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Physiology II Exam 2 > GI Secretory Functions, Digestion, Absorption > Flashcards

GI Secretory Functions, Digestion, Absorption Flashcards

1
Q

What stimulates gland secretion?

A
  • food contact and local epithelial stimulation
  • autonomic stimulation (parasympathetic)
  • higher brain centers
  • hormonal stimulation
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2
Q

What are the functions of mucus?

A
  • adheres to food and other particles
  • spreads thin films over surfaces
  • coats wall of gut, preventing actual contact with food
  • causes fecal particles to adhere to one another
  • resistance to digestion by GI enzymes
  • has atmospheric properties making it useful for buffering small amounts of acids and bases
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3
Q

What are the functions of saliva?

A
  • initial starch digestion and initial triglyceride digestion
  • lubrication of food and protection of mouth and esophagus
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4
Q

What is the composition of saliva?

A
  • high K and bicarbonate conc
  • low Na and Cl
  • hypotonicity
  • presence of alpha-amylase, lingual lipase, and kallikrein
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5
Q

What is the difference between low and high saliva flow rates?

A

Low:

  • lowest osmolarity
  • lowest Na, Cl, and CO3-
  • highest K

High:
-composition closest to plasma

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6
Q

What are the salivary gland secretions?

A
  • parotid gland secretions are almost entirely serous

- submandibular and sublingual secretions are mixed

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7
Q

What occurs during the first stage of salivary secretion?

A
  • occurs in acini
  • secretion contains ptyalin (alpha-amylase)
  • composition is isotonic with ionic conc similar to plasma
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8
Q

What happens during stage two of salivary secretion?

A
  • occurs in salivary ducts
  • active reabsoption of Na
  • active secretion of K
  • active/passive secretion of bicarbonate ions
  • passive reabsoption of Cl due to -70mv in ducts
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9
Q

What controls salivary gland secretion?

A
  • cranial nerves VII and IX
  • production if controlled mostly by parasympathetic system, but also by sympathetic system, both can result in saliva production
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10
Q

Describe how parasympathetic stimulation of salivary gland secretion functions.

A
  • muscarinic cholinergic receptors on both acinis and ducts

- second messenger is inositol 1,4,5-triphosphate (IP3) and increased Ca+2

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11
Q

Describe how sympathetic stimulation of salivary gland secretion functions.

A
  • B-adrenergic receptors

- second messenger is cAMP

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12
Q

What are some factors the increase saliva production?

A
  • food in mouth (para)
  • smells
  • conditioned reflexes
  • nausea
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13
Q

What are some factors that decrease saliva production?

A 
-inhibition of parasympathetic system:
   \+sleep
   \+dehydration
   \+fear
   \+anticholinergic drugs
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14
Q

What are the secretory cells in gastric glands?

A
  • mucus neck cells
  • chief/peptic cells
  • parietal/oxyntic cells
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15
Q

What are the different gland types?

A
  • unicellular mucous
  • crypts of Lieberkuhn
  • tubular glands
  • complex glands -> salivary glands, pancreas, liver
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16
Q

What do chief cells do?

A
  • secrete pepsinogen (pH range of activity: 1.8-3.5)

- secrete intrinsic factor

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17
Q

What stimulates the release of pepsinogen?

A
  • ACh from vagus nerves or gastric enteric nervous plexus

- response to acid in stomach

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18
Q

What do parietal cells secrete?

A

-secrete HCl

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19
Q

What is the mechanism of secretion for HCl?

A
  • dissociation of water inside the cell into H+ and OH-
  • OH- + CO2 -> HCO3 +H+

-HCO3 is exchanged for Cl- ions
+HCO3 increases blood pH and will even tail be secreted by pancreas to neutralize H+ in duodenum

  • Cl- ions are secreted through chloride channels into the canaliculi
  • H+ are pumped out of the cell in exchange for K+
  • K+ leaks outside the cell but is transported back in via H-K ATPase pump
  • Na+ is reabsorbed into the cell due to Na-K basolateral pump
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20
Q

What are the two pathways that parasympathetic stimulation increases H+ secretion?

