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Pancreas
SECTION I
2
GROSS ANATOMY
Surgeons must thoroughly understand the anatomy of the pancreas and its
surrounding structures in order to minimize inadvertent injury and subse-
quent complications. In an adult, the pancreas weighs between 75 and 100 g
and is about 15 to 20 cm long. However, the shape, size, and texture of the
pancreas are quite variable. e pancreas is located in the retroperitoneum
and is obliquely oriented with the tail being more superior than the head.
e pancreas is covered with a ne connective tissue but lacks a true sero-
sal surface or capsule. Pain from inammation of the pancreas is therefore
typically described as being located in the midepigastrium and penetrat-
ing to the back. e inammatory process in acute pancreatitis can spread
inferiorly through the retroperitoneum down the perinephric and paracolic
gutters rather than diusely throughout the peritoneal cavity.
e pancreas is commonly divided into ve regions: the head, unci-
nate process, neck, body, and tail. e head is the thickest part of the pan-
creas and lies to the right of the superior mesenteric vessels. It is attached to
the second and third portions of the duodenum, and the two organs share a
common blood supply from the pancreaticoduodenal arcade. e anterior
surface of the pancreatic head is near the rst portion of the duodenum
and the transverse mesocolon while the posterior surface is close to the
hilum and medial border of the right kidney, right ureter, the inferior vena
cava, right renal vein and artery, and the right crus of the diaphragm. In
the majority of patients, the common bile duct passes through the pancre-
atic head and is covered by varying amounts of parenchyma before joining
with the main duct of Wirsung and emptying into the second portion of the
duodenum.
e uncinate process is an extension of the pancreatic head. It passes
posterior, inferior, and slightly to the left from the head, staying adjacent to
the third and fourth portions of the duodenum. It then continues behind
the superior mesenteric vessels. On a sagittal section, the uncinate process
can be seen between the aorta and the superior mesenteric artery (SMA).
e uncinate process varies in size and shape. It may be absent in some, or
it may completely encircle the superior mesenteric vessels in others. Short
fragile vessels from the SMA and vein supply the uncinate process and must
be divided during resection of the pancreatic head. Pancreatitis or cancer
can make the attachments between the uncinate process and surrounding
mesenteric vessels dicult to separate intraoperatively.
e neck of the pancreas is dened as the portion of the gland located
anterior to the superior mesenteric vein (SMV) and splenoportal conu-
ence. It is the thinnest portion of the gland and is covered anteriorly by
the pylorus. e second lumbar vertebra is just posterior to the neck of
the pancreas, which can be crushed against this bony structure with blunt
Pancreatic Anatomy and
Physiology
Somala Mohammed and William E. Fisher
1
Chapter 1 / Pancreatic Anatomy and Physiology 3
anterior–posterior abdominal trauma. e gastroduodenal artery passes
superiorly to inferiorly to the right of the pancreatic neck and at the infe-
rior margin of the pancreas gives rise to the right gastroepiploic artery and
then terminates as the superior pancreaticoduodenal artery. An ulcer in the
posterior duodenal bulb can erode through the posterior wall of the duode-
num into the gastroduodenal artery, which must sometimes be oversewn
to stop the bleeding. Behind the pancreatic neck at its inferior border, the
SMV joins the splenic vein and then continues toward the porta hepatis as
the portal vein (PV).
Once the gastrocolic omentum is divided, the body and tail of the
pancreas can be seen occupying the oor of the lesser sac, posterior to
the stomach. e celiac axis is located superior to the proximal body of
the pancreas with the hepatic artery coursing to the right, and the splenic
artery coursing to the left along the superior border of the body and tail
of the pancreas. e splenic artery is often tortuous and may pass directly
through the pancreatic parenchyma or even anterior to the pancreatic tail.
e transverse mesocolon attaches to the inferior edge of the body and tail
of the pancreas. e body of the pancreas begins to the left of the superior
mesenteric vessels. is portion of the pancreas measures approximately
4 to 5 cm in width and 1.5 to 2 cm in thickness. It lies anterior to the aorta
at the origin of the SMA and is anterior to the left renal vessels, the left crus
of the diaphragm, and the splenic vein. Resection of the pancreatic tail with
division at the neck is equivalent to a 60% to 70% resection, while division
at the proximal body is equivalent to a 50% to 60% resection.
e tail of the pancreas refers to the portion of the pancreas that is
anterior to the left kidney and left adrenal gland. In about 50% of patients,
the pancreatic tail extends into the hilum of the spleen and can thus be
injured during a splenectomy. e tail of the pancreas is attached to the
splenic exure of the colon. erefore, the pancreas can also be injured
d
uring a left colectomy.
