GI Flashcards
1
Q
Two components of Digestive system
A
1) Luminal or tubular gastrointestinal tract: luminal structures from oral cavity to anus (mouth, pharynx, esophagus, stomach, small intes, large intes.) - part of external environment
2) Accessory components: teeth, tongue, salivary glands, liver, gallbladder, pancreas - produce secretions
2
Q
sphincters
A
- Upper Esophageal sphincter (skeletal): controls part of the processes of deglutition (swallowing)
- Lower Esophageal sphincter: remains tonically contracted prevents regurgitation of stomach content (prevents gastroesophageal reflux)
- Hepatopancreatic sphincter (Sphincter of Oddi): between duodenum and the outlet of the accessary structures of liver, gallbladder, and pancreas + controls release of bile and pancreatic juice (containing digestive enzymes and HCO3-)
- Pyloric sphincter: controls stomach emptying (prevents gastric contents > 2 mm from entering duodenum) + prevents excessive acidic gastric content from entering the duodenum
- Ileocolic sphincter: (between ileum and cecum) prevents colonic contents (gut flora) from entering small intestine
- Internal anal sphincter:
- External anal sphincter (skeletal):
3
Q
4 sections of large intestine
A
cecum, colon, rectum, and anal canal
4
Q
tasks performed by GI tract
A
- sustains adequate calories (basal rate @ 30kcal/kg) + growth
1) Ingestion
2) Secretion - 7L into lumen per day
3) Motility: churning, segmentation, peristalsis propulsion + rhythmic contraction of smooth muscles
4) Digestion
5) Absorption
6) Defecation - stercolibin (RBC catabolism)
5
Q
pharynx function
A
swallows
6
Q
stomach function
A
- stores and churns food
- pepsin digests protein
- HCl activates enzymes, breaks up food, kills germs
- mucus protects stomach wall
- limited absorption
7
Q
Liver
A
- breaks down and builds up many biological molecules
- stores vitamins and iron
- destroys old blood cells
- destroys poisons
- produces bile
8
Q
gallbladder
A
stores and concentrates bile
9
Q
pancreas
A
- hormones regulate blood glucose levels
- bicarbonate neutralizes stomach acid
- trypsin and chymotrypsin digest proteins
- amylase digests polysaccharides
- lipase digests lipids
10
Q
small intestine
A
- completes digestion
- mucus protects gut wall
- absorbs nutrients, most water
- peptidase digests protein
- sucrases digest sugars
- amylase digests polysaccharides
11
Q
large intestine
A
- reabsorbs some water and ions
- forms and stores feces
12
Q
rectum
A
stores and expels feces
13
Q
anus
A
opening for elimination of feces
14
Q
Average duration of GI contents in each section of GI tract
A
- mouth: 1 minute
- esophagus: 4-8 seconds
- stomach: 2-4 hrs
- small intestine: 3-5 hours
- large intestine: 10 hours to several days
15
Q
Digestion in mouth
A
Carbohydrates by lingual amylase, fats by lingual lipase (minimal)
16
Q
3 phases of swallowing (deglutition)
A
- voluntary
- pharyngeal
- esophageal
17
Q
voluntary phase of swallowing
A
- moistened food shaped into bolus
- tongue moves upward and backward against the palate
- bolus forced into oropharynx
18
Q
pharyngeal phase of swallowing
A
- bolus activates pressure receptors in the oropharynx and triggers swallowing reflex by sending impulses to the deglutition center in medulla and lower pons
- soft palate and uvula move up to close off nasopharynx
- epiglottis covers the larynx to seal off airways
19
Q
esophageal phase of swallowing
A
- bolus enters esophagus
- coordinated contractions and relaxations (peristalsis) of the circular and longitudinal layers of the muscularis
20
Q
esophagus
A
- collapsible tube
- smooth muscle layers provide peristaltic movements to move food down to stomach
- mucus secretion by mucosal and submucosal (protects eso + lubricates food)
- squamous epithelium (protective function - replaced every 5-7 days)
- sphincters prevent air from entering esoph and stomach during breathing
