W14 - GI system II (5.4, 5.5) Flashcards Preview

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Flashcards in W14 - GI system II (5.4, 5.5) Deck (39):
1

How much saliva is produced daily?

Main functions?

800 - 1500 ml/day
(90% during eating)

  • initiates digestion of lipids carbs 
  • lubrication + dilution of food to create bolus

further functions on "constituents of saliva" card

2

Describe the general structure of the salivary glands.

Differentiate.

tubualveolar structure
= consists of acini + ducts

  • parotid: purely serous
  • sublingual gland: mainly mucous
  • submandibular gland: mixed seromucous

3

Generally...

How is the secretory function of the GI tract regulated?


 

  • enteric NS: direct stimulation or action of ENS cells
  • autonomic NS: 
    • parasymp. → ↑ GI activity
    • symp. → ↓ or ↑ GI activity (dep. on tissue)
  • secretogogues: substances that cause secr., either holo-/endocrine

⇒ act on slayer of myoepithelium around acini 
→ contraction

4

What are the major constituents of saliva?

  • water
  • electrolytes (Na+, K+, HCO3-, Cl-)
  • proteins 
    • α-amylase, lipase → digestion
    • IgA, lysosyme → antimicrobial effect
    • Ca2+ binding protein → provides ↑[Ca2+] around teeth
    • R-protein → vit B12 absorption

5

Describe the two-stage model of salivary secretion.

Another name for this mechanism?

= sequential secretion

  1. primary saliva = isotonic
    produced/secreted from acinar cells
  2. secondary saliva = hypotonic
    modified as it passes through duct cells, reabsorb Na+/Cl-, secrete K+, HCO3- → alkaline 

6

Describe the ion transport in acinar cells.

  1. Cl- diffuses via CFTR into lumen of acinus, causes negative potential
  2. Na+/H2O follow paracellularly
  3. Na/K-ATPase and Na/K/Cl cotransporter maintain balance on basolat. surface
  4. proteins exocytosed

isotonic primary saliva

 

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7

Briefly describe the characteristics of the CFTR channel.

cystic fibrosis transmemb. conductance regulator

  • Cl- channel
  • ABC transporter → needs ATP
    if ATP present: passive transport

 

8

Describe the ion transport in duct cells.

  • Na+ reabsorbed via ENaC
  • HCO3- and K+ secreted
  • no aquaporins → impermeable to water

hypotonic secondary saliva

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9

How does the flow rate affect the ionic composition of saliva?

the higher the flow rate (eating) the less time for compositional modification by duct cells

↑ [Na+], ↓[K+]
= more similar to that of plasma

ONLY exception: 
[HCO3-] is HIGHEST at HIGH flow rates 
bc secreted selectively upon parasymp. stim.

10

Describe the 3 mechanisms of parasymp. regulation of salivary secretion.

via n. VII, IX

  • ACh binds to m3-R (Gq) on acinar cells
    ⇒ ↑ fluid and enzyme secretion
  • VIP causes vasodilation via Gs
    ⇒ ↑ secretion due to ↑ blood flow
  • incr. parasymp. activity (↑ACh) from smelling or seeing food, nausea

11

Why does atropin only selectively inhibit the saliva production?

only blocks ACh receptors

→ VIP can still enhance saliva prod.

12

Describe the 2 mechanisms of symp. regulation of salivary secretion.

NE binds to acinar/duct cells via

  • α1-R (Gq)
    → vasoconstriction + ↓ fluid production
  • β2-R (Gs)
    → ↑ mucin secretion

13

How much gastric juice is secreted daily?

Functions w/ to its constituents.

1000 - 1500ml/day
incr. production after meal

  • mucous/HCO3-: protection of mucosa
  • H+: antimicrobial effect, activates pepsinogen to pepsin
  • lipase, pepsinogen: digestion
  • intrinsic factor: only essential secretion → necessary for absoprption of vit B12

 

14

As a summary..

Differentiate btw the most important cell types of the stomach, their location and function.

3 functional regions w/ unique functions:

  • LES + cardia
  • fundus + body
  • antrum + pylorus

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15

Differentiate btw the 2 states of parietal cells.

Function?

resting state - active state
translocation of membranes/transporters to luminal surface for ↑↑ gastric acid secretion

  • H+, Cl- secretion
  • intrinsic factor production

16

Describe the mechanism of gastric acid secretion.

initiated by translocation of H/K pump

  1. CO2 + H2O form H+ + HCO3-
  2. HCO3- exported, Cl- imported by antiporter on basolateral surface
  3. H+/K+ pump on lum. surface exports H+,
    Cl- leaves via CFTR
  4. K+ leaves again via channels

NOTE: blood that leaves stomach = alkaline due to ↑[HCO3-]

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17

Which mechanisms regulate the activity of parietal cell secretion?

stimulated by

  • neural: ACh by vagus → via m3-R
  • hormonal: histamine by ECL cells → via H2-R (Gs)
  • paracrinegastrin by G cells → via CCKB-R, also ECL cells

18

How is the release of gastrin stimulated?

  • distension of stomach → ACh
  • ACh from vagus → GRP 
  • AAs, peptides
  • enteric reflexes
  • too high symp. tone → β2-R

19

How is the release of gastrin inhibited?