EMBRYOLOGY AND PANCREATIC DUCT ANATOMY
e pancreas is derived as an outpouching of the primitive foregut endo-
derm and is formed by the fusion of a ventral and dorsal pancreatic bud.
e ventral bud develops into the inferior portion of the head and the unci-
nate process while the dorsal bud develops into the body and tail. e ven-
tral bud rotates behind the duodenum from the right to the left and fuses
with the dorsal bud by the 6th to 8th week of gestation. e fusion of the
buds results in fusion of the two ductal systems in most individuals. e
embryonal ventral duct connects directly with the common bile duct and
becomes the main duct of Wirsung. e embryonal dorsal duct arises from
the duodenum and becomes the accessory duct of Santorini. With gut rota-
tion, the two ducts fuse in the head of the pancreas such that in most cases,
the majority of the pancreas drains through the duct of Wirsung into the
common channel formed from the bile duct and pancreatic duct. A “com-
mon channel” greater than 10 mm has been termed “pancreaticobiliary
maljunction” (PBM). Patients with PBM are felt to have an increased inci-
dence of biliary malignancy. e length of the common channel is variable.
e junction of the main duct and the accessory duct occurs at a major
bend in the main duct termed the “genu.”
Two pancreatic ducts that drain into the duodenum are the main duct
of Wirsung and the accessory duct of Santorini. roughout the tail and
body, the main duct runs midway between the superior and inferior borders
of the pancreas, slightly closer to the posterior side. After passing into the
neck, the main duct travels inferior and posterior before joining the distal
Section I / Pancreas4
common bile duct and entering the second portion of the duodenum. e
pancreatic duct is usually only 2 to 4 mm in diameter, with its widest por-
tion in the head. Pancreatic cancer within the head of the pancreas typi-
cally obstructs both the bile duct and pancreatic duct resulting in jaundice,
and imaging shows dilation of both bile and pancreatic ducts, the “double
duct sign.” Pancreatic cancer in the body or tail of the gland often obstructs
the pancreatic duct only; patients typically present after months of vague
abdominal pain radiating to the back.
e normal pancreatic duct contains around 20 secondary branches,
which drain the tail, body, head, and uncinate process. ese branches
enter the main duct at right angles and alternate in location on each side of
the duct. e pressure of the pancreatic ductal system is about twice that in
the common bile duct, and this pressure dierential prevents reux of bile
into the pancreatic duct. e muscle bers around the ampulla form the
sphincter of Oddi, which controls the ow of pancreatic and biliary secre-
tions into the duodenum. e sphincter’s contraction and relaxation are
regulated by complex neural and hormonal factors including cholecysto-
kinin (CCK) from the duodenal mucosa, which causes sphincter relaxation.
e accessory duct of Santorini drains the superior and anterior
portions of the pancreatic head. In 60% of individuals, the accessory duct
enters separately into the duodenum via the minor papilla, which is located
approximately 2 cm proximal and anterior to the ampulla of Vater. In 30% of
individuals, either no minor papilla is present or there is a minor papilla but
the terminal portion of the accessory duct is diminutive in size and does
not permit the passage of pancreatic uid. In 10% of patients, no connec-
tion is present between the accessory duct and the main duct. In the latter
group of patients, contrast medium injected into the main duct would not
delineate the anatomy of the minor duct. e normal pancreatic ductal sys-
tem is quite small. Two to three milliliters of contrast medium can ll the
main pancreatic duct and 7 to 10 mL can ll its branches and smaller ducts
as well. Injection of contrast material forcefully and at large volumes risks
distention of the ducts and postprocedural pancreatitis.
Various congenital anomalies can result from the failure of rotation or
fusion of the two pancreatic buds and their associated ducts. Pancreas divi-
sum is the most common congenital variant of the ductal system, occurring
in 5% to 15% of the population, and resulting from failure of the dorsal and
ventral buds to fuse. Subsequently, the duct of Santorini from the dorsal bud
drains most of the pancreas via the minor papilla. In these patients, the infe-
rior portion of the head and uncinate process continues to drain separately
through the duct of Wirsung via the major papilla (Fig. 1.1). Most patients
with pancreas divisum are asymptomatic. However, in some patients the
minor papilla may be inadequate to handle the ow of pancreatic uid from
the majority of the gland. is theoretically leads to outow obstruction and
potentially, pancreatitis.
