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MD3002 > GI Tract Secretion > Flashcards

GI Tract Secretion Flashcards

1
Q

How are GI secretions regulated ?

A

Through hormonal and neural signals, which can be chemical (sight and presence of food), or physical (stretch receptors) + input from higher centers.

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2
Q

Identify the main stages of digestion. How many hours does each take ?

A

Cephalic (thinking about food): 1 hour
Gastric (food is in stomach)
Intestinal (food is in intestines)

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3
Q

Identify the primary regulatory events in the cephalic phase.

A
  • Initiated by taste, thought, smell of food (acquired reflex)
  • Brain higher integrating centers send signals to parasympathetic and sympathetic branches.
  • Changes in the mouth: PSNS branch increases salivary secretions while SNS branch increases blood flow to salivary glands and increases salivary V
  • Changes in stomach: PSNS increases secretions in stomach, including HCl, mucus, pepsin, and gastrin secretions (and blood factors such as intrinsic factor), while SNS branch increases blood flow to stomach
  • Other changes: PSNS stimulates bile ducts, hepatocytes, and pancreatic acinar cells
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4
Q

Identify the primary regulatory events in the gastric phase.

A

• Initiated by food entering stomach causing release of gastrin by G cells due to:

  • Parasympathetic pathways
  • Decreased acidity in stomach due to buffering of food (especially caffeine, alcohol, calcium which are all basic)
  • Distention of antrum
  • Proteins, peptides, and AAs
  • Release of gastrin causes increased gastric motility, increased parietal cell secretions (including HCl), and trophic maintenance of GI epithelium
  • Low pH (e.g. HCl, stimulated by gastrin release) causes negative feedback and inhibits release of gastrin
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5
Q

Identify the primary regulatory events in the intestinal phase.

A

• Initiated by presence of food in the duodenum

• If the pH > 3, duodenal peptides/AAs cause release of gastrin
If pH < 2, gastric inhibition and intestinal stimulation occurs
• Duodenal fats and breakdown products cause release of CCK and GIP
• Acid entering the intestine causes secretin release

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6
Q

What is a secretion ? Identify the main secretions of the GI (and accessory organs).

A

Addition of substances (enzymes, ions, fluids) into the GI lumen

  • Saliva (salivary glands)
  • Gastric secretions (cells of gastric mucosa)
  • Pancreatic secretions (Exocrine cells of pancreas)
  • Bile (liver)
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7
Q

SALIVA

  • Composition
  • Factors which increase it
  • Factors which decrease it
A

SALIVA
-Composition: High HCO3-, high K+, alpha-amylase and lingual lipase
(hypotonic)
-Factors which increase it: PSNS and SNS
-Factors which decrease it: Atropine, sleep, dehydration

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8
Q

How much saliva do we produce per day approximately ?

A

We approximately produce 1 L of saliva per day

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9
Q

What are the main functions of saliva ?

A
  • Initial digestion of starches
  • Dilution and buffering of ingested food
  • Protection of teeth and gums
  • Lubrication of ingested food with mucous (mucin) so we can swallow the food
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10
Q

Identify the main salivary glands, identify the location of each, and the type of fluid secreted.

A
  • Submandibular gland: under lower edge of mandible, mixed glands containing mucous and serous cells
  • Sublingual gland: deep in floor of mouth, underneath tongue, mostly mucous cells
  • Parotid: below ear and over the masseter (jaw muscle), serous cells secreting an aqueous fluid composed of water, ions, and enzymes
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11
Q

Identify and explain the main stages of salivary production.

A

Saliva is produced by salivary glands in 3 stages:

1) Acinar cells (furthest away from mouth) secrete primary secretion (isotonic), rich in NaCl, as well as amylase and mucin. This secretion also contains K+ and HCO3-.
2) Myoepithelial cells are then stimulated by neural input to eject saliva into the duct
3) Duct cells perform secondary modification, which consists of reabsorption of Na+ and Cl-, and addition of K+. The concentration of HCO3- is also modified depending on the flow rate; if high flow rate, saliva has ↑ HCO3- whereas if low, more HCO3- is extracted so ↓ HCO3-.

In the mouth, saliva is hypotonic (more dilute than plasma).

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12
Q

Describe the main salivary reflexes.

