Pancreas: Anatomy

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Pancreas Anatomy Physiology

• Sanjay Munireddy
• Sinai Hospital
• 11/21/2006

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Pancreas- Brief History

• Herophilus, Greek surgeon first described
  pancreas.
• Wirsung discovered the pancreatic duct in 1642.
• Pancreas as a secretory gland was investigated by
  Graaf in 1671.
• R. Fitz established pancreatitis as a disease in
  1889.
• Whipple performed the first pancreatico-duodenecto
  my in 1935 and refined it in 1940.

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Pancreas

• Gland with both exocrine and endocrine functions
• 6-10 inch in length
• 60-100 gram in weight
• Location retro-peritoneum, 2nd lumbar vertebral
  level
• Extends in an oblique, transverse position
• Parts of pancreas head, neck, body and tail

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Embryology of pancreas

• Endodermal origin
• Develops from ventral and dorsal pancreatic buds
• Ventral bud becomes the uncinate process and
  inferior head of pancreas
• Dorsal bud becomes superior head, neck, body and
  tail
• Ventral bud duct fuses with dorsal bud duct to
  become mail pancreatic duct (Wirsung)

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Embryology of Pancreas
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Pancreas
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Head of Pancreas

• Includes uncinate process
• Flattened structure, 2 3 cm thick
• Attached to the 2nd and 3rd portions of duodenum
  on the right
• Emerges into neck on the left
• Border b/w head neck is determined by GDA
  insertion
• SPDA and IPDA anastamose b/w the duodenum and the
  rt. lateral border

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Neck of Pancreas

• 2.5 cm in length
• Straddles SMV and PV
• Antero-superior surface supports the pylorus
• Superior mesenteric vessels emerge from the
  inferior border
• Posteriorly, SMV and splenic vein confluence to
  form portal vein
• Posteriorly, mostly no branches to pancreas

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Pancreas
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Body of Pancreas

• Elongated, long structure
• Anterior surface, separated from stomach by
  lesser sac
• Posterior surface, related to aorta, lt. adrenal
  gland, lt. renal vessels and upper 1/3rd of lt.
  kidney
• Splenic vein runs embedded in the post. Surface
• Inferior surface is covered by tran. mesocolon

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Tail of Pancreas

• Narrow, short segment
• Lies at the level of the 12th thoracic vertebra
• Ends within the splenic hilum
• Lies in the splenophrenic ligament
• Anteriorly, related to splenic flexure of colon
• May be injured during splenectomy (fistula)

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Pancreatic Duct

• Main duct (Wirsung) runs the entire length of
  pancreas
• Joins CBD at the ampulla of Vater
• 2 4 mm in diameter, 20 secondary branches
• Ductal pressure is 15 30 mm Hg (vs. 7 17 in
  CBD) thus preventing damage to panc. duct
• Lesser duct (Santorini) drains superior portion
  of head and empties separately into 2nd portion
  of duodenum

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Arterial Supply of Pancreas

• Variety of major arterial sources (celiac, SMA
  and splenic)
• Celiac ? Common Hepatic Artery ? Gastroduodenal
  Artery ? Superior pancreaticoduodenal artery
  which divides into anterior and posterior
  branches
• SMA ? Inferior pancreaticoduodenal artery which
  divides into anterior and posterior branches

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Arterial Supply of Pancreas

• Anterior collateral arcade b/w anterosuperior and
  anteroinferior PDA
• Posterior collateral arcade b/w posterosuperior
  and posteroinferior PDA
• Body and tail supplied by splenic artery by about
  10 branches
• Three biggest branches are
• Dorsal pancreatic artery
• Pancreatica Magna (midportion of body)
• Caudal pancreatic artery (tail)

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• Arterial Supply of Pancreas

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Venous Drainage of Pancreas

• Follows arterial supply
• Anterior and posterior arcades drain head and the
  body
• Splenic vein drains the body and tail
• Major drainage areas are
• Suprapancreatic PV
• Retropancreatic PV
• Splenic vein
• Infrapancreatic SMV
• Ultimately, into portal vein

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• Venous Drainage of Pancreas

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Lymphatic Drainage

• Rich periacinar network that drain into 5 nodal
  groups
• Superior nodes
• Anterior nodes
• Inferior nodes
• Posterior PD nodes
• Splenic nodes

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Innervation of Pancreas

• Sympathetic fibers from the splanchnic nerves
• Parasympathetic fibers from the vagus
• Both give rise to intrapancreatic periacinar
  plexuses
• Parasympathetic fibers stimulate both exocrine
  and endocrine secretion
• Sympathetic fibers have a predominantly
  inhibitory effect

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Innervation of Pancreas

• Peptidergic neurons that secrete amines and
  peptides (somatostatin, vasoactive intestinal
  peptide, calcitonin gene-related peptide, and
  galanin
• Rich afferent sensory fiber network
• Ganglionectomy or celiac ganglion blockade
  interrupt these somatic fibers (pancreatic pain)

