CHAPTER 16
Digestive Glands and Associated Organs
305
Figure 16-12A
Liver Acinus
Figure 16-12B
Portal Triad, Liver
Figure 16-12C
Clinical Correlation: Alcoholic Fatty Liver (Steatosis)
Figure 16-13A
Hepatocytes and Hepatic Sinusoids, Liver
Figure 16-13B
Space of Disse, Hepatocyte
Figure 16-14A
A Representation of Bile Canaliculi and Hepatocytes
Figure 16-14B
Bile Canaliculus, Hepatocytes
Gallbladder
Figure 16-15A
Gallbladder
Figure 16-15B
Epithelial Cells Lining the Gallbladder
Figure 16-16A
Clinical Correlation: Hepatitis C
Figure 16-16B
Clinical Correlation: Gallstones
Synopsis 16-2
Pathological and Clinical Terms for the Digestive Glands and Associated Organs
Introduction and Key Concepts
for the Digestive Glands and Associated
Organs
Digestive glands
and
associated organs
include the
major
salivary glands
,
pancreas
,
liver
, and
gallbladder
. These organs
are located outside the wall of the digestive tract. Their secretory
products are delivered into the digestive tract via a duct
system.
Major Salivary Glands
The
major salivary glands
produce saliva and empty into the
oral cavity. Saliva is 99% water and contains protein, enzymes,
glucose, cholesterol, urea, uric acid, ions (e.g., Na
+
, K
+
, Ca
++
,
HCO
3
-
), and antibacterial agents (lactoferrin, lysozyme, and
IgA). Saliva is produced by both serous and mucous cells in
the salivary glands. It plays important roles in aiding digestion,
lubrication, protection, buffering, wound healing, maintaining
the integrity of the esophagogastric epithelium, perception of
taste, and in hardening of the enamel of the teeth. There are
three major salivary glands: the
parotid
,
submandibular
, and
sublingual glands
. These are paired glands and have similar
structures of secretory units (acini) and duct systems, including
intralobular ducts (intercalated and striated ducts), interlobular
ducts, lobar ducts, and a main duct (Figs. 16-1 and 16-8B).
1.
The parotid gland
is the largest of the three major salivary
glands. This gland is surrounded by a connective tissue
capsule. Its secretory unit is composed of only serous cells,
which produce watery proteinaceous fl uid (Figs. 16-3B and
16-4A).
2.
The submandibular gland
is the second largest salivary
gland. This gland is surrounded by a connective tissue cap-
sule. It is a mixed gland, although the majority of cells are
serous cells. The secretory unit is composed of both serous
and mucous cells.
Serous demilunes
(serous cell caps on the
mucous cells) are present in the submandibular gland (Figs.
16-5A to 16-7B).
3.
The sublingual gland
is the smallest salivary gland. This
gland is not surrounded by a connective tissue capsule. This
is also a mixed gland, but is predominately mucous. Acini
that are completely serous are few, but serous demilunes are
commonly present. Striated ducts are not as obvious as in
the other two types of salivary glands (Fig. 16-8A,B).
Pancreas
The
pancreas
has
endocrine
and
exocrine
portions
. The
endocrine portion
(
islets of Langerhans
) secretes blood glucose–
regulating hormones (
insulin
,
glucagon
,
somatostatin
, and
pan-
creatic polypeptide
), which are released into the bloodstream
(see Chapter 17, “Endocrine System”). The
exocrine portion
produces
pancreatic secretions (juice)
, which are carried by
pancreatic ducts
. Most of these secretions go to the
main pan-
creatic duct
, which joins the
hepatopancreatic ampulla (ampulla
of Vater)
and then enters the duodenum through the
major duo-
denal papilla (papilla of Vater)
. The ampulla is surrounded by
smooth muscle called the
sphincter of the ampulla
(
sphincter
of Oddi
[Fig. 16-9A]). A small portion of the pancreatic secre-
tion is carried by the accessory pancreatic duct and enters the
duodenum through the minor duodenal papilla. The pancreatic
duct system includes intralobular ducts, interlobular ducts, and
a main duct. The pancreas does not have striated ducts, and
the smallest intralobular ducts are intercalated ducts. The initial
portions of the intercalated ducts are lined by
centroacinar cells
.
The initial pancreatic secretions (enzymes) are produced by
pan-
creatic acinar cells
, and a large volume of fl uid (water, sodium,
and bicarbonate) is added by intercalated duct cells. Pancreatic
secretions contain enzymes for digesting proteins, carbohy-
drates, and fats. The major components include
water
,
sodium
,
bicarbonate ions
,
trypsinogen
,
chymotrypsinogen
,
procarboxy-
polypeptidase
,
amylase
,
lipase
,
cholesterol esterase
,
phospho-
lipase
, and
nucleases
. The main enzymes for digesting
protein
are trypsinogen, chymotrypsinogen, and procarboxypolypepti-
dase; the main enzyme for
digesting
carbohydrates
is amylase;
and the main enzymes for digesting
fat
are lipase, cholesterol
esterase, and phospholipase. The nucleases degrade nucleic
acids. When the pancreatic digestive enzymes are ± rst synthe-
sized by the pancreatic cells, they are in an inactive stage. They
become activated after they enter the duodenum and come into
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