A

1) Simple (unconditioned) reflex: Chemo/Pressure receptors in mouth activated in presence of food (or other stimulus) → Impulses sent via afferent nerves → Salivary centers in medulla → Impulses via extrinsic autonomic nerves (SNS and PSNS stimulation) → Salivary glands increase saliva production
2) Acquired (conditioned) reflex: Think about/hear/see preparation of food → Cerebral cortex Salivary centers in medulla → Impulses via extrinsic autonomic nerves (SNS and PSNS stimulation) → Salivary glands increase saliva production

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13
Q

How are the three stages of saliva production regulated ?

A

By the SNS and PSNS. In health, PSNS more active whereas in stressful times, SNS more active. Either way, affect acinar and ductal cells and therefore result in slightly different salivary compositions.

  • In health, mainly PSNS → large V, watery, enzyme rich saliva
  • In stressful times, mainly SNS → small V, thick, mucousy saliva
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14
Q

Describe the function of the oesophageal secretions.

A

No real role in digestion, only secretes mucous for protection and lubrication.

  • Main body lined with simple mucous glands (protects against mechanical damage)
  • Gastric end (especially) has compound mucous glands (protects against chemical damage)
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15
Q

Identify the main secretions of the stomach, along with the name of the cells secreting each secretion, and the location in the stomach of each cells.

A

►Parietal cells (in body): Intrinsic Factor + HCl to stomach
►Chief cells (in body): Pepsinogen to stomach
►G cells (in antrum): Gastrin to circulation
►Mucous cells (in antrum): Mucous to stomach

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16
Q

Explain the function of each of the main gastric secretions.

A
  • Pepsinogen: Protein digestion
  • Intrinsic Factor: Vitamin B12 absorption (in ileum)
  • HCl: Protein digestion (a bit), mainly for pepsinogen activation at acid pH (creates pH approximately 2)
  • Mucous: Protection and lubrication
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17
Q

Identify factors which increase, and decrease secretion of HCl.

A
  • Gastrin increases secretions of HCl

- H+ in stomach decreases secretions of HCl

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18
Q

Identify factors which increase, and decrease secretion of pepsinogen.

A
  • Acetylcholine increases secretions of pepsinogen

- Chyme in the duodenum decreases secretions of pepsinogen

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19
Q

Identify factors which increase, and decrease secretion of intrinsic factor.

A
  • Histamine and parasympathetics increase secretions of intrinsic factor
  • Atropine, Somatostatin, Omeprazole, Cimetidine all decrease secretions of intrinsic factor
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20
Q

Define gastric pit.

A

Indentations in the stomach which denote entrances to oxyntic/gastric glands.
These pits increase the stomach SA.

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21
Q

Identify the type of cell which gives rise to each gastric secretion. Where in the gastric pits are these located ?

A
  • Mucous neck cells (closest to stomach lumen) → Mucus
  • Oxyntic/parietal cells → HCl and Intrinsic Factor
  • Peptic/chief cells → Pepsinogen (furthest away from stomach lumen)
  • G cells → Gastrin
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22
Q

Explain the process of HCl production by parietal/oxyntic cells in the stomach.

A

1) In apical membrane
- H+ is secreted into the lumen via the H+-K+ ATPase
- Cl- follows by diffusion through an apical channel

This process (secretion of H+ and Cl-) occurs in separate stages, because otherwise formation of HCl in parietal cells would denature them. 
In the lumen, H+ and Cl- can freely associate and dissociate chemically without a problem (protection present, and HCl actually needed there).

2) Basolateral membrane
- HCO3- (out of parietal cell, into blood) is exchanged for Cl- (into parietal cell) via the chloride-bicarbonate exchanger (alkaline tide)
- Eventually HCO3- is secreted back into GI tract in pancreatic secretions

23
Q

What proportion of gastric secretions occurs in each phase of digestion ?

A

Cephalic- 30%
Gastric- 60%
Intestinal- 10%

24
Q

Describe the gastric effects in the cephalic phase, explaining the nervous pathways involved.

A
  • Direct stimulation of parietal cells by vagus (hence production of HCl and intrinsic factor)
  • Indirect stimulation of parietal cells by gastrin (vagal gastrin releasing peptide GPR stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl).
25
Q

Describe the gastric effects in the gastric phase, explaining the nervous pathways involved.