Annular pancreas (AP) is an uncommon variant characterized by a
thin band of normal pancreatic tissue surrounding the second portion of
the duodenum. Annular pancreas occurs due to incomplete rotation of the
ventral pancreatic bud, so that it remains on the right side of the duodenum.
e incidence of AP is approximately 1 out of 20,000 individuals. More than
60% of patients with this anomaly present during the neonatal period with
features of gastric outlet obstruction. Many children with annular pancreas
have other congenital abnormalities such as Down syndrome and esopha-
geal or duodenal atresias. Children with AP typically present with duode-
nal obstruction (often diagnosed by prenatal ultrasound). is obstruction
is treated by duodenostomy, as dividing the pancreatic annulus may lead
to pancreatic stula from dividing a main pancreatic duct. In contrast to
Chapter 1 / Pancreatic Anatomy and Physiology 5
children, adults presenting with AP often have dierent and more challeng-
ing pancreatobiliary disease such as pancreatitis or biliary obstruction.
ARTERIAL BLOOD SUPPLY
e pancreas has a rich blood supply derived from both the celiac trunk and
the SMA (Fig. 1.2). e celiac trunk gives rise to the splenic artery, the left
gastric artery, and the common hepatic artery. e common hepatic artery
gives rise to the gastroduodenal artery before continuing as the proper
hepatic artery. e gastroduodenal artery gives o the right gastric artery
FIGURE 1.1 A. Normal pancreatic ductal anatomy. B. Failure of the ventral and dorsal
buds to fuse results in pancreas divisum, in which case the majority of the pancreas drains
through the duct of Santorini into the minor papilla. In these patients, the inferior portion of
the head and the uncinate process continue to drain separately through the duct of Wirsung
into the major papilla. (From Mulholland MW, Lillemoe KD, Doherty GM, et al. Greeneld’s
surgery: scientic principles and practice, 4th ed. Philadelphia, PA: Lippincott Williams &
Wilkins, 2005:1939.)
A
B
Section I / Pancreas6
superior to the duodenum, travels inferiorly, anterior to the pancreatic neck
and posterior to the duodenum, and gives rise to the right gastroepiploic
artery at the inferior border of the duodenum, and then continues as the
superior pancreaticoduodenal artery. is branches into the anterior and
posterior superior pancreaticoduodenal arteries. e SMA gives rise to the
inferior pancreaticoduodenal artery, which also divides into anterior and
posterior branches. e pancreaticoduodenal arcades are always present
and form an extensive network of blood vessels that supply both the pancre-
atic head and the second and third portions of the duodenum. is shared
blood supply renders duodenal-preserving pancreatectomy a complex feat.
A rim of pancreatic tissue containing the arcade must be left intact.
Patients with celiac stenosis may derive all hepatic arterial blood
ow retrograde from the SMA via collateral pancreaticoduodenal arcades.
erefore, it is prudent to temporarily occlude the gastroduodenal artery
prior to dividing this vessel during pancreatic head resection. is maneu-
ver ensures adequate antegrade hepatic arterial ow is present from the
celiac artery.
e neck, body, and tail of the pancreas are supplied by many branches
from the splenic artery and the SMA. e inferior pancreatic artery usually
arises from the SMA and runs to the left along the inferior border of the
body and tail of the pancreas, parallel to the splenic artery. ree vessels run
FIGURE 1.2 Arterial anatomy of the pancreas. (From Moore KL, Agur AM, Dalley AF.
Clinically oriented anatomy, 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins,
2013:266.)
Chapter 1 / Pancreatic Anatomy and Physiology 7
perpendicular to the long axis of the pancreatic body and tail and connect
the splenic artery and inferior pancreatic artery. ey are, from medial to
lateral, the dorsal, great, and caudal pancreatic arteries. At the pancreatic
neck, the dorsal pancreatic artery arises from the splenic artery and gives
o both right and left branches. e right branch supplies the head of the
pancreas and usually joins the posterior arcade. e left branch passes
through the body and tail of the pancreas, often making connections with
branches of the splenic artery or left gastroepiploic artery. ese arteries
form arcades within the body and tail of the pancreas, and account for the
rich blood supply of the organ. Detailed knowledge of the blood supply to
the neck, body, and tail of the pancreas is important when performing a dis-
tal pancreatectomy with splenic preservation or a central pancreatectomy.
Equally important is recognizing the broad arterial variability present.