- upper third: striated skeletal muscle
- lower 2/3: smooth muscle cells
21
Q
Gastroesophageal reflux disease (GERD)
A
Reflux of the stomach contents back into the esophagus
22
Q
sphincter with highest resting pressure
A
Upper esophageal sphincter (150 mmHg)
23
Q
Pressure changes in the esophagus during swallowing
A
- UES has highest resting pressure (150 mmHg)
- higher luminal press in skeletal section (90 mmHg) than smooth (60 mmHg)
- press increases again near LES (100 mmHg)
- LES relaxes at the onset of swallowing
24
Q
peristalsis in esophagus types
A
- Primary peristalsis: initiated by swallowing
- Secondary peristalsis: caused by distension of esophagus
25
peristalsis contractions in esophagus regulated by
- medullary swallowing center
- intramural esophageal reflexes
- vagus nerve
- intrinsic myogenic processes
26
Peristalsis contractions elicited by distention of the esophagus alone are mediated by:
Intramural esophageal plexuses and intrinsic myogenic processes
27
Segmental Innervation of the esophagus
under the control of vagus nerve
- Somatic motor neurons in the vagus nerve innervate the skeletal muscle directly
- Autonomic nerve fibers in the vagus innervate the smooth muscle indirectly via the myenteric plexus
28
Stomach
- motility: peristaltic mixing and propulsion
- Secretion: HCl and intrinsic factor (parietal cells); pepsinogen and gastric lipase (chief cells); mucous and HCO3- (surface mucous cells); gastrin (G cells); histamine (enterochromaffin-like cells)
- digestion: proteins (by pepsin - endopeptidase breaks peptide bonds of nonterminal amino acids), fats (by lipase)
- absorption: lipid-soluble substances like alcohol
29
3 functions of stomach
- storage: approx 1L can be stored
- digestion: mainly proteins (by pepsin - as an endopeptidase that breaks peptide bonds of nonterminal amino acids) and fats digested into chyme
- protection: destroys bacteria
30
Gastric accommodation (aka receptive relaxation)
vagovagal reflex causes relaxation of stomach (afferent sends sensory info to CNS; efferent vagal fibers transmitsignal from CNS to stomach)
- gastric accom reduced after vagotomy - higher gastric intraluminal pressure
31
Two important functions in the stomach contraction
1) mixing of food with gastric juice into liquid-like state and reducing them into 2mm or less in size
2) emptying of gastric contents into the duodenum - 3 mL emptied at a time (through relaxation of pyloric sphincter) + upper intestine contracts to push stuff forward
32
contractions during gastric emptying
- stomach mostly inactive when fasting
- Phasic contractions of variable intensity occur after meal
- Peristaltic contraction appears in the mid-section of stomach and moves toward the gastroduodenal junction with increase in force
- Contractions increase in both the FORCE and VEOLCITY as they approach the antrum
- Contraction repeats itself at intervals of 12 to 20 seconds
33
Retropulsion
promotes thorough mixing of the gastric contents + reduces size of solid particles
the backward movement of food from the pylorus of the stomach back into the body of the stomach
34
Longest of all three regions of SI
ileum
35
shortest of all three regions of SI
duodenum
36
functions of the small intestine
- mix chyme with digestive enzymes
- bring food in contact with mucosa for absorption
- completes digestion of carbs, proteins and lipids
- begins and completes digestion of nucleic acids
- Absorbs vitamin B12 (at the ilium)
- Absorbs about 90% of nutrients and water that pass through the GI tract
37
Vitamin B12 absorption
at the ileum
38
SI mobility
mixing and propulsion by SEGMENTATION (alternate segments contract with little or no net forward movement) and PROPULSION (forward movement of the bolus - contraction of circular muscles behind food mass + contraction of longitudinal muscles ahead of food mass)
39
