, and lipids are absorbed into the
epithelial cells by diffusion and active and passive trans-
port processes (Figure 14.8). The removal of these ma-
terials from the lumen creates a gradient that allows
water to be absorbed by osmosis. Vitamins are absorbed
by simple diffusion. This can happen directly, as is the
case with water-soluble vitamins (B, C), or indirectly—
along with lipids—for the fat-soluble vitamins (A, D, E,
and K).
T h e liv e r a n d p a n c r e a s s e c r e t e s u b s t a n c e s in t o t h e d u o d e n u m •
F ig u r e 1 4 . 7
Bile is made in the liver, stored in the gallbladder, and subsequently released into
the duodenum through the common duct. Pancreatic juice gets secreted from
exocrine cells of the pancreas called acinar cells. Pancreatic juice flows into the
duodenum through the pancreatic duct.
Duodenum
Key:
Liver
□ Gallbladder
Pancreas
410 CHAPTER 14
The Digestive System, Nutrition, and Metabolism
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