A

Distention causes:

  • Direct vagal stimulation of parietal cells
  • Indirect stimulation of parietal cells via gastrin (direct effect of AAs and small peptides on G cells stimulates gastrin release)
  • Local reflexes in the antrum that stimulate gastrin release
26
Q

Describe the gastric effects in the intestinal phase, explaining the nervous pathways involved.

A

Following presence of breakdown products of proteins in the duodenum, activity in the stomach pretty much ceases (residual activity, i.e. 10% of gastric secretions, are for lubrication and protection)

27
Q

When and how is HCl secretion inhibited ?

A

When: HCl secretion is inhibited when HCl is no longer needed to convert pepsinogen into pepsin. This occurs after the chyme moves into the small intestine and the H+ buffering capacity of the food is no longer a factor

How: SOMATOSTATIN

  • Direct pathway: binds to receptors on parietal cells and inhibits Adenylate Cyclase via Gi protein so inhibits HCl release
  • Indirect pathway: inhibits hiaatamine release from stomach and gastrin release from G cells so HCl inhibited
28
Q

What factors stimulate pepsinogen secretion ?

A
  • Vagal stimulation

- H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen

29
Q

Briefly explain how pepsinogen is activated ?

A
  • Pepsinogen inactive (which prevents gastric pit cells from being digested).
  • Activated by pH of 2-3 (when comes into contact with HCl)
30
Q

What amount of pancreatic secretions are secreted into the duodenum per day ?

A

Approximately 1 L of exocrine pancreatic secretions are secreted into the duodenum per day

31
Q

Describe the composition of pancreatic secretions into the duodenum. What is the function of this secretion ?

A
  • Aqueous solution containing enzymes (pancreatic lipase, amylase, and proteases) and high concentration of HCO3- (isotonic). Enzymes are stored in condensed zymogen granules until released (to avoid digestion of pancreas itself)
  • Function of this secretion is to neutralise stomach H+, and enzymes digest carbs, proteins and lipids
32
Q

Name the factors that increase pancreatic secretions, as well as any factors which inhibit pancreatic secretions.

A

Secretin
Cholecystokinin (CCK, potentates secretin)
Parasympathetic innervation from the vagus

Sympathetic innervation inhibits pancreatic secretions

33
Q

Why don’t the pancreatic enzymes digest the pancreas itself ?

A

Because they are stored in condensed zymogen granules until released.

34
Q

Describe the process of formation of pancreatic secretion.

A

1) Aqueous component of secretion released from centroacinar cells and ductal cells. This is isotonic and contains Na+, K+, Cl-, and HCO3-. Modification of the ion composition by the ductal cells (bicarbonate chloride exchanger bringing bicarbonate into ductal lumen for chloride which goes into pancreatic ductal cell) results in a fluid secretion rich in HCO3-
2) Enzymatic component of secretion is released from acinar cells. Pancreatic amylase and lipase are secreted as active enzymes. Pancreatic proteases are secreted in an inactive form and activated in the duodenum.

Final pancreatic secretion is also isotonic.

35
Q

Identify the reaction catalysed by each kind of pancreatic enzyme.

A
  • Pancreatic amylase: Polysaccharde → Disaccharide
  • Pancreatic lipase: Triglycerides → Monoglycerides + FA
  • Pancreatic proteases: Proteins → AAs and small peptides
36
Q

Which of salivary or pancreatic amylase is more important ?

A

Pancreatic amylase is much more important than salivary amylase

37
Q

Identify the different kinds of proteolytic pancreatic enzymes. Are these active or inactive ? Why ?

A
  • Trypsinogen
  • Chymotrypsinogen
  • Procarboxypeptidase

These are all inactive (in zymogens) to prevent self-digestion

38
Q

Explain how proteolytic pancreatic enzymes are activated.

A

Trypsinogen is activated by enteropeptidase (aka enterokinase) in luminal border of cells lining duodenal mucosa, forming Trypsin.

Trypsin is the duodenal lumen:

1) activates more trypsinogen to form more Trypsin
2) activates chymotrypsinogen into chymotrypsin
3) activates procarboxypeptidase into carboxypeptidase

39
Q

Explain the regulation of pancreatic secretions in the three phases of digestion.

