Lecture 15: Secretions of GI Tract and Pancreas Flashcards by Spandana Sama

What are the functions of saliva?

Initial digestion of starches and lipids
Dilution and buffering of ingested food
Lubrication of ingested food with mucus

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Describe the parotid gland.

Composed of serous cells

- Secretes 25% of daily output of saliva

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Describe the sublingual and submaxillary gland.

Composed serous and mucus cells
Secretes 75% of saliva
Secrete aqueous fluid and mucin glycoprotein

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Describe the structure of salivary gland.

Acinus (blind end): secrete initial isotonic saliva
Myoepithelial cells
Intercalated duct
Striated duct: modification of saliva

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What type of cells line the striated duct?

Columnar epithelial cells (ductal cells)

-modify initial saliva

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What is saliva composed of?

H2O
Electrolytes
α-amylase
Lingual lipase
Kallikrein
Mucus

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Describe the electrolyte composition of saliva.

Hypotonic (compared to plasma):

Increased potassium and bicarbonate concentration
Decreased sodium and chloride concentration

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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

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What are ductal cells impermeable to so that the saliva is hypotonic?

Impermeable to water

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Where do the presynaptic parasympathetic nerves originate at for innervation of salivary glands?

Facial and Glossopharyngeal nerves

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What NT is utilized in parasympathetic innervation of salivary glands?

Acetylcholine that is recognized by type III mACh receptors

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Where do the preganglionic sympathetic nerves originate at for innervation of salivary glands?

Cervical ganglion/T1-T3

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What NT is utilized in sympathetic innervation of salivary glands?

Norepinephrine that is recognized by β-adrenergic receptors

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Does parasympathetic or sympathetic innervation dominate the regulation of salivary secretion?

Parasympathetic

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What are the components of gastric mucosa?

HCl/H+
Pepsinogen
Mucus
Intrinsic Factor
Water

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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

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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

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What do chief cells secrete and what does the compound do?

Chief cells: secrete pepsinogen

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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

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What do mucus cells secrete and what do the compounds do?

Mucus: protects stomach wall from damage and is a lubricant
Bicarbonate (HCO3-)
Pepsinogen

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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?`

1. 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?

1. Cephalic phase 2. Gastric phase 3. 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?

1. 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?

1. 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-