A

DIRECT:

  • CN X innervates parietal cells
  • stimulates H+ secretion directly
  • utilizes ACh and muscarinic receptor

INDIRECT:

  • CN X innervates G cells
  • stimulates gastrin secretion
  • gastrin stimulates secretion of H+
  • neurotransmitter is GRP (gastrin releasing peptide)
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21
Q

How is gastrin secreted?

A
  • secreted by gastrin G) cells in pyloric glands
  • two forms: G-34 and G-17 (more abundant)
  • released in response to presence of protein in pylorus
  • causes enterochromaffin-like cells to release histamine -> stimulates H+ secretion
  • second messenger on parietal cells is IP3/Ca+2
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22
Q

How is histamine stimulated?

A
  • stimulates secretion of H+ by activating H2 receptors on parietal cell membrane
  • H2 receptor is coupled to adenyl cyclase via Gs protein
  • second messenger is cAMP
23
Q

What are the two pathways that somatostatin inhibits gastric H2 secretions?

A

DIRECT:

  • somatostatin binds to receptors on the parietal cell that are coupled to adenyl cyclase via a GI protein
  • antagonistic to stimulatory action of histamine

INDIRECT:
-inhibits the release of histamine and gastrin

24
Q

What do prostaglandins do?

A

-inhibits gastric H2 secretions by activating G1 protein, inhibiting adenyl cyclase

25
Q

What are the three phases of gastric secretions?

A
  • cephalic phase (via vagus, parasympathetic so excite pepsin and acid production)
  • gastric phase (1. Local nervous secretory reflexes 2. Vagal reflexes 3> gastrin-histamine stimulation)
  • intestinal phase (1. Nervous mechanisms 2. Hormonal mechanisms)
26
Q

What are the digestive enzymes for proteins?

A

-trypsin, chymotrypsin, and carboxypolypeptidase

-trypsin inhibitor
+secreted by the glandular cells
+necessary to prevent the action of trypsin on the pancreatic tissues themselves

27
Q

What is the digestive enzyme for carbs?

A

-pancreatic amylase

28
Q

What are the digestive enzymes for fat?

A

-pancreatic lipase, cholesterol esterase, phospholipase

29
Q

What are some characteristics of pancreatic secretions?

A
  • high volume
  • isotonic
  • same Na and K ion concs as plasma
  • higher bicarbonate conc than plasma
  • Lowe Cl- conc than plasma
  • low flow rates: isotonic fluid composed mostly of Na and Cl
  • high flow rates: isotonic fluid composed mostly of Na and bicarbonate
30
Q

How do the acinar cells help form pancreatic secretions?

A

-produce small volume of pancreatic secretion composed mainly of Na and Cl

31
Q

How do ductal cells help in the formation of pancreatic secretions?

A
  • secrete bicarbonate ion and reabsorb CL via a Cl-HCO3 exchange mechanism
  • ducts are permeable to water, so water moves into ducts to make secretion isotonic
32
Q

What does ACh do in the regulation of the pancreatic secretions?

A
  • from parasympathetic nerves and enteric nervous system
  • released in response to H+, small peptides, aas, and FA in duodenum
  • stimulates enzyme secretion by acinar Cole sand potentiates the effect of secretin
33
Q

What does CCK do for pancreatic secretions?

A
  • release is stimulated by presence of food in upper intestine, especially small peptides, aas, and FAs
  • secreted by duodenal and upper jejunal mucosal cells
  • results in dramatic increase in secretion on pancreatic enzymes
  • potentiates effect of secretin on ductal cells to stimulate bicarbonate secretion
  • second messengers are IP3 and increased intracellular Ca
34
Q

How does secretin regulate pancreatic secretions?

A
  • release is stimulated by presence of acidic foods in upper intestine
  • secreted by duodenal and upper jejunal S mucosal cells
  • stimulates release of large amounts of HNaCO3 by ductal cells
  • second messenger is cAMP
35
Q

What factors stimulate the release of bile from the gallbladder?

A

-presence of fatty food in duodenum

36
Q

In the gallbladder, ow is the bile concentrated?

