Gastric Secretions and Control

Gastric Secretions

• Intrinsic factor (parietal glands) necessary for vitamin B12 absorption, deficiency leads to pernicious anemia
• Mucus creates a 1 mm wide alkaline barrier
• Gastrin (pyloric glands) stimulates acid secretion, releases histamine, and is controlled by gastrin and the enteric nervous system

Control of Gastric Secretions

• Stimulation of acid secretion:
  • Enterochromaffin-like cells (ECL cells) release histamine
  • Controlled by gastrin and the enteric nervous system
• Stimulation of pepsinogen secretion:
  • Acetylcholine from vagus or enteric nervous system
  • Response to acid in the stomach
• Inhibition of gastric secretion:
  • Reverse enterogastric reflex by enteric nervous system and extrinsic sympathetic
  • Hormonal inhibition by secretin, GIP, VIP, and somatostatin

Phases of Gastric Secretions

• Cephalic phase (30% secretion):
  • Neurogenic upper signals that respond to stimuli
  • Conducted by the vagus nerves from the dorsal motor nuclei
• Gastric phase (60% secretion):
  • Extrinsic long vasovagal reflex to the brain
  • Local enteric reflexes
  • Gastrin mechanism
• Intestinal phase (10% secretion):
  • Small secretion caused by food in the duodenum
  • Probably mediated by gastrin

Pancreatic Secretions

• Enzymes (Acini):
  • Amylase
  • Proteases (trypsin, chymotrypsin, procarboxypeptidase)
  • Lipases (pancreatic lipase, cholesterol esterase, phospholipase)
• Bicarbonate and water (Ducts)

Regulation of Pancreatic Secretions

• Acetylcholine:
  • Stimulates the acini
  • From parasympathetic and enteric nervous system
• Cholecystokinin:
  • Stimulates the acini
  • Released by I cells in the duodenal and proximal jejunal mucosa in response to proteoses, peptones, and long-chain fatty acids
• Secretine:
  • Stimulates the ducts to produce HCO3+
  • Released by S cells in the duodenal and proximal jejunal mucosa
  • Increases pH to 7.0-8.0

Phases of Pancreatic Secretion

• Cephalic and gastric phases:
  • Conducted by parasympathetic system
  • Initial and limited production of enzymes at the acini
• Intestinal phase:
  • Production of secretin and CCK by chyme at the duodenum
  • Copious production of pancreatic juice

Bile Components

• Water
• Bile salts
• Bilirubin
• Cholesterol
• Fatty acids
• Lecithin
• Na+, K+, Ca++, Cl-, HCO3-

Regulation of Bile Excretion

• Cholecystokinin:
  • Stimulates the contractions of the gallbladder and opening of the Sphincter of Oddi
• Secretine:
  • Stimulates the duct cells to produce water and sodium bicarbonate
• Acetylcholine:
  • Stimulates the gallbladder contractions
  • From parasympathetic and enteric nervous system

Small Intestine Secretions

• Brunner's glands:
  • Alkaline mucus
  • Stimulated by tactile or irritating stimuli, vagal stimulation, secretin, and sympathetic inhibition
• Lieberkühn crypts:
  • Globet cells
  • Mucus
  • Enterocytes
  • Water and electrolytes (1800 ml/d)
  • Digestive enzymes (peptidases, sucrase, maltase, lactase, intestinal lipase)

Colonic Secretions

• Lieberkühn crypts:
  • Globet cells
  • Mucus
  • Enterocytes
  • Water and electrolytes
• Regulation:
  • Parasympathetic and enteric nervous system
  • Tactile stimulation

Salivary Secretions

• Parotid glands:
  • Serous secretion
  • Ptyalin (amylase)
• Submandibular glands:
  • Mix secretion
  • Mucin
• Sublingual glands:
  • Mix secretion
  • Water
• Buccal glands:
  • Mucus
• Composition:
  • Water
  • Electrolytes (low ClNa, high KHCO3)
  • Antibodies
  • Proteolytic enzymes (lysozyme)
  • Thiocyanate ions

Esophageal Secretions

• Mucus:
  • Upper two-thirds: simple mucous glands
  • Lower third: compound mucous glands

Gastric Secretions

• Glands:
  • Mucous cells
  • Tubular glands
  • Oxyntic or gastric glands
  • Peptic or chief cells
• Acid secretion:
  • Enzymes involved (H+-K+ ATPase, carbonic anhydrase, Na+-K+ ATPase, anion exchanger)
  • Gastric fluid composition (ClH, ClK, ClNa)
  • Gastric barrier (tight intercellular junctions, mucus)

Other Gastric Secretions

• Pepsinogens (peptic glands):
  • Inactive at pH > 5
  • Activated in acid environment

Types of Secretory Glands

• Unicellular glands:
  • Mucous (goblet) glands
  • Tubular glands
• Multicellular glands:
  • Crypts of Lieberkühn
  • Complex glands (salivary glands, pancreas, liver)

Stimulation of Secretory Glands

• Local stimulation:
  • Direct contact
  • Enteric nervous system
• Autonomic innervation:
  • Parasympathetic stimulation
  • Sympathetic dual effect (by reduction of blood supply)

Secretions

• Organic substances:
  • Endoplasmic reticulum
  • Golgi complex
  • Vesicles
• Mucus:
  • Composition (water, electrolytes, polysaccharides)
  • Properties (adherent, compact, slippy, resistant, amphoteric)

This quiz covers the functions and regulation of gastric secretions, including intrinsic factor, mucus, gastrin, and histamine. Learn about the role of these substances in vitamin B12 absorption and acid secretion.