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The pancreas is a
mixed exocrine-endocrine gland that extends transversely in the upper
abdomen and is cradled between the loop of the duodenum and hilum of
the spleen.
It is
retroperitoneal, behind the lesser omental sac and the stomach, a
location that renders it largely inaccessible to physical examination
and other modalities of direct clinical assessment.
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The adult
pancreas is 10 to 15 cm long and weighs from 60 to 150 gm.
It is divided
into three subdivisions :
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- The head, which lies in the concavity of the
duodenum and extends to the superior mesenteric vessels immediately
behind the organ;
- The body, which includes most of the gland; and
- A
tapered tail, which ends at the hilum of the spleen.
The secretions of
the exocrine pancreas drain via the duct of Wirsung, which begins by
the convergence of several small ducts in the tail and extends into
the head, collecting secretions from ductal tributaries along the way.
It then turns
downward and backward, where it empties into the duodenum at the
ampulla of Vater.
Occasionally, in
addition to the major duct, an accessory duct of Santorini represents
the duct of the embryonic ventral pancreas.
The major
pancreatic duct may enter the duodenum directly or, more commonly,
drain into the common bile duct immediately proximal to the ampulla of
Vater.
The common
channel that carries bile and pancreatic secretions is invested with a
circular complex of smooth muscle fibers, which condense as they pass
through the duodenal wall into the sphincter of Oddi.
Exocrine tissue,
comprising 80 to 85% of the pancreas, consists of secretory cells
organized in acini that connect with ductules.
These in turn
merge into small ducts that empty into medium and large ducts, and
finally form the main pancreatic duct.
Acinar
cells
synthesize
a wide range of digestive enzymes, which
are secreted into the intestine following both neural and hormonal
stimulation.
Stimulation of
the vagus nerves increases the flow of pancreatic juice.
Amino acids and a
duodenal - jejunal pH of less than 3 trigger the release of the
polypeptide hormone cholecystokinin, and antral distension stimulates
that of secretin.
Cholecystokinin
and secretin bind to surface receptors on acinar and duct cells,
respectively, stimulating the secretion of digestive enzymes from acinar cells and of bicarbonate ions and water from the duct cells.
Bicarbonate ions
serve to neutralize the highly acidic gastric chyle in the intestine
and to achieve an optimum pH for the function of pancreatic digestive
enzymes.
The daily
secretion of about 1.5 to 2 liters of pancreatic juice attests to the
remarkable synthetic and secretory capacity of acinar cells and the
transport of ions and water by ductal cells.
The endocrine
pancreas consists of cells organized into islets that are distributed
throughout the organ.
These endocrine
islets comprise only about 2% of the total pancreas.
Islets contain
several cell types, each of which synthesizes one or more hormones,
including insulin and glucagons, among others.
Histology Images
Following the
proper stimulus the hormones are secreted directly into the blood.
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Major Anatomic Landmarks:
The
pancreas and first three parts of the duodenum are retroperitoneal
structures and therefore their posterior relation is the posterior
abdominal wall and their relation is the peritoneal surface lining
the lesser sac. The superior mesenteric artery and vein lie within
a groove between the head and uncinate process of the pancreas.The
lower end of the common bile duct enters the superior margin of
the head of the pancreas inferior to the distal greater gastric
curvature and is usually joined by the main pancreatic duct
shortly before draining via the ampulla of Vater, at the junction
of the second and third parts of the duodenum. The pancreatic duct
may drain separately into the duodenum via an accessory duct.
Lymphatic drainage occurs via several regional lymph node groups. |
Visit:
Pancreatic Pathology Online
;
Normal Islets of Langerhans
; The Apud Concept.
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