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Flashcards in Gastrointestinal - Physiology Deck (21):
1

GI regulatory substances:
Cholecystokinin

  • Source
  • Action
  • Regulation

  • Source
    • I cells (duodenum, jejunum)
  • Action
    • Increased pancreatic secretion
    • Increased gallbladder contraction
    • Decreased gastric emptying
    • Increased sphincter of Oddi relaxation
    • Acts on neural muscarinic pathways to cause pancreatic secretion
  • Regulation
    • Increased by fatty acids, amino acids

2

GI regulatory substances:
Gastrin

  • Source
  • Action
  • Regulation

  • Source
    • G cells (antrum of stomach)
  • Action
    • Increased gastric H+ secretion
    • Increased growth of gastric mucosa
    • Increased gastric motility
  • Regulation
    • Increased by stomach distention / alkalinization, amino acids, peptides, vagal stimulation
    • Decreased by stomach pH < 1.5
    • Really increased in Zollinger-Ellison syndrome.
    • Increased by chronic PPI use.
    • Phenylalanine and tryptophan are potent stimulators.

3

GI regulatory substances:
Glucose-dependent insulinotropic peptide

  • Source
  • Action
  • Regulation
  • AKA
  • Oral vs. IV glucose

  • Source
    • K cells (duodenum, jejunum)
  • Action
    • Exocrine: decreased gastric H+ secretion
    • Endocrine: increased insulin release
  • Regulation
    • Increased by fatty acids, amino acids, oral glucose
  • AKA
    • Also known as gastric inhibitory peptide (GIP).
  • Oral vs. IV glucose
    • An oral glucose load is used more rapidly than the equivalent given by IV due to GIP secretion.

4

GI regulatory substances:
Motilin

  • Source
  • Action
  • Regulation
  • Motilin receptor agonists

  • Source
    • Small intestine
  • Action
    • Produces migrating motor complexes (MMCs)
    • Motilin receptor agonists (e.g., erythromycin) are used to stimulate intestinal peristalsis
  • Regulation
    • Increased in fasting state
  • Motilin receptor agonists
    • e.g., erythromycin
    • Used to stimulate intestinal peristalsis.

5

GI regulatory substances:
Secretin

  • Source
  • Action
  • Regulation

  • Source
    • S cells (duodenum)
  • Action
    • Increased pancreatic HCO3– secretion
      • Increased HCO3– neutralizes gastric acid in duodenum, allowing pancreatic enzymes to function.
    • Decreased gastric acid secretion
    • Increased bile secretion
  • Regulation
    • Increased by acid, fatty acids in lumen of duodenum

6

GI regulatory substances:
Somatostatin

  • Source
  • Action
  • Regulation
  • Notes

  • Source
    • D cells (pancreatic islets, GI mucosa)
  • Action
    • Inhibitory hormone.
      • Decreased gastric acid and pepsinogen secretion
      • Decreased pancreatic and small intestine fluid secretion
      • Decreased gallbladder contraction
      • Decreased insulin and glucagon release
    • Antigrowth hormone effects (inhibits digestion and absorption of substances needed for growth).
  • Regulation
    • Increased by acid
    • Decreased by vagal stimulation

7

GI regulatory substances:
Nitric oxide

  • Action
  • Loss of NO secretion is implicated in...

  • Action
    • Increased smooth muscle relaxation, including lower esophageal sphincter (LES)
  • Loss of NO secretion is implicated in...
    • Increased LES tone of achalasia.

8

GI regulatory substances:
Vasoactive intestinal polypeptide (VIP)

  • Source
  • Action
  • Regulation
  • VIPoma

  • Source
    • Parasympathetic ganglia in sphincters, gallbladder, small intestine
  • Action
    • Increased intestinal water and electrolyte secretion
    • Increased 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.
    • Copious Watery Diarrhea, Hypokalemia, and Achlorhydria (WDHA syndrome).

9

GI regulatory substances:
Intrinsic factor

  • Source
  • Action

  • Source
    • Parietal cells (stomach)
  • Action
    • Vitamin B12–binding protein (required for B12 uptake in terminal ileum)
    • Autoimmune destruction of parietal cells Ž--> chronic gastritis and pernicious anemia.

10

GI regulatory substances:
Gastric acid

  • Source
  • Action
  • Regulation
  • Gastrinoma

  • Source
    • Parietal cells (stomach)
  • Action
    • Decreased stomach pH
  • Regulation
    • Increased by histamine, ACh, gastrin
    • Decreased by somatostatin, GIP, prostaglandin, secretin
  • Gastrinoma
    • Gastrin-secreting tumor that causes high levels of acid secretion and ulcers refractory to medical therapy.