Preoperative planning for patients with pancreatic cancer includes
high-quality computed tomography imaging to evaluate the primary
tumor or any sites of distant metastases, assess the patency of nearby ves-
sels, and delineate their relationship to the primary lesion. Tumors can
then be classied as resectable, locally advanced, or metastatic. A subset
of tumors blurs the distinction between resectable and locally advanced.
ese tumors of borderline resectability include those that abut the SMA,
celiac axis, or hepatic artery (<180 degrees) or display short-segment occlu-
sion of the SMV, portal vein, or conuence of the two vessels with suitable
remaining vessels for reconstruction. Locally advanced, surgically unre-
sectable tumors include those that encase the celiac axis, hepatic artery, or
SMA (>180 degrees), or that occlude the SMV, portal vein, or its conuence
leaving no technical options for reconstruction. Encasement is dened as
involvement of greater than 50% of the circumference of the vessel whereas
abutment refers to less than 50% involvement.
Preoperative imaging also delineates aberrant vascular anatomy. e
most common arterial variant is a replaced right hepatic artery that arises
from the SMA instead of the proper hepatic artery. is variant is found in
10% to 15% of patients and usually courses posteriorly and superiorly from
the SMA around the posterior side of the portal vein and then up to the
porta hepatis on the right side. e artery may be involved by pancreatic
head tumors. It must also be dierentiated from the inferior pancreatico-
duodenal arteries, which also arise from the SMA and take a similar course
to the replaced right hepatic. Inadvertent injury or resection of a replaced
right hepatic artery can lead to hepatic ischemia or compromise of the bili-
ary enteric anastomosis. A replaced left hepatic arter y, present in 10% of the
population, typically arises from the left gastric artery and travels along the
superior border of the lesser omentum. A replaced left hepatic artery is usu-
ally distant from pancreatic head masses but may be involved with tumors
of the pancreatic body. Less common arterial variants include accessory
left and right hepatic arteries, which are similar to the replaced hepatic
arteries but are found in addition to the typical hepatic arterial anatomy.
VENOUS BLOOD SUPPLY
e venous drainage of the pancreas follows the arterial supply. e veins
are generally located anterior to the arteries, and both veins and arteries
lie posterior to the pancreatic ducts. All pancreatic veins ultimately drain
into the portal vein, splenic vein, SMV, or inferior mesenteric vein. Just as an
arterial arcade supplies the pancreatic head, a venous arcade of pancreati-
coduodenal vessels drains this region as well. e anterior superior pancre-
aticoduodenal vein joins the right gastroepiploic vein, which also receives
the middle colic vein, and drains directly into the SMV. Anterior traction on
Section I / Pancreas8
the transverse colon during colectomy or pancreatectomy can avulse these
veins, which then retract behind the pancreas and can be
dicult to control.
e posterior superior pancreaticoduodenal vein enters the portal
vein above the superior margin of the pancreas. e anterior and posterior
inferior pancreaticoduodenal veins enter the SMVs together or separately.
Typically, numerous small venous branches drain from the pancreatic
parenchyma directly into the lateral posterior aspect of the portal vein.
With few exceptions, the veins generally enter along the lateral and poste-
rior sides of the SMV or portal vein and there are usually no anterior venous
tributaries. erefore, a plane can be developed behind the pancreatic neck
and the underlying superior mesenteric/portal vein during pancreatic
resection, unless the tumor is invading the vein.
ree major venous branches drain the body and tail of the pancreas:
the inferior pancreatic vein, the caudal pancreatic vein, and the great pan-
creatic vein. All these vein branches drain into the splenic vein, which runs
in a groove on the dorsal pancreas. Many other unnamed venous branches
from the pancreatic parenchyma also drain into the splenic vein, and these
branches must be divided when performing a distal pancreatectomy with
splenic preservation. e inferior mesenteric vein courses behind the pan-
creas and usually joins the splenic vein. In some cases (about 15%), the
inferior mesenteric vein joins the left side of the SMV or directly with the
portal vein at the splenoportal conuence (Fig. 1.3). is variable anatomy
has implications for pancreaticoduodenectomy with venous resection.
Ideally the splenic vein–portal vein junction should be preserved if possible,
especially if the inferior mesenteric vein needs to be ligated and divided or
FIGURE 1.3 Venous anatomy of the pancreas. (From Moore KL, Agur AM, Dalley AF.
Clinically oriented anatomy, 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins,
2013:266.)
Chapter 1 / Pancreatic Anatomy and Physiology 9
if the inferior mesenteric vein enters the SMV. is may require reimplanta-
tion of the splenic vein into the interposition graft (usually the jugular vein).