SI secretion
enzymes, mucus, hormones
40
SI digestion
Carbohydrates, fats, polypeptides, nucleic acids
41
SI absorption
➢ Small peptides, amino acids, monosaccharides, fatty acids, monoglycerides, cholesterol, water, electrolytes, minerals, and vitamin
→ Small intestine is the only portion of GI tract that is capable of absorbing both the lipids, carbohydrates, and amino acids
42
4 major sections of LI
cecum, colon, rectum, anal canal
43
appendix attached to
cecum
44
4 sections of colon
ascending, transverse, descending, sigmoidal
45
rectum
Section between sigmoid colon and anal canal
46
Functions of the large intestine
- driving contents of colon into rectum
- Producing some B vitamins and vitamin K with the help of gut flora
- converting some indigested food particles into absorbable constituents by gut flora
- absorption of some water and ions (in the proximal half), and vitamins
- formation of feces
- defecation (emptying rectum)
47
LI motility
segmental mixing (less than in SI) and mass movement (peristaltic wave occurring 3-4 times a day with strong contraction)
48
LI secretion
mucus by goblet cells
49
LI digestion
none except by bacteria
50
LI absorption
ions, water, vitamins, minerals, small organic molecules produced by bacteria
51
Movement of large intestine contents
- presence of food in the stomach (gastrocolic reflex) and chyme in the duodenum (duodenocolic reflex) stimulate mass movement in the colon
- local reflexes initiated by enteric plexus
52
when do mass movements occur
- 15 minutes after breakfast
- usually persist for 10-30 minutes
53
Defecation reflex
- initiated by distension of rectal wall: initiates action potentials going to the sacral spinal cord through the afferent nerve fibers + action potentials through efferent nerve fibers reinforce peristaltic contraction in the lower colon and rectum
- local reflexes causing weak contractions of the rectum + relaxation of internal anal sphincter
- parasympathetic reflexes causing strong rectal contraction
54
time required to empty stomach contents
2-4 hours
55
Peristaltic contraction in stomach is associated with the processes of:
✓ Mixing
✓ Propulsion (forward movement)
✓ Grinding (churning - no forward motion)
✓ Retropulsion
56
intraluminal pressure in stomach
highest as you move down
57
When gastric content is mixed with acidic gastric juice
- salivary amylase is inactivated
- lingual lipase is activated
58
carb digestion in stomach
Without mixing, food may remain in fundus for as long as an hour without being mixed with gastric juice
- allowing digestion of carbohydrate by salivary amylase to continue
59
fat digestion in stomach
30% - without action of bile - through lingual lipase (activated by gastric juice) and gastric lipase (secreted by chief cells in the fundic mucosa)
60
parietal cell location
within the gland in the gastric wall containing proton pump (H-K pump)
61
Acidic gastric juice 2 functions
1) unfold proteins - expose proteolytic sites for digestion by pepsin
2) conversion of pepsinogen (zymogen) into pepsin (by breaking off N-terminal) - when ph is between 3-5
- when ph is below 3.5, pepsin itself converts pepsinogen into pepsin (positive feedback)
62
helicobacter pylori
- gram negative
- survives acidic gastric environment
- colonizes within mucosal layer of gastric wall
- causes inflammation
- causes gastric and duodenal ulcers
63
2 types of regulation of GI tract
1) Neural: mostly under the control of enteric nervous system; function of ENS modulated by CNS and ANS (brain-gut axis)
2) Hormonal: hormones expressed by cell types in stomach and small intestine
64
Enteric nervous system
- brain of the gut
- part of Peripheral nervous system
- involuntary control of GI function
- millions of neurons in the form of enteric plexuses
- communicates with the CNS - CNS modulates GI function through ANS
- controls contractions and secretion of the GI