A

-by active transport of Na followed by secondary absorption of Cl-, water, and other diffusible constituents

37
Q

What are the functions of bile salts?

A
  • emulsification

- complex with lipids to form micelles for absorption across intestinal mucosa

38
Q

How are bile acids formed?

A
  • colic acid and chenodeoxycholic acid are synthesized from cholesterol by hepatocytes
  • bacteria in the intestine convert the primary bile acids into secondary bile salts (deoxycholic and lithocholic acid)
39
Q

How is bile formed?

A
  • bile acids are conjugated with Gly and taurine
  • electrolytes and water are added to bile
  • bile is concentrated in the gallbladder as a result of isomotic absorption of solutes and water
40
Q

What are the different cell types in the crypts of Lieberkuhn?

A
  • goblet cells
  • enterocytes (absorptive cells)
  • paneth cells (secrete antimicrobial proteins to limit bacteria-enterocyte contact)
  • enteroendocrine cells (secrete peptide hormones controlling several GI system functions)
41
Q

What is the difference between hydrolysis and condensation reactions?

A
  • condensation reactions are used to remove H+ and OH- ions from building blocks in order to allow the binding monomers into polymers
  • hydrolysis is the reverse, incorporating water molecules in such a way that polymers are broken down into monomers
42
Q

Describe the absorption of carbs.

A
  • only monosaccharides are absorbed
  • absorption of glucose and galactose in SI
  • absorption of fructose is entirely by facilitated diffusion in SI
  • absorption of lactose requires lactase in brush border
43
Q

What are the causes of gall stones?

A
  1. Too much absorption of water from bile
  2. Too much absorption of bile acids from bile
  3. Too much cholesterol in bile
  4. Inflammation of epithelium
44
Q

Describe the absorption of glucose and galactose.

A

-active transport of Na followed by secondary active transport involving a sodium co-transport mechanism (SGLT 1) in luminal membrane

45
Q

Describe the absorption of fructose.

A

-cannot be absorbed against a concentration gradient

46
Q

Describe the absorption of lactose.

A
  • lactase breaks lactose down into glucose and galactose

- absence of lactase results in lactose intolerance, causing lactose to remain in lumen and cause osmotic diarrhea

47
Q

What are proteins absorbed as?

A

-amino acids or peptides

48
Q

How are aas and peptides absorbed into the SI?

A
  • protein digestive products are absorbed as aas, dipeptides, and tripeptides
  • sodium-aa cotransporter mechanism in luminal membrane
  • facilitated transport from enterocyte into blood
  • four separate transports: neutral, acidic, basic, imino aa
  • hydrogen ion-dipeptide/tripeptide so-transporters on luminal membrane, cytoplasmic peptidases then convert them to aas
49
Q

What does CCK do in terms of lipid digestion?

A

-bile acids emulsify lipids in SI

-pancreatic lipases hydrolyze lipids to FAs, monoglycerides, cholesterol, and lysolecithin
+pancreatic lipase, cholesterol ester hydrolase, phospholipase A2

  • these products of enzymatic digestion are hydrophobic and are solubilized in micelles by bile acids
  • micelles bring the products of digestion into contact with cell membrane of enterocyte and contents diffuse into cells
  • intracellularly, products of lipid digestion are converted into triglycerides, cholesterol ester, and phospholipids
  • these products are combined with apoproteins to form chylomicrons
  • chylomicrons are exocytosis and transferred to lymph vessels
50
Q

How is NaCl absorbed?

A
  • passive diffusion of Na through Na channels
  • Na-glucose or Na-aa cotransporters
  • Na-H ion exchange
  • passive absorption of Cl
  • Na-Cl cotransport
  • Cl-HCO3 ion exchange
51
Q

Absorption and secretion of K

A
  • K can be absorbed passively via paracellular route

- K secretion in colon is stimulated by aldosterone

52
Q

Absorption of water

A
  • secondary to solute absorption
  • isomotic in SI and gallbladder
  • permeability to water is much lowere in colon than in SI -> feces may be hypertonic
53
Q

Secretion of electrolytes

A

-secretory mechanisms are located in the crypts (absorption is in the villi)

Physiology II Exam 2 (6 decks)

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