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+.
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
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
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
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.
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.
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)