- plexuses contain motor neurons, interneurons, sensory neurons
- motor neurons of the myenteric plexus innervate the longitudinal and circular smooth muscle of the muscularis - control frequency and strength of contraction (motility)
- motor neurons of the submucosal plexus innervate the secretory cells of the mucosal epithelium (only in intestine) - controlling secretion of intestinal system
65
brain-gut axis
- sensory receptors (chemoreceptors and mechanoreceptors)
- affect GI endocrine system (hormones and paracrine regulators) and local blood circulation
66
Parasympathetic nervous innervation:
- vagus (X) nerves supply parasympathetic fibers to most parts of the GI tract
- exception: last half of the large intestine (parasympathetic fibers supplied by the sacral spinal cord through the pelvic nerve)
67
Parasympathetic nervous innervation in GI effects
increased GI secretion and motility by increasing the activity of ENS neurons
- motility and defecation reflexes in distal LI controlled by pelvic nerve/sacral spinal cord
68
Sympathetic nervous innervation in GI
- nerves arising from thoracic and upper lumbar regions of spinal cord
- postganglionic neurons synapsing with neurons in myenteric and submucosal plexuses
69
sympathetic nervous innervation in GI effects
- decreases secretion and motility and increases sphincter tone (tightens sphincter to prevent food movement forward) by inhibiting the neurons of the ENS
- celiac postganglionic neurons: liver, gallbladder, smooth muscle of esophagus, stomach, duodenum
- superior mesenteric postganglionic neurons: jejunum, ileum, ascending and transverse colon
- inferior mesenteric postganglionic neurons: distal section of the transverse colon and the rest of the large intestine
70
gastric secretions
- surface mucous cells: HCO3- and mucous
- mucous neck cell: mucous
- parietal cell: HCl and intrinsic factor (fundus and body)
- chief cell: gastric lipase and pepsinogen
- G cell: gastrin (antrum)
- vagus nerve: gastrin releasing peptide (stimulates gastrin release, which stimulates HCl release)
- D cells: somatostatin (inhibit cell secretions in stomach)
71
gastric secretion amount
2-3 L per day by mucous neck cells, parietal cells, and chief cells
- amount and type of food influence gastric secretion
- regulated by both neural and hormonal
72
Effect of eating on gastric acid secretion
- empty stomach: [H+] high; gastric acid secretion low
- gastric emptying: gastric acid secretion begins to rise
73
basal state of acid secretion
- interdigestive phase
- low rate of gastric acid secretion (pH between 4 to 6 with empty stomach)
- gastric acid secretion low in the morning, high at night (varies w person and time of day)
- low pH because no food to buffer
74
cephalic state of acid secretion
- Primarily mediated by the Vagus nerve
- sensory stimuli activates the dorsal motor nucleus of the vagus nerve in the medulla --> activates parasympathetic preganglionic efferent nerves
- 30% of total acid secretion
- ACh (released by vagal postganglionic muscarinic nerves) binds to M3 receptors on parietal cells (directly stimulating acid secretion) and ECL cells (triggers histamine release, which binds to H2 on parietal cells)
- gastrin releasing peptide (GRP) released by the parasympathetic vagal nerve endings - induces G cells
- vagus nerve inhibits D cells, reducing somatostatin release (reducing the background inhibition of gastrin release)
75
gastrin
hormone binds to CCK2 (cholecystokinin receptor) on the parietal cells
76
Food or cephalic reflexes
initiate gastric secretion of
gastrin, histamine, and acid
77
Acid secretion in the stomach uses
- H+/K+ ATPase (exchanger)
- HCO3-/Cl- (exchanger - reabsorption of HCO3-)
- chloride channel
- carbonic hydrase makes H+ and HCO3- from CO2 and H20
78
gastric phase