  • mainly feedback regulation
    ↑[H+] → D cells produce ↑[somatostatin] (Gi) → ↓[gastrin] → ↓[H+]
  • ALSO: inhibited by PGE2
    omeprazole for treatment of gastric cancer

20

Differentiate btw mucous cells of the stomach.

Function?

surface/neck mucous cells

⇒ secrete Na+, Cl-, HCO3-, mucin to buffer HCl
cells surface remains near pH 7

21

Which substances are secreted by chief cells?

What is the necessary stimulus?

​ACh, secretin causes

⇒ secretion of pepsinogen, gastric lipase

22

What can you say about the ionic composition of gastric acid w/r/t its rate of secretion?

ALWAYS isotonic

  • ↑[Cl-], [K+] w/ incr. rate of secretion
  • ↑↑↑[H+] w/ incr. rate of secretion
  • ↓↓↓[Na+] w/ incr. rate of secretion

23

Values for peak acid output in male and females.

  • PAO - male = 25mmol/h
  • PAO - female = 16mmol/h

> in males due to higher no. of parietal cells

24

What are the 3 phases of gastric secretion?

  1. cephalic phase
  2. gastric phase
  3. intestinal phase

25

As a summary..

What happens during the cephalic phase of gastric secretion?

thought of food, smell, taste, chewing, swallowing
= 40% of total acid secretion

vagus → ACh → GRP → G cells → HCl secretion
(ACh also directly stimulates parietal cells)

NOTE: atropine-resistant pathway bc acting via GRP

26

As a summary..

What happens during the gastric phase of gastric acid secretion?

food reached stomach, hence gastric distension
= 50% of total gastric acid secretion

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27

What happens during the intestinal phase of gastric acid secretion?

food reaches small intestine
= 10% of total acid secretion

  • protein digestion digested in stomach → free AAs 
  • distension → signals intestinal endocrine cells to secrete enterooxyntin

⇒ stimulus to parietal cells

 

28

What are mechanisms other than the pathway using somatostatin to inhibit the release of gastric acid?

Where do they happen?

  • antrum: low pH directly inhibits gastrin release
  • duodenum: low pH causes release of  secretin/neural reflex
  • duodenum/jejunum: hyperosmotic sol. and FAs cause release of GIP and CCK

⇒ inhibited rel. of gastrin/HCl rel. by parietal cells

29

What happens during the small intestinal phase?

food reaches small intestine
secretions of pancreas, liver, small intestine

30

How much pancreatic juice is secreted daily?

Functions?

Tonicity.

700 - 900ml/day
98% exocrine, 2% endocrine

  • exocrine:
    • neutralization of acids
    • digestive enzymes
  • endocrine: regulation of blood sugar 

ALWAYS isotonic

NOTE: large reserve, even 10% would be enough
 

31

How does the sequential secretion in the pancreas differ from that in salivary glands?

  1. acini secrete isotonic acinar fluid
    and lots of enzymes
  2. intralobular ductal system: spontaneous secretion → incr. [HCO3-], [K+]
  3. extralobular ductal system: stimulated by secretin to further secrete [HCO3-]

32

List important enzymes that are produced by the pancreas.

Where?

Examples.

pancreatic acinar cells

  • zymogen proteases: chymo-/trypsinogen
  • starch-digesting enzymes: amylase
  • lipid-digesting enzymes/precursors: lipase
  • nucleases
  • regulatory factors: procolipase

33

Describe the main features of ion transport in pancreatic duct cells.

  • in apical membr.: Cl-/HCO3 exchanger
  • in basolat. membr.: Na/K-ATPase and an Na/H-exchanger
  1. carbonic anhydrase
    CO2 + H2O → H2CO3 → H+ and HCO3
  2. Cl-/HCO3 exchanger
    HCO3 secreted into pancreatic juice
  3. Na/H-exchanger
    H+ is transported into the blood

⇒ net secretion of HCO3 into pancreatic ductal juice and net absorption of H+
(acidification of pancr. venous blood)

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34

What is cystic fibrosis?

 

genetic disease, caused by mutation in CFTR
→ impaired Cl- secretion

⇒ clog secretion (since secretion of mucous is not suspended), hence impaired function of a variety of epithelial organs

if CFTR in pancreas defect, bicarbonate cannot be secr.

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35

Which substance is the most important stimulant for enzyme secretion in acinar cells?

CCK (secreted by I cells of duodenum)

36

How is the pancreatic secretion regulated?

Mechanisms.

 

neurally and hormonally

activated by:

  • hormonally: CCK/gastrin, secretin
  • neurally: VIP, GRP, ACh

​inhibited by: somatostatin

 

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37

What are the 3 phases of pancreatic secretion?

Which one is the most important one?

  • cephalic phase = sight, smell, taste of food
  • gastric phase = distention of stomach
  • intestinal phase (cf. own card)

⇒ 80% of secretion happens in intestinal phase

38

Which stimuli induce pancreatic secretion in the intestinal phase?

Mediator/mechanism.

  • acidic duodenum (pH → secretin stimulates duct cells
  • AAs, FAs, Ca2+
    → CCK stimulates aff. arm of vagovagal reflexes to acinar and duct cells
  • distention of duodenum, hypertonicity in duodenum
    → enteropancr. reflexes stimulate both cell types

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39

Describe the feedback loop that responds to a fall in luminal pH in the duodenum.

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