Vascular involvement by tumor no longer represents an absolute con-
traindication to pancreatectomy. Cancer located at the inferior aspect of the
pancreatic head or at the uncinate process may involve either the portal vein
or the SMV with or without involvement of one of its two primary branches,
the jejunal and ileal veins. e infrapancreatic venous anatomy is variable. e
main trunk of the SMV is observed in over 90% of patients, but in the remain-
ing 10% of patients, the jejunal and ileal veins merge at the level of the splenic
vein without forming a common trunk. ese veins are smaller and more
fragile than the main trunk, and either one of them can safely be ligated and
resected if the other is of sucient caliber to allow for collateral venous drain-
age of the gut. Involvement of the conuence of the two smaller veins along
with the main trunk is managed by ligation of the jejunal branch as well as
segmental resection and reconstruction of the SMV trunk and proximal ileal
branch. Reconstruction of this branch is preferred because the jejunal branch
is usually more posterior and technically dicult to ac cess for reconstruction.
LYMPHATIC DRAINAGE
A diuse and widespread network of lymphatic channels is closely asso-
ciated with the blood vessels supplying the pancreas. Pancreatic cancer is
metastatic to the lymph nodes in 50% to 70% of patients. ere are ve main
groups of lymph nodes for the pancreas: superior, inferior, anterior, poste-
rior, and splenic nodes (Fig. 1.4). e anterior and posterior superior half of
the pancreatic head drains to the superior nodes, which are located along
the superior border of the pancreas and celiac trunk. e anterior and pos-
terior inferior half of the pancreatic head and body drain to inferior nodes,
located near the groove between the pancreas and duodenum. Additional
drainage is to nodes along the hepatoduodenal ligament, including those
along the portal vein and the hepatic artery.
e anterior portions of the pancreatic head also drain to a group
of anteriorly located nodes that ultimately drain to the right of the celiac
trunk and SMA, while the posterior portions of the head drain to posteriorly
located nodes. e upper part of the pancreatic body drains into the supe-
rior pancreatic nodes, while the lower part drains into inferior pancreatic,
superior mesenteric, and para-aortic nodes. Tumors of the body and tail
may be locally unresectable due to metastases to nodes in the transverse
mesocolon or jejunal mesentery. e tail of the pancreas drains to splenic
nodes located along the splenic vessels.
INNERVATION OF THE PANCREAS
Pancreatic innervation comes from the sympathetic division of the auto-
nomic nervous system through the splanchnic nerves and from the para-
sympathetic division through the vagus nerve. ese nerves follow the
blood vessels and lymphatics. ey contain a mixture of motor eerent and
sensory aerent nerve bers from both autonomic systems. e parasym-
pathetic supply begins in the dorsal motor nucleus of the vagus nerve in
the medulla of the brain. ese bers travel in the vagus nerve and pass
through the celiac plexus. ey then travel with arteries that branch from
the celiac trunk and ultimately enter the pancreatic parenchyma and syn-
apse with terminal ganglion cells within the gland. e postganglionic
bers terminate at pancreatic islet cells. Almost 90% of the bers carried by
the vagus nerve are sensory in function, related to stretch, chemoreceptors,
osmoreceptors, and thermoreceptors.
Section I / Pancreas10
e sympathetic supply begins with ganglia in the thoracic spinal cord.
Fibers pass through the sympathetic chain and descend in the greater and
lesser splanchnic nerves. e former is composed of preganglionic eerent
bers from the 5th through 9th thoracic segments while the latter is com-
posed of bers from the 10th and 11th segments. ese bers pass through
the diaphragmatic crura and synapse on cell bodies in the celiac or superior
mesenteric ganglia. Postganglionic bers then travel with branches of the
arteries to reach the pancreas.
Some aerent bers cross midline in the celiac plexus. Interconnections
between aerent bers from the pancreas and other sensory bers from the
abdominal wall also exist. is anatomy may explain the referred pain associ-
ated with pancreatic diseases. e etiology of pain secondary to pancreatic
cancer is poorly understood, however. Explanations include inltration of
nerve sheaths by malignancy, increased ductal pressure, and gland inam-
mation. Pancreatic pain is generally transmitted through the celiac plexus,
located near the emergence of the celiac trunk from the aorta at the level of the
rst lumbar vertebra. Celiac plexus nerve block can be performed for patients
with pain secondary to cancer as well as for patients with chronic pancreatitis.
PHYSIOLOGY
The pancreas is both an endocrine and an exocrine organ. The exo-
crine component of the pancreas accounts for 80% to 90% of the organ’s
mass, while the endocrine component accounts for approximately 2%.
FIGURE 1.4 Lymphatic drainage of the pancreas. (From Moore KL, Agur AM, Dalley
AF. Clinically oriented anatomy, 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins,
2013:243.)