food distends the gastric mucosa --> activates vagovagal reflex/vagus nerve + local enteric nervous system reflexes
- local enteric nervous system reflexes release ACh --> parietal cells release gastric acid --> pepsinogen converted to pepsin --> partially digested proteins/amino acids in the antrum --> stimulate G cells --> gastrin
- 50-60% of total gastric acid secretion
79
gastric secretion during the gastric phase is regulated by the hormone
gastrin
80
gastrin is released by the G cells of the gastric glands in the antrum and duodenum in response to
- distension of the stomach
- partially digested proteins in chyme
- high pH of chyme
- caffeine
- ACh released from parasympathetic neurons
81
gastrin enters the bloodstream and reaches the digestive system to promote
- stimulates gastric glands to secrete gastric juice
- strengthens the contraction of the lower esophageal sphincter to prevent acid reflux
- increases motility of the stomach
- relaxes pyloric sphincter to promote gastric emptying
- activation of pepsinogen into forming pepsin
- provides optimal low pH for killing of microbes + denaturing of proteins
82
gastric inhibitory polypeptide (GIP)
- aka glucose-dependent insulinotropic peptide
- released by K cells of intestinal glands in the duodenum and proximal jejunum
- stimulated by acidic chyme + fatty acids and other lipids in the duodenum
- slows down gastric emptying
- inhibits parietal cell acid secretion
- stimulates insulin release from pancreatic islet cells in response to duodenal glucose and fatty acids
83
intestinal phase
- presence of amino acids and partially digested peptides in the proximal portion of the small intestine stimulates acid secretion (Partially digested peptides stimulate duodenal G cells to secrete gastrin like they stimulate antral G cells in the gastric phase
- acidic chyme entering duodenum stimulates secretin release from S cells in small intestine (inhibits secretion of gastric acid by parietal cells + causes buffering of chyme acid in duodenum by stimulating ductal cells of pancreas and bile duct to increase production of bicarbonate)
- 5-10% of total gastric acid secretion
84
pancreas
- heterocrine organ (contains both endocrine and exocrine acinar cells
- endocrine acinar cells - for controlling blood sugar levels
- 99% of the pancreatic mass is of compound exocrine glands
- exocrine acinar cells release zymogens for digestion of proteins (protease/peptidase) - packaged within zymogen granules and released through calcium mediated exocytosis
- amylase, lipase, colipase, nucleases (RNAase and DNAase) are released in their active forms
85
HCO3- secretion by pancreatic exocrine glands
pancreatic duct cells (non-acinar cells) produce isotonic fluid rich in bicarbonate ion (neutralizes acidic gastric content when it enters duodenum) - duodenum also secretes a bit of HCO3-
86
HCO3- secretion by pancreatic exocrine glands MECHANISM (secreting HCO3- into lumen of pancreas from duct cells)
- CO2 enters the duct cells
- combines with H20 to form carbonic acid (gives you HCO3- and H+)
- HCO3- excreted by Cl-/HCO3- exchanger at the apical membrane
- after influx, CL- leaves cell through apical cystic fibrosis transmembrane conductance regulator channel (CFTR) - CHLORIDE RECYCLING (maintains driving force for bicarbonate secretion by maintaining low Cl- concentration inside cell)
-
87
pancreatic digestive enzymes
pancreatic acinar cells secrete into duodenum:
- zymogens (trypsinogen, chymotrypsinogen, proelastase (endopeptidase); procarboxypeptidase A and B (exopeptidase))
- active digestive enzymes: alpha amylase, carboxyl ester lipase, lipase, RNAase, DNAase) - active enzymes do not involve protein digestion
88
Endopeptidase
enzyme that hydrolyzes interior peptide bonds of a protein molecule
89
Exopeptidase
enzyme that hydrolyzes the terminal peptide bond of a protein to release amino acid, di and/or tripeptide
90
