Carlos Shared "Gastrointestinal Physiology" - 7 Picmonics

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Gastrointestinal Physiology

Stomach
Gastrin
SOURCE: G cells (antrum of stomach, duodenum)
ACTION: increased gastric H+ secretion, increased growth of gastric mucosa, and increased gastric motility
REGULATION: Increased by stomach distention/ alkalinization, amino acids, peptides, vagal stimulation via gastrin-releasing peptide (GRP). Decreased by pH < 1.5
Gastrin increases acid secretion primarily through its effects on enterochromaffin-like (ECL) cells (leading to histamine release) rather than through its direct effect on parietal cells.
BICARBONATE
SOURCE: Mucosal cells (stomach, duodenum, salivary glands, pancreas)
ACTION: Neutralizes acid
REGULATION: Increased by pancreatic and biliary secretion with secretin
Trapped in mucus that covers the gastric epithelium.
Somatostatin
SOURCE: D cells (pancreatic islets, GI mucosa)
ACTION: Decrease gastric acid and pepsinogen secretion
Decrease pancreatic and small intestine fluid secretion
Decreases gallbladder contraction
Decreases insulin and glucagon release
REGULATION: Increased by acid, decreased by vagal stimulation
Intrinsic factor
SOURCE: Parietal cells (stomach)
ACTION: Vitamin B12–binding protein (required for B12 uptake in terminal ileum)
Gastric acid
SOURCE: Parietal cells (stomach)
ACTION: Decreases stomach pH
REGULATION: Increased by histamine, ACh, gastrin
Decreased by somatostatin, GIP, prostaglandin, secretin
Pepsin
SOURCE: Chief cells (stomach)
ACTION: Protein digestion
REGULATION: Increased by vagal stimulation, local acid
Pepsinogen (inactive) is converted to pepsin (active) in the presence of H+.
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Duodenum
Covered by Brush Border Enzymes
Cholecystokinin
SOURCE: I cells (duodenum, jejunum)
ACTION: Increase pancreatic secretion
Increase gallbladder contraction
Decreases gastric emptying
Increases sphincter of Oddi relaxation
REGULATION: Increased by fatty acids and amino acids
Acts on neural muscarinic pathways to cause pancreatic secretion.
Secretin
SOURCE: S cells (duodenum)
ACTION: Increases pancreatic HCO3 – secretion
Decreases gastric acid secretion
Increases bile secretion
REGULATION: Increased by acid, fatty acids in lumen of duodenum
Increased HCO3 – neutralizes gastric acid in duodenum, allowing pancreatic enzymes to function.
Glucosedependent insulinotropic peptide (GIP)
SOURCE: K cells (duodenum, jejunum)
ACTION: Exocrine: Decreases gastric H+ secretion
Endocrine: increases insulin release
REGULATION: Increased by fatty acids, amino acids, oral glucose
BICARBONATE
SOURCE: Mucosal Cells and Brunner Glands
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Vasoactive intestinal polypeptide (VIP) and VIPoma
SOURCE: Parasympathetic ganglia in sphincters, gallbladder, small intestine
ACTION: Increases intestinal water and electrolyte secretion
Increases relaxation of intestinal smooth muscle and sphincters
REGULATION: Increased by distention and vagal stimulation
Decreased by adrenergic input
VIPoma
non-α, non-β islet cell pancreatic tumor that secretes VIP.
Men 1 Association
Watery Diarrhea that persists with fasting: often greater than 3 L/day (pancreatic cholera)
Hypokalemia
Achlorhydria
Treatment: Octreotide
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Pancreatic secretions
Isotonic fluid
Low flow → high Cl−
High flow → high HCO3 −
α-amylase
Starch digestion
Secreted in active form
Lipases
Fat digestion
Proteases
Protein digestion
Includes trypsin, chymotrypsin, elastase, carboxypeptidases
Secreted as proenzymes also known as zymogens
Trypsinogen
Converted to active enzyme trypsin, leading to activation of other proenzymes and cleaving of additional trypsinogen molecules into active trypsin (positive feedback loop)
Converted to trypsin by enterokinase/ enteropeptidase, a brush-border enzyme on duodenal and jejunal mucosa
nucleases
Pancreatic Nucleases breakdown DNA/RNA
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Carbohydrate absorption
Only monosaccharides (glucose, galactose, fructose) are absorbed by enterocytes.
Glucose and galactose are taken up by SGLT1 (Na+ dependent).
Fructose is taken up by facilitated diffusion by GLUT5.
All are transported to blood by GLUT2
D-xylose absorption test: distinguishes GI mucosal damage from other causes of malabsorption.
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Peyer patches
Unencapsulated lymphoid tissue found in lamina propria and submucosa of ileum.
Contain specialized M cells that sample and present antigens to immune cells.
B cells stimulated in germinal centers of Peyer patches differentiate into IgA-secreting plasma cells, which ultimately reside in lamina propria.
IgA receives protective secretory component and is then transported across the epithelium to the gut to deal with intraluminal antigen.
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Bile
composed of
Bile salts (bile acids conjugated to glycine or taurine, making them water soluble)
Phospholipids
Cholesterol
Bilirubin
Water
Ions
Cholesterol 7α-hydroxylase catalyzes rate-limiting step of bile acid synthesis
Functions
Digestion and absorption of lipids and fat-soluble vitamins
Cholesterol excretion (body’s 1° means of eliminating cholesterol)
Antimicrobial activity (via membrane disruption)
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