878
UNIT 4
Maintenance of the Body
23
reach their destinations. Te gallbladder rests in a recess on the
inferior surface of the right liver lobe (Figure 23.24b).
Te traditional scheme of defining liver lobes (outlined
above) has been criticized because it is based on superficial fea-
tures of the liver. Hepatic surgeons prefer to discuss the liver in
terms of eight segments based on its internal anatomy relative
to its vascular and biliary supply. Tis system permits them to
remove sections while encountering the fewest major vascular
structures and the lowest risk.
Bile leaves the liver through several bile ducts that ultimately
fuse to form the large
common hepatic duct
, which travels
downward toward the duodenum. Along its course, that duct
fuses with the
cystic duct
draining the gallbladder to form the
bile duct
(see Figure 23.21).
Microscopic Anatomy of the Liver
Te liver is composed of sesame seed–sized structural and
functional units called
liver lobules
. Each lobule is a roughly
hexagonal (six-sided) structure consisting of plates of
liver cells
,
or
hepatocytes
(hep
9
ah-to-sīts), organized like bricks in a gar-
den wall
(Figure 23.25)
. Te hepatocyte plates radiate outward
from a
central vein
running in the longitudinal axis of the lob-
ule. ±o make a rough model of a liver lobule, open a thick pa-
perback book until its two covers meet: Te pages represent the
plates of hepatocytes and the hollow cylinder formed by the
rolled spine represents the central vein.
If you keep in mind that the liver’s main function is to filter
and process the nutrient-rich blood delivered to it, its histology
makes a lot of sense. At each of the six corners of a lobule is a
portal triad
(
portal tract
region), so named because it contains
three basic structures (Figure 23.25c):
A branch of the
hepatic artery
(supplying oxygen-rich arte-
rial blood to the liver)
A branch of the
hepatic portal
vein (carrying venous blood
laden with nutrients from the digestive viscera)
A
bile duct
Between the hepatocyte plates are enlarged, heavily fenestrated
liver sinusoids
. Blood from both the hepatic portal vein and the
hepatic artery percolates from the triad regions through these si-
nusoids and empties into the central vein. From the central veins
blood eventually enters the hepatic veins, which drain the liver,
and empties into the inferior vena cava. Forming part of the sinu-
soid walls are star-shaped
stellate
macrophages
, also called
he-
patic macrophages
(Figure 23.25c). Tey remove debris such as
bacteria and worn-out blood cells from the blood as it flows past.
Te versatile hepatocytes have large amounts of both rough
and smooth ER, Golgi apparatus, peroxisomes, and mitochon-
dria. Equipped in this way, the hepatocytes can secrete or gener-
ate some 900 ml of bile daily. Te hepatocytes also:
Process bloodborne nutrients in various ways (e.g., they store
glucose as glycogen and use amino acids to make plasma
proteins)
Store fat-soluble vitamins
Play important roles in detoxification, such as ridding the
blood of ammonia by converting it to urea (Chapter 24)
Check Your Understanding
30.
What common advantage do circular folds, villi, and
microvilli provide to the digestive process? Which of these
modifications causes chyme to spiral through the lumen and
slows its passage?
31.
What are brush border enzymes?
32.
What is a lacteal and what is its function?
33.
Name three secretory products that help protect the
intestinal mucosa from bacterial damage.
For answers, see Appendix H.
The Liver and Gallbladder
Describe the histologic anatomy of the liver.
State the role of bile in digestion.
Describe the role of the gallbladder.
Te
liver
and
gallbladder
are accessory organs associated with
the small intestine. Te liver, one of the body’s most important
organs, has many metabolic and regulatory roles. However, its
digestive
system function is to produce bile for export to the
duodenum. Bile is a fat emulsifier that breaks down fats into
tiny particles to make them more readily digestible. We will de-
scribe this process when we discuss the digestion and absorp-
tion of fats later in the chapter. Although the liver also processes
nutrient-laden venous blood delivered to it from the digestive
organs, this is a metabolic rather than a digestive role (see Chap-
ter 24.) Te gallbladder is chiefly a storage organ for bile.
Gross Anatomy of the Liver
Te ruddy, blood-rich
liver
is the largest gland in the body,
weighing about 1.4 kg (3 lb) in the average adult. Shaped like a
wedge, it occupies most of the right hypochondriac and epigas-
tric regions (see Figure 1.12), extending farther to the right of
the body midline than to the le². Located under the diaphragm,
the liver lies almost entirely within the rib cage, which provides
some protection (see Figure 23.1 and
Figure 23.24
).
Te liver has four primary lobes. Te largest, the
right lobe
,
is visible on all liver surfaces and separated from the smaller
lef lobe
by a deep fissure (Figure 23.24a). Te posteriormost
caudate lobe
and the
quadrate lobe
, which lies inferior to the le²
lobe, are visible in an inferior view of the liver (Figure 23.24b).
A mesentery, the
falciform ligament
, separates the right and le²
lobes anteriorly and suspends the liver from the diaphragm and an-
terior abdominal wall. Running along the inferior edge of the falci-
form ligament is the
round ligament
, or
ligamentum teres
(te
9
rēz;
“round”), a fibrous remnant of the fetal umbilical vein. Except for
the superiormost liver area (the
bare area
), which touches the dia-
phragm, the entire liver is enclosed by the visceral peritoneum.
As we mentioned earlier, a ventral mesentery, the lesser
omentum, anchors the liver to the lesser curvature of the stom-
ach (see Figure 23.30b). Te
hepatic artery
and the
hepatic
portal vein
, which enter the liver at the
porta hepatis
(“gateway
to the liver”), and the common hepatic duct, which runs infe-
riorly from the liver, all travel through the lesser omentum to
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