Products of pancreatic digestive zymogens after activation
zymogen, activator, active enzyme
- trypsinogen --> enterokinase and trypsin --> trypsin + peptide
- chymotrypsinogen --> trypsin --> chymotrypsin + peptide
- proelastase --> trypsin --> elastase + peptide
- procarboxypeptidase --> trypsin --> carboxypeptidase + peptide
- prohospholipase A --> trypsin --> phospholipase A + peptide
91
protein digestion by pancreatic enzymes
- Trypsin and chymotrypsin split peptides further down into smaller peptide
- Carboxypolypeptidase splits some peptides into amino acids, di and/or tripeptide
92
carbohydrate digestion by pancreatic enzymes
pancreatic amylase hydrolyzes most carbohydrates (not cellulose) to form disaccharides
93
fat digestion by pancreatic enzymes
- Pancreatic lipase hydrolyzes neutral fat into fatty acids and monoglycerides
- Cholesterol esterase hydrolyzes cholesterol esters
- Phospholipase splits phospholipid into fatty acids and other lipophilic components
94
pancreatic enzyme secretion under fasted basal condition (such as interdigestive period)
- pancreas releases low levels of pancreatic enzymes (indicated by the low levels of trypsin found in the small intestine)
- The basal levels of trypsin output
oscillate synchronously with the migrating motor complexes in small intestine (GI motility related)
95
pancreatic enzyme secretion under fed state (such as interdigestive period)
- higher pancreatic secretion
- massive and sustained increase in the rate of trypsin output
- multiple stimuli regulate pancreatic secretion (redundant mechanism) - don't require all receptors for all stimuli to be activated to trigger a response
96
CCK (cholecystokinin)
used to assess relationship between cytosolic calcium and exocytosis of pancreatic granules
- cytosolic calcium oscillations happens at physiological dose of CCK
- high dose of CCK generates calcium spike without oscillation, releasing pancreatic digestive enzymes through exocytosis
- Ca2+ as the second messenger for
the secretion of proteins by
pancreatic acinar cells
97
exocytosis of pancreatic granules can be triggered by
- Secretin secreted in the S cells of the duodenum
- VIP (vasoactive intestinal peptide) secreted by ENS neurons
- CCK (cholecystokinin) released by gastric I cell and duodenum after meal
- Ach by vagus nerves
- Ach and gastrin during cephalic and gastric phase
- during the intestinal phase, presence of lipids in duodenum stimulate release of CCK, which stimulates vagal afferents and the release of Ach to act on acinar cells
98
secretin causes release of ____ and CCK causes release of ______
- bicarbonate secretions
- enzymes
99
fluid balance in the GI tract
- fluid load of approximately 8.5 L to the small intestine per day (2L from oral intake)
- ileocecal flow is 2.0 L (6.5 got absorbed by SI)
- Maximal absorption capacity in SI: Up to 20 L/day
- maximal colonic fluid absorption capacity: 5L (occurs when there is a significant increase in ileocecal flow)
caused by a decrease in small intestinal absorption - to prevent diarrhea
- stool water < 0.2 L per day so normal colonic absorption rate is ~ 1.9 L per day
100
observations in the GI tract regarding secretion
- HCO₃⁻ is secreted by SI (K⁺ is not but enters the GI lumen via gastric, pancreatic, and biliary secretions --> more than diet)
- Secretion of both the HCO3- and K+ are observed in colon
101
observations in the GI tract regarding absorption
- Water, Na+, K+ (absorbed passively with water through solvent drag), and Cl- absorption are observed in small intestine
- Absorption of water, Na+, and Cl- are observed in colon, only small amount for K+ at the distal colon
102
Secretion as an adaptive mechanism for GI protection
Epithelial secretion as a protective mechanism to avoid absorption of noxious agents or bacterial toxin
- detection of noxious agents or bacterial toxin within the lumen
