Lecture 15: Secretions of GI Tract and Pancreas Flashcards
What are the functions of saliva?
- Initial digestion of starches and lipids
- Dilution and buffering of ingested food
- Lubrication of ingested food with mucus
Describe the parotid gland.
- Composed of serous cells
- Secretes 25% of daily output of saliva
Describe the sublingual and submaxillary gland.
- Composed serous and mucus cells
- Secretes 75% of saliva
- Secrete aqueous fluid and mucin glycoprotein
Describe the structure of salivary gland.
- Acinus (blind end): secrete initial isotonic saliva
- Myoepithelial cells
- Intercalated duct
- Striated duct: modification of saliva
What type of cells line the striated duct?
Columnar epithelial cells (ductal cells)
-modify initial saliva
What is saliva composed of?
H2O Electrolytes α-amylase Lingual lipase Kallikrein Mucus
Describe the electrolyte composition of saliva.
Hypotonic (compared to plasma):
- Increased potassium and bicarbonate concentration
- Decreased sodium and chloride concentration
What are important transport mechanisms utilized to produce the hypotonic saliva?
Apical side:
- Cl-/HCO3-
- Na+/H+
- H+/K+ exchange
Basal Side:
- Na+/K+ ATPase
- Cl- channels
What are ductal cells impermeable to so that the saliva is hypotonic?
Impermeable to water
Where do the presynaptic parasympathetic nerves originate at for innervation of salivary glands?
Facial and Glossopharyngeal nerves
What NT is utilized in parasympathetic innervation of salivary glands?
Acetylcholine that is recognized by type III mACh receptors
Where do the preganglionic sympathetic nerves originate at for innervation of salivary glands?
Cervical ganglion/T1-T3
What NT is utilized in sympathetic innervation of salivary glands?
Norepinephrine that is recognized by β-adrenergic receptors
Does parasympathetic or sympathetic innervation dominate the regulation of salivary secretion?
Parasympathetic
What are the components of gastric mucosa?
- HCl/H+
- Pepsinogen
- Mucus
- Intrinsic Factor
- Water
What are two areas of gastric mucosa and what do they do?
- Oxyntic gland: found in body and fundus of stomach and secretes acid
- Pyloric gland: found in antrum of stomach and secretes gastrin
What do parietal cells secrete and what do the compounds do?
- Intrinsic Factor: required for Vit B12 absorption
2. HCl: Initiates protein digestion and converts pepsinogen to pepsin
What do chief cells secrete and what does the compound do?
Chief cells: secrete pepsinogen
What do G cells secrete and what does the compound do?
Gastrin: stimulates secretion of gastric juice and secreted into bloodstream in response to food
- stimulate pepsinogen production
- induces pancreatic secretions and gallbladder emptying
What do mucus cells secrete and what do the compounds do?
- Mucus: protects stomach wall from damage and is a lubricant
- Bicarbonate (HCO3-)
- Pepsinogen
What drug can be used to inhibit parietal cells from secreting HCl?
Omeprazole
-inhibits K+/H+ ATPase
What composes gastric juice?
Non-parietal:
- sodium, chloride, potassium
- bicarbonate
Pariteal:
-chloride
What stimulates parietal cells?
- M3 receptor activated by acetylcholine from Vagus nerve
- Gastin from G cells
- Histamine from ECL cells
What inhibits parietal cells?
- Somatostatin from D cells
2. Prostaglandins
What can inhibit acetylcholine from M3 receptor on parietal cells?
Atropine
What can inhibit histamine on parietal cells?
Cimetidine: antagonist of H2 receptors
What are the two ways for parietal cells to be activated?`
- Direct: innervation from Vagus Nerve
2. Indirect: Vagus Nerve –> G cells –> gastrin –> parietal cells
What stimulates release of somatostatin?
Gastrin
H+
What are the three phases of gastric HCl secretion?
- Cephalic phase
- Gastric phase
- Intestinal phase
Describe the cephalic phase.
30% of HCl secreted
- stimulated by smell/taste of food
- Vagus nerve plays big role: stimulate parietal and G cells
Describe the gastric phase.
60% of HCl secreted
-stimulated by distention and vagus nerve
Describe the intestinal phase.
10% of HCl secreted
-distention of small intestine and stimulates acid secretion
When does pepsinogen convert to pepsin?
Very low pH (from parietal cells)
- vagus nerve stimulation
- positive feedback loop: pepsin converts more pepsinogen to pepsin
What is the role of pepsin?
- Degrades proteins into peptides
- Converts pepsinogen to pepsin
Where is Vitamin B12 absorbed?
Distal ileum
How does pernicious anemia occur?
Failure to secrete IF
-lack of B12 absorbed
What can lead to disruptions in the absorption of Vitamin B12?
- Gastrectomy
2. Gastric bypass
What protects the gastric mucosal epithelium from HCl and pepsin?
Mucus and bicarbonate (main)
- prostaglandins
- mucosal blood flow
- gastin
- growth factors
What attacks the gastric mucosal epithelium from HCl and pepsin?
HCl and pepsin (main)
- NSAIDs
- H. pylori
- alcohol
- bile
- stress
- smoking
Describe Zollinger-Ellison syndrome.
Increased secretion of gastrin by duodenal or pancreatic tumors leading to gastrinomas
- increased H+ and parietal cells
- inhibition of sodium and water absorption
- ulcers formed by increased hydrogen overwhelming bicarbonate
- damage of intestinal epithelial cells/villi
How can you test for Zollinger-Ellison syndrome?
Secretin stimulation test
-injecting secretin will lead to unexplained increase of gastrin
What causes peptic ulcer disease?
- Helicobacter pylori
- use of NSAIDs
What are the two most common types of ulcers?
Gastric ulcer: defective mucosal barrier
Duodenal: increased H+ secretion rates
How does H. pylori work?
1) Releases cytotoxins to breakdown mucosal barrier
2) Uses urease to convert urea to ammonia, which alkalinizes local env
3) NH4+ buildup can lead to cytotoxicity
What is found in pancreatic juice?
HCO3-: neutralizes acidity from stomach
Enzymes: digest carbs, proteins, and lipids
How is the exocrine pancreas organized?
- Acinus: blind end secreting major enzymes
2. Ducts: secrete bicarbonate
Where are pancreatic enzymes converted to active forms?
Lumen of duodenum
How can cystic fibrosis affect the pancreas?
Due to improper chloride channels, HCO3- secretion might not occur properly
What induces the release of pancreatic enzymes?
CCK
What does secretin induce?
Secretion of HCO3-
