882
UNIT 4
Maintenance of the Body
23
to expel bile into the
cystic duct
. From there bile flows into the
bile duct. Te gallbladder, like most of the liver, is covered by
visceral peritoneum.
Homeostatic Imbalance
23.11
Bile is the major vehicle for excreting cholesterol from the body,
and bile salts keep the cholesterol dissolved within bile. ±oo much
cholesterol or too few bile salts allows the cholesterol to crystal-
lize, forming
gallstones
or
biliary calculi
(bil
9
e-a
0
re kal
9
ku-li),
which obstruct the flow of bile from the gallbladder. When the
gallbladder or its duct contracts, the sharp crystals cause agoniz-
ing pain that radiates to the right thoracic region.
Gallstones are easy to diagnose because they show up well
with ultrasound imaging. ±reatments for gallstones include
dissolving the crystals with drugs, pulverizing them with ul-
trasound vibrations (lithotripsy), vaporizing them with lasers,
and the classical treatment, surgically removing the gallbladder.
When the gallbladder is removed, the bile duct enlarges to as-
sume the bile-storing role.
Bile duct blockage prevents both bile salts and bile pigments
from entering the intestine. As a result, yellow bile pigments
accumulate in blood and eventually are deposited in the skin,
causing it to become yellow, or
jaundiced
. Jaundice caused by
blocked ducts is called
obstructive jaundice
, but jaundice may
also reflect liver disease (in which the liver is unable to carry out
its normal metabolic duties).
Check Your Understanding
34.
What is a portal triad?
35.
What is the importance of the enterohepatic circulation?
36.
What is the role of the stellate macrophages of the liver?
For answers, see Appendix H.
The Pancreas
State the role of pancreatic juice in digestion.
Te
pancreas
(pan
9
kre-as;
pan
5
all,
creas
5
flesh, meat) is
a so², tadpole-shaped gland that extends across the abdomen
from its
tail
(abutting the spleen) to its
head
, which is encircled
by the C-shaped duodenum (see Figures 23.1 and 23.21). Most
of the pancreas is retroperitoneal and lies deep to the greater
curvature of the stomach.
An accessory digestive organ, the pancreas is important to
the digestive process because it produces enzymes that break
down all categories of foodstuffs. Tis exocrine product, called
pancreatic juice
, drains from the pancreas into the duodendum
via the centrally located
main pancreatic duct
. Te pancreatic
duct generally fuses with the bile duct just as it enters the duo-
denum (at the hepatopancreatic ampulla). A smaller
accessory
pancreatic duct
empties directly into the duodenum just proxi-
mal to the main duct.
Within the pancreas are the
acini
(as
9
ĭ-ni; singular: acinus), clus-
ters of secretory acinar cells surrounding ducts
(Figure 23.26a)
.
Tese cells are full of rough endoplasmic reticulum and exhibit
deeply staining
zymogen granules
(zi
9
mo-jen; “fermenting”)
products,
stercobilin
(ster
9
ko-bi
0
lin), gives feces a brown color. In
the absence of bile, feces are gray-white and have fatty streaks be-
cause essentially no fats are digested or absorbed.
The Gallbladder
Te gallbladder is a thin-walled green muscular sac about 10 cm
(4 inches) long. Te size of a kiwi fruit, it snuggles in a shallow
fossa on the inferior surface of the liver (see Figures 23.1 and
23.24) from which its rounded fundus protrudes.
Te gallbladder stores bile that is not immediately needed
for digestion and concentrates it by absorbing some of its water
and ions. When empty, its mucosa is thrown into honeycomb-
like folds (see Figure 23.21) that, like the rugae of the stomach,
allow the organ to expand as it fills. Its muscular wall contracts
Small
duct
Acinar cell
Basement
membrane
Duct cell
One acinus
Zymogen
granules
Rough
endoplasmic
reticulum
(a)
(b)
Acinar cells
Pancreatic
duct
Figure 23.26
Structure of the enzyme-producing tissue of
the pancreas.
(a)
Schematic view of one acinus (a secretory unit).
The acinar cells contain abundant zymogen (enzyme-containing)
granules and dark-staining rough ER (typical of gland cells producing
large amounts of protein for export).
(b)
Photomicrograph of
pancreatic acinar tissue (155
3
).
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