GI Secretory Functions, Digestion, Absorption Flashcards
(53 cards)
1
Q
What stimulates gland secretion?
A
- food contact and local epithelial stimulation
- autonomic stimulation (parasympathetic)
- higher brain centers
- hormonal stimulation
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
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
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
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
6
Q
What are the salivary gland secretions?
A
- parotid gland secretions are almost entirely serous
- submandibular and sublingual secretions are mixed
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
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
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
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
11
Q
Describe how sympathetic stimulation of salivary gland secretion functions.
A
- B-adrenergic receptors
- second messenger is cAMP
12
Q
What are some factors the increase saliva production?
A
- food in mouth (para)
- smells
- conditioned reflexes
- nausea
13
Q
What are some factors that decrease saliva production?
A
-inhibition of parasympathetic system: \+sleep \+dehydration \+fear \+anticholinergic drugs
14
Q
What are the secretory cells in gastric glands?
A
- mucus neck cells
- chief/peptic cells
- parietal/oxyntic cells
15
Q
What are the different gland types?
A
- unicellular mucous
- crypts of Lieberkuhn
- tubular glands
- complex glands -> salivary glands, pancreas, liver
16
Q
What do chief cells do?
A
- secrete pepsinogen (pH range of activity: 1.8-3.5)
- secrete intrinsic factor
17
Q
What stimulates the release of pepsinogen?
A
- ACh from vagus nerves or gastric enteric nervous plexus
- response to acid in stomach
18
Q
What do parietal cells secrete?
A
-secrete HCl
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
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)
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
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
What are the three phases of gastric secretions?
- 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
What are the digestive enzymes for proteins?
-trypsin, chymotrypsin, and carboxypolypeptidase
-trypsin inhibitor
+secreted by the glandular cells
+necessary to prevent the action of trypsin on the pancreatic tissues themselves
27
What is the digestive enzyme for carbs?
-pancreatic amylase
28
What are the digestive enzymes for fat?
-pancreatic lipase, cholesterol esterase, phospholipase
29
What are some characteristics of pancreatic secretions?
- 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
How do the acinar cells help form pancreatic secretions?
-produce small volume of pancreatic secretion composed mainly of Na and Cl
31
How do ductal cells help in the formation of pancreatic secretions?
- 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
What does ACh do in the regulation of the pancreatic secretions?
- 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
What does CCK do for pancreatic secretions?
- 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
How does secretin regulate pancreatic secretions?
- 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
What factors stimulate the release of bile from the gallbladder?
-presence of fatty food in duodenum
36
In the gallbladder, ow is the bile concentrated?
-by active transport of Na followed by secondary absorption of Cl-, water, and other diffusible constituents
37
What are the functions of bile salts?
- emulsification
| - complex with lipids to form micelles for absorption across intestinal mucosa
38
How are bile acids formed?
- 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
How is bile formed?
- 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
What are the different cell types in the crypts of Lieberkuhn?
- 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
What is the difference between hydrolysis and condensation reactions?
- 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
Describe the absorption of carbs.
- 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
What are the causes of gall stones?
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
Describe the absorption of glucose and galactose.
-active transport of Na followed by secondary active transport involving a sodium co-transport mechanism (SGLT 1) in luminal membrane
45
Describe the absorption of fructose.
-cannot be absorbed against a concentration gradient
46
Describe the absorption of lactose.
- 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
What are proteins absorbed as?
-amino acids or peptides
48
How are aas and peptides absorbed into the SI?
- 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
What does CCK do in terms of lipid digestion?
-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
How is NaCl absorbed?
- 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
Absorption and secretion of K
- K can be absorbed passively via paracellular route
| - K secretion in colon is stimulated by aldosterone
52
Absorption of water
- 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
Secretion of electrolytes
-secretory mechanisms are located in the crypts (absorption is in the villi)
