673
CHAPTER SEVENTEEN
Digestive System
Bloodstream
Acidic chyme
enters
duodenum
1
Bicarbonate ions
neutralize acidic
chyme
5
Pancreatic juice rich
in bicarbonate ions
passes down
pancreatic ducts
to the duodenum
4
Secretin stimulates
pancreas to secrete
bicarbonate ions
3
Intestinal mucosa
releases secretin
into bloodstream
2
Hormonal signals
released into bloodstream
Stimulation of effector organ
17.8
LIVER
The
liver,
the largest internal organ, is located in the upper
right quadrant of the abdominal cavity, just inferior to the
diaphragm. It is partially surrounded by the ribs and extends
from the level of the F
fth intercostal space to the lower mar-
gin of the ribs. It is reddish brown in color and well supplied
with blood vessels (
f
gs. 17.25, 17.26, 17.27
and reference
plates 8, 17, 24).
Liver Structure
A F brous capsule encloses the liver, and connective tissue
divides the organ into a large
right lobe
and a smaller
left
lobe.
The
falciform ligament
is a fold of visceral peritoneum
that separates the lobes and fastens the liver to the abdomi-
nal wall anteriorly. The liver also has two minor lobes, the
quadrate lobe,
near the gallbladder, and the
caudate lobe,
close to the vena cava (see F
g. 17.26). The area where the
four lobes meet and blood vessels and ducts enter or exit the
liver is the
porta hepatis.
A fold of visceral peritoneum called the
coronary ligament
attaches the liver to the diaphragm on its superior surface. Each
lobe is separated into many tiny
hepatic lobules,
the liver’s
functional units (F g. 17.27). A lobule consists of many
hepatic
FIGURE 17.24
Acidic chyme entering the duodenum from the stomach stimulates the release of secretin, which, in turn, stimulates the release of
pancreatic juice.
cells
radiating outward from a
central vein.
Vascular channels
called
hepatic sinusoids
separate platelike groups of these
cells from each other. Blood from the digestive tract, carried
in the
hepatic portal vein
(see chapter 15, pp. 603–604), brings
newly absorbed nutrients into the sinusoids
(f g. 17.28)
. At the
same time, oxygenated blood from the hepatic artery mixes
freely with the blood containing nutrients, then fl ows through
the liver sinusoids and nourishes the hepatic cells.
Often blood in the portal vein contains some bacte-
ria that have entered through the intestinal wall. However,
large
Kupffer cells,
F
xed to the inner lining (endothelium)
of the hepatic sinusoids, remove most of the bacteria from
the blood by phagocytosis. Then the blood passes into the
central veins
of the hepatic lobules and exits the liver via the
hepatic vein.
Within the hepatic lobules are many F ne
bile canaliculi,
which carry secretions from hepatic cells to
bile ductules.
The
ductules of neighboring lobules unite to form larger bile ducts,
which then converge to become the
hepatic ducts.
These
ducts merge, in turn, to form the
common hepatic duct.
Liver Functions
The liver carries on many important metabolic activities. ±rom
Science to Technology 17.1 discusses a bioengineered liver.
Recall from chapter 13 (pp. 509–511) that the liver plays a key
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