Exocrine Pancreas & Salivary Gland

Question 1

Sources of saliva

Answer 1

• salivary glands:
• submandibular (70%)
• parotid (25%)
• sublingual (5%)

Question 2

Major components of saliva

Answer 2

• mucins = large glycoproteins
  
  • fxn = lubricate food & facilitate swallowing
• enzymes = amylase and lingual lipase
  
  • fxn = digestion of starches & fats
• NaHCO₃
  
  • fxn = 1. maintain optimal pH for enzyme activity
  • 2. reduce Ca²⁺ solubility (==> reduced loss from teeth to oral fluids)
• antibacterial agents = IgA, lysozyme, lactoferrin

Question 3

Salivary antibacterial agents

Answer 3

• immunoglobulin A (lgA)
• lysozyme which destroys bacterial cell walls
• Iactoferrln which chelates iron thus preventing the growth of bacteria that require iron

Question 4

Structure of salivary glands/saliva production

Answer 4

• composed of serious and mucous acini
• serous acini = secrete fluid, electrolytes, enzymes
• mucous acini = secrete mucins
• saliva passes from acini ==> short intercalated ducts ==> striated duct = modifies inorganic ion composition

Question 5

Saliva @ high flow rates vs. low flow rates

Answer 5

• @ high flow rates, saliva is slightly hypotonic and rich in bicarbonate
• @ low flow rates it becomes very hypotonic
  
  • striated ducts to modify the secretion
  • Na+ and Cl- ions are transported out of the lumen of the duct.
  • HCO3- and K+ ions are secreted into the lumen of the duct

Question 6

Stimuli of salivary secretion

Answer 6

• parasympathetic input = strong influence
• sympathetic input

Question 7

General characteristics of pancreatic exocrine secretion

Answer 7

• ezymatic & aqueous components
• acinar cells = produce and secrete digestive enzymes
  
  • enzymes/zymogens released via exocytosis
• majority of protein synthesis = inactive secretory enzymes (“zymogens”)

Question 8

Zymogens produced by pancrease

Answer 8

• all inactive until activated by proteolytic cleavage:
• Trypsinogen
  
  • activated by enterokinase @ small intestinal mucosa
• Chymotrypsinogen
• RNase/amylase/lipase

Question 9

Role of Cholecystokinin (CCK)

Answer 9

• stimulated by arrival of chyme @ small intestine
  
  • also stimulated by ACh & gastrin
• acts as stimulus for acinar cell secretion
• cellular mechanisms
  
  • 2nd messenger system w/ IP₃ & Ca²⁺
    *

Question 10

Characteristics of aqueous component of pancretic secretion

Answer 10

• water and bicarbonate
• produced by duct cells
• fxn = neutralize acid @ duodenum
  
  • prevents damage to duodenum
  • controls pH for optimal enzyme fxn
• Secretin = released by endocrine cells upon arrival of acid @ small intestine ==> increased NaHCO₃ production/release

Question 11

Mechanism of bicarbonate-rich fluid secretion from duct cells

Answer 11

• ~reverse of acid-producing process of parietal cells
• primary active transport process = Na+/K+-ATPase @ basolateral membrane
• H+ is transported into the plasma by secondary active transport using the energy of the inward Na+ gradient
• low H+ concentration leads to high OH- and consequently high HCO3- ==> lumen via CI-/HCO3- exchange
• most CO2 used to produce bicarbonate comes from the blood
• Na from plasma ==> lumen via paracelular

Question 12

Pancreatic juice composition: low flow rate vs. high flow rate

Answer 12

• @ low flow rates: HCO3- is exchanged for Cl- as the solution moves down the duct
  
  • final concentration of bicarbonate is low and Cl- (NaCl) is relatively high
• @ high flow rates: fluid moves through duct too fast to allow for significant exchange
  
  • HCO3- is high, Cl- is relatively low

Question 13

Phases of pancreatic secretion

Answer 13

• Cephalic phase accounts for 25-50% of secretion
  
  • stimulated by sight/thought/smell/taste of food ==> vagal ACh ==> increase pancreatic enzyme secretion
• Gastric phase accounts for 10% of secretion
  
  • stimulated by gastric distention ==> vaso-vagal reflexes
• Intestinal phase is most important, accounting for 50-100% of secretion
  
  • food @ intestine ==> CCK increase ==> increased secretion