11

GI regulatory substances:
Pepsin

  • Source
  • Action
  • Regulation

  • Source
    • Chief cells (stomach)
  • Action
    • Protein digestion
  • Regulation
    • Increased by vagal stimulation, local acid
    • Inactive pepsinogen -->Ž pepsin by H+

12

GI regulatory substances:
HCO3-

  • Source
  • Action
  • Regulation

  • Source
    • Mucosal cells (stomach, duodenum, salivary glands, pancreas) and Brunner glands (duodenum)
  • Action
    • Neutralizes acid
  • Regulation
    • Increased by pancreatic and biliary secretion with secretin
    • HCO3- is trapped in mucus that covers the gastric epithelium

13

Locations of GI secretory cells

  • Gastrin
  • Atropine
  • Vagal stimulation of G cells

  • 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.
  • Atropine
    • Blocks vagal stimulation of parietal cells.
  • Vagal stimulation of G cells
    • Unaffected, as a different transmitter (gastrin-releasing peptide or GRP) is used, not ACh.

14

Gastric parietal cell

15

Brunner glands

  • Located in duodenal submucosa.
  • Secrete alkaline mucus.
  • Hypertrophy seen in peptic ulcer disease.

16

Pancreatic secretions

  • Fluid
  • α-amylase
  • Lipase, phospholipase A, colipase
  • Proteases
  • Trypsinogen

  • Fluid
    • Isotonic fluid
    • Low flow -->Ž high Cl-
    • High flow -->Ž high HCO3-
  • α-amylase
    • Role in starch digestion
    • Secreted in active form
  • Lipase, phospholipase A, colipase
    • Role in fat digestion
  • Proteases
    • Role in protein digestion
    • Includes trypsin, chymotrypsin, elastase, carboxypeptidases
    • Secreted as proenzymes also known as zymogens
  • Trypsinogen
    • Converted to active enzyme trypsin -->Ž 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 the duodenal and jejunal mucosa

17

Carbohydrate absorption

  • Monosaccharide absorption
  • Transportation
    • Glucose and galactose
    • Fructose
    • All monosaccharides
  • D-xylose absorption test

  • Monosaccharide absorption
    • Only monosaccharides (glucose, galactose, fructose) are absorbed by enterocytes.
  • Transportation
    • Glucose and galactose are taken up by SGLT1 (Na+ dependent).
    • Fructose is taken up by facilitated diffusion by GLUT-5.
    • All monosaccharides are transported to blood by GLUT-2.
  • D-xylose absorption test
    • Distinguishes GI mucosal damage from other causes of malabsorption.

18

Vitamin/mineral absorption

  • Iron
  • Folate
  • B12

  • Iron
    • Absorbed as Fe2+ in duodenum.
  • Folate
    • Absorbed in jejunum and ileum.
  • B12
    • Absorbed in terminal ileum along with bile acids, requires intrinsic factor.
  • Iron Fist, Bro

19

Peyer patches

  • Definition
  • Immunoglobin

  • Definition
    • Unencapsulated lymphoid tissue [A]
    • Found in lamina propria and submucosa of ileum. 
    • Contain specialized M cells that sample and present antigens to immune cells.
  • Immunoglobin
    • 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.
  • Think of IgA, the Intra-Gut Antibody.
    • And always say “secretory IgA.”

20

Bile

  • Composition
  • Functions

  • Composition
    • Composed of bile salts (bile acids conjugated to glycine or taurine, making them water soluble), phospholipids, cholesterol, bilirubin, water, and ions.
    • Cholesterol 7a-hydroxylase catalyzes rate‑limiting step of bile synthesis.
  • Functions
    • ƒƒDigestion and absorption of lipids and fat-soluble vitamins
    • Cholesterol excretion (body’s only means of eliminating cholesterol)
    • Antimicrobial activity (via membrane disruption)

21

Bilirubin (349)

  • Definition
  • Direct vs. indirect

  • Definition
    • Product of heme metabolism.
    • Bilirubin is removed from blood by liver, conjugated with glucuronate, and excreted in bile.
  • Direct vs. indirect
    • Direct bilirubin
      • Conjugated with glucuronic acid
      • Water soluble.
    • Indirect bilirubin
      • Unconjugated
      • Water insoluble.

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