A

Cephalic phase: some secretion initiated by vagus nerve (mainly enzymatic secretion)

Gastric phase: some secretion initiated by stomach distention and mediated by vagus nerve (mainly enzymatic secretion)

Intestinal phase: initiated by distention of duodenum, accounts for 80% of pancreatic secretions (both enzymatic and aqueous secretions are stimulated)

40
Q

Describe the pancreatic response in the intestinal phase.

A

1) Acinar cells
-Duodenal I cells secrete CCK in response to the presence of AAs, small peptides, and FAs in the small intestine lumen
-Vagal release of ACh potentiates CCK action
This all triggers acinar cells to produce enzymes

2) Ductal cells
-Secretin released by S cells of the duodenum is the major stimulus for aqueous rich HCO3- secretion
-Secretin release is triggered by the arrival of acidic chyme in the duodenum
-ACh and CCK potentiate secretin action
This triggers ductal cells to produce Na+, K+, Cl-, HCO3- (bicarbonate will hence buffer acidic chyme to prevent it damaging duodenum)

41
Q

What are the main functions of the gallbladder ?

A
  • Stores bile
  • Concentrates bile (epithelial cells lining gallbladder absorb ions and water iso-osmotically)
  • Ejects bile
42
Q

Which cells produce bile ? How does bile flow from here to the gallbladder ?

A

Hepatocytes

Through bile ducts

43
Q

How soon after a meal does bile start to get ejected ? What is the main stimulus for bile ejection ?

A

Bile ejections starts about 30 minutes after a meal

Major stimulus for ejection is release of CCK from I cells in duodenum and jejunum

44
Q

What is the function of bile ?

A

Essential for digestion and absorption of lipids; bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into packets called micelles.

45
Q

What are the main components of bile ?

A

By decreasing order of presence

Bile acids, phospholipids, cholesterol, bile pigments (e.g. bilirubin), HCO3- (NO ENZYMES)

46
Q

Describe the regulation of bile secretion in between meals.

A
  • Bile salts are recirculated to the liver via the enterohepatic system (i.e. more bile produced)
  • Bile is then stored and concentrated in the gallbladder.
47
Q

Describe the regulation of bile secretion in the cephalic phase.

A

Neural (parasympathetic) stimulation via the vagus nerve to increase bile flow towards duodenum.

48
Q

Describe the regulation of bile secretion during/after a meal.

A

Chyme in duodenum stimulates release of CCK and secretin.

  • CCK triggers release of stored bile through gall bladder contraction + Sphincter of Odi relaxation
  • Secretin triggers bile secretion, especially NaHCO3- (bicarbonate ions)

Hence, the bile enters the duodenum.

49
Q

Summarise the regulation of biliary secretions by secretin and CCK.

A

• Chyme in duodenal lumen in intestinal phase
• Secretin release from duodenal mucosa
(Secretin carried by blood)
• Hepatocytes and bile duct cells
• ↑HCO3- rich bile secretion
(Neutralisation of duodenal H+ and micelle formation)
• Back to top
——–
• Chyme in duodenal lumen in intestinal phase
• CCK release from duodenal mucosa
(CCK carried by blood)
• Gallbladder contraction and sphincter of Odi relaxation
(Digestion of lipid rich chyme contents, presence of food results in flow of bile)
• Back to top

50
Q

Identify all stimuli for biliary secretion.

A
  • Secretin (bile production)
  • CCK (gallbladder contraction and sphincter of Odi relaxation)
  • Some central and enteric nerve input
51
Q

Identify the main secretion of the large intestine. What is the function of this secretion ?

A

Alkaline mucus (no digestive enzymes, high [K+] and [HCO3-])

Functions: protection and lubrication + neutralisation of H+ produced by gut bacteria

52
Q

What is the trigger to the secretion of the alkaline mucus in the large intestine ?

A
  • Greatest trigger is distention/mechanical stimulation of walls
  • Increased secretion triggered by ACh and vasoactive intestinal peptide
  • Decreased secretion triggered by adrenaline and somatostatin
53
Q

What is the result of extreme parasympathetic stimulation on alkaline mucus secretion in the large intestine ?

A

Extreme/increased PSNS stimulation → Excess mucus → Increased defecation (e.g. nervousness, stress) (since amount of mucus impacts on motility in large intestine)

54
Q

True or false: sometimes in disease, mucus production can increase, which can result in diarrhea.

A

True. Sometimes in disease, mucus production can increase, which can result in diarrhea.

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