- increase electrolytes secretion (increase osmotic gradient)
- followed by water secretion (causes intestinal wall distension)
- myenteric and submucosal activation (increased intestinal motility and secretion)
- Enhance defecation reflex to expel harmful agents
103
Cellular mechanisms of Na+ absorption
- nutrient-coupled absorption
- Na+/H+ exchanger
- Na+-H+ & Cl--HCO3- exchanger
- Epithelial Na+ channels
104
1) Cellular mechanisms of Na+ absorption
(nutrient coupled)
- associated with nutrient-coupled Na+ absorption within the small intestine (greater in post-prandial phase - lil contribution during interdigestive phase)
- Na+-Glucose and Na+-Amino acid
cotransporters
- Considered as 2ndary active transporters
- Utilizing high extracellular Na+ concentration gradient (via Na+-K+-ATPase) as the driving force - high intracellular [glucose & amino acid]
- jejunum and ileum
105
Absorption of carbohydrate in the small intestine
- glucose and galactose taken up with Na+ on SGLT1 (exits on GLUT2 into the bloodstream)
- fructose enters on GLUT5 (exits on GLUT2 into the bloodstream)
106
2) Cellular mechanisms of Na+ absorption
Na+/H+ exchanger
- Na+ absorption is increased due to an increase in luminal HCO3- concentration (mainly due to pancreatic bicarbonate secretion)
- Activity enhanced by both decreases in intracellular pH and increases in extracellular pH
- duodenum and jejunum
107
3) Cellular mechanisms of Na+ absorption
Na+-H+ & Cl--HCO3- exchanger
- both small and large intestine (ileum and proximal colon)
- parallel co-exchangers closely linked by changes in intracellular pH
- NaCl absorption leads to secretion of H+ and HCO3-
- primary mechanism of Na+ absorption between meals
108
4) Cellular mechanisms of Na+ absorption
Epithelial Na+ channels
- distal colon
- related to electrogenic Na+ absorption, highly specific for Na+
- can absorb Na+ against high concentration gradient
- important in Na+ conservation
- Aldosterone enhances electrogenic apical Na+ absorption and basolateral Na+-K+-ATPase activity
109
Cellular mechanisms of Cl- absorption
- passive Cl- absorption
- Cl-/HCO3- exchanger
- Na+/H+ and Cl-/HCO3- exchanger
110
Cellular mechanisms of Cl- absorption
1) passive Cl- absorption
- paracellular or transcellular
- resulting from the electrogenic Na+ absorption
- passive process driven by the electrochemical gradient
- jejunum, ileum, distal colon
111
Cellular mechanisms of Cl- absorption
2) Cl-/HCO3- exchanger
- electroneutral process
- ileum, proximal colon, distal colon
112
Cellular mechanisms of Cl- absorption
3) Na+-H+ & Cl--HCO3- exchanger
- parallel co-exchangers powered by pH
- primary mechanism of Cl- absorption between meals
- ileum and proximal colon
113
Cellular mechanisms of electrogenic Cl- secretion
- low activity in basal state but can be secreted via secretagogues
- secretagogues: ACh and any agent that increases cAMP, cGMP, or cytosolic calcium channels - also increase CFTR channels on apical membrane
- low intracellular Na+ drives Cl- uptake through the BASOLATERAL Na/K/Cl cotransporter - increases intracellular Cl- levels - secreted from apical side through CFTR channel
- also promotes Na+ to undergo paracellular transport into the lumen
114
Cellular mechanisms of K+ absorption
- passive K+ absorption (substantial amount): occurs in SI via solvent drag (move with bulk water movement) - jejunum and ileum
- active K+ absorption: distal colon - enhanced by reduction in dietary K+ - energy dependent process in which a proton (H+) is exchanged for K+
115
Cellular mechanisms of K+ secretion
passive K+ secretion: main process for K+ secretion - driven by an electrochemical gradient - occurs via paracellular route - highest activity in distal colon (also in proximal)
- active K+ secretion: smaller role - can be stimulated by cAMP and cytosolic calcium levels (proximal and distal colon)
