675
CHAPTER SEVENTEEN
Digestive System
role in digestion is to secrete bile.
Table 17.7
summarizes
the major functions of the liver. Clinical Application 17.3 dis-
cusses hepatitis, an infl
ammation of the liver.
PRACTICE
36
Locate the liver.
37
Describe a hepatic lobule.
38
Review liver functions.
39
How does the liver aid in digestion?
Composition of Bile
Bile
(bı¯l) is a yellowish-green liquid that hepatic cells con-
tinuously secrete. In addition to water, it contains
bile salts,
bile pigments, cholesterol,
and
electrolytes.
Of these, bile salts
are the most abundant. They are the only bile substances
that have a digestive function.
Bacteria in the intestine produce ammonia, which is carried in the
blood to the liver, where it reacts to yield urea. When this liver func-
tion fails, concentration of blood ammonia sharply rises, causing
hepatic coma,
a condition that can lead to death.
The liver also stores many substances, including glyco-
gen, iron, and vitamins A, D, and B
12
. Extra iron from the
blood combines with a protein (apoferritin) in liver cells,
forming
ferritin.
The iron is stored in this form until blood
iron concentration falls, when some of the iron is released.
Thus, the liver is important in iron homeostasis.
Liver cells help destroy damaged red blood cells and
phagocytize foreign antigens. The liver removes toxic sub-
stances such as alcohol and certain other drugs from the
blood (detoxiF cation). The liver can also serve as a blood
reservoir, storing 200 to 400 milliliters of blood. The liver’s
Bile duct
Branch of
hepatic portal vein
Branch of hepatic artery
Plates of liver cells
Central vein
Branch of
hepatic portal vein
Bile canaliculi
Bile ductule
Bile duct
Branches of
hepatic artery
Sinusoids
(b)
Hepatic
sinusoids
(a)
(c)
FIGURE 17.27
Hepatic lobule. (
a
) Cross section of a hepatic
lobule. (
b
) Enlarged longitudinal section of a hepatic lobule. (
c
) Light
micrograph of hepatic lobules in cross section (150×).
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