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Histology-Exocrine Pancreas

• 2 major components acinar cells and ducts
• Constitute 80 to 90 of the pancreatic mass
• Acinar cells secrete the digestive enzymes
• 20 to 40 acinar cells coalesce into a unit called
  the acinus
• Centroacinar cell (2nd cell type in the acinus)
  is responsible for fluid and electrolyte
  secretion by the pancreas

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Histology-Exocrine Pancreas

• Ductular system - network of conduits that carry
  the exocrine secretions into the duodenum
• Acinus ? small intercalated ducts ? interlobular
  duct ? pancreatic duct
• Interlobular ducts contribute to fluid and
  electrolyte secretion along with the centroacinar
  cells

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Histology-Endocrine Pancreas

• Accounts for only 2 of the pancreatic mass
• Nests of cells - islets of Langerhans
• Four major cell types
• Alpha (A) cells secrete glucagon
• Beta (B) cells secrete insulin
• Delta (D) cells secrete somatostatin
• F cells secrete pancreatic polypeptide

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Histology-Endocrine Pancreas

• B cells are centrally located within the islet
  and constitute 70 of the islet mass
• PP, A, and D cells are located at the periphery
  of the islet

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Physiology Exocrine Pancreas

• Secretion of water and electrolytes originates in
  the centroacinar and intercalated duct cells
• Pancreatic enzymes originate in the acinar cells
• Final product is a colorless, odorless, and
  isosmotic alkaline fluid that contains digestive
  enzymes (amylase, lipase, and trypsinogen)

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Physiology Exocrine Pancreas

• 500 to 800 ml pancreatic fluid secreted per day
• Alkaline pH results from secreted bicarbonate
  which serves to neutralize gastric acid and
  regulate the pH of the intestine
• Enzymes digest carbohydrates, proteins, and fats

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Bicarbonate Secretion

• Centroacinar cells and ductular epithelium
  secrete 20 mmol of bicarbonate per liter in the
  basal state
• Fluid (pH from 7.6 to 9.0) acts as a vehicle to
  carry inactive proteolytic enzymes to the
  duodenal lumen
• Sodium and potassium concentrations are constant
  and equal those of plasma
• Chloride secretion varies inversely with
  bicarbonate secretion

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Bicarbonate Secretion

• Bicarbonate is formed from carbonic acid by the
  enzyme carbonic anhydrase
• Major stimulants
• Secretin, Cholecystokinin, Gastrin, Acetylcholine
• Major inhibitors
• Atropine, Somatostatin, Pancreatic polypeptide
  and Glucagon
• Secretin - released from the duodenal mucosa in
  response to a duodenal luminal pH lt 3

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Enzyme Secretion

• Acinar cells secrete isozymes
• amylases, lipases, and proteases
• Major stimulants
• Cholecystokinin, Acetylcholine, Secretin, VIP
• Synthesized in the endoplasmic reticulum of the
  acinar cells and are packaged in the zymogen
  granules
• Released from the acinar cells into the lumen of
  the acinus and then transported into the duodenal
  lumen, where the enzymes are activated.

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Enzymes

• Amylase
• only digestive enzyme secreted by the pancreas in
  an active form
• functions optimally at a pH of 7
• hydrolyzes starch and glycogen to glucose,
  maltose, maltotriose, and dextrins
• Lipase
• function optimally at a pH of 7 to 9
• emulsify and hydrolyze fat in the presence of
  bile salts

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Enzymes of Pancreas

• Proteases
• essential for protein digestion
• secreted as proenzymes and require activation for
  proteolytic activity
• duodenal enzyme, enterokinase, converts
  trypsinogen to trypsin
• Trypsin, in turn, activates chymotrypsin,
  elastase, carboxypeptidase, and phospholipase
• Within the pancreas, enzyme activation is
  prevented by an antiproteolytic enzyme secreted
  by the acinar cells

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Insulin

• Synthesized in the B cells of the islets of
  Langerhans
• 80 of the islet cell mass must be surgically
  removed before diabetes becomes clinically
  apparent
• Proinsulin, is transported from the endoplasmic
  reticulum to the Golgi complex where it is
  packaged into granules and cleaved into insulin
  and a residual connecting peptide, or C peptide

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Insulin

• Major stimulants
• Glucose, amino acids, glucagon, GIP, CCK,
  sulfonylurea compounds, ß-Sympathetic fibers
• Major inhibitors
• somatostatin, amylin, pancreastatin,
  a-sympathetic fibers

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Glucagon

• Secreted by the A cells of the islet
• Glucagon elevates blood glucose levels through
  the stimulation of glycogenolysis and
  gluconeogenesis
• Major stimulants
• Aminoacids, Cholinergic fibers, ß-Sympathetic
  fibers
• Major inhibitors
• Glucose, insulin, somatostatin, a-sympathetic
  fibers

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Somatostatin

• Secreted by the D cells of the islet
• Inhibits the release of growth hormone
• Inhibits the release of almost all peptide
  hormones
• Inhibits gastric, pancreatic, and biliary
  secretion
• Used to treat both endocrine and exocrine
  disorders