Intestinal Electrolyte & Water Absorption

Question 1

function organization of villus in the small intestine

Answer 1

*villi = absorptive zone
-brush border hydrolases
-solute & nutrient transporters
-low water permeability

*crypts = secretory zone
-little nutrient permeabiity
-high water permeability

Question 2

the villus of the small intestine as an anatomic unit of absorption

Answer 2

*interior of villus contains network of blood vessels and a central lacteal (vessel for absorption of lymph)
*also contains actin filaments that rhythmically contract to cause microvillus movement that promotes absorption

Question 3

general principles of intestinal electrolyte/water absorption and secretion

Answer 3

*water absorption in the small intestine is PASSIVE
*dependent on absorption of ions (Na+ and Cl-) and solutes (sugars, amino acids) into basolateral space
*water moves passively into the basolateral space to keep it isosmotic with plasma
*water enters capillaries along hydrostatic gradient

Question 4

mechanisms of sodium absorption in the small intestine

Answer 4

*main driving force for water ABSORPTION
*proximal gut: Na+/H+ exchange & Na+/solute co-transport
*distal gut: coupled Na+/Cl- absorption & Na+ channels

Question 5

mechanisms of chloride absorption in the small intestine

Answer 5

*whole gut: PD-dependent Cl- transport
*distal gut: coupled Na+/Cl- absorption & HCO3-dependent Cl- absorption

Question 6

mechanisms of chloride secretion via CFTR in small intestine

Answer 6

*main driving force for water SECRETION

1. serosal Na+/K+/Cl- cotransporter loads cells with chloride ions
2. Na+ ions are pumped back out
3. CFTR is an apical Cl- gate and channel
4. multiple extracellular signals can open CFTR via cAMP and PKA (lets chloride OUT of the cells)
5. Na+ ions passively follow Cl- ions via paracellular pathways
6. water passively follows the increased flux of Cl- and Na+ ions (water exiting the cell)

Question 7

mechanisms of potassium absorption in the small intestine

Answer 7

*proximal gut: driven by H2O absorption
*distal gut: active transport

Question 8

mechanisms for potassium secretion in the gut

Answer 8

*mainly in the COLON
*active & passive transport

Question 9

factors that stimulate electrolyte secretion and absorption in the small intestine

Answer 9

1. circulating hormones
2. enteroendocrine cells
3. enteric neurons
4. mast cells
5. macrophages

Question 10

diarrhea

Answer 10

*increase in the VOLUME & FREQUENCY of bowel movements with a change in the consistency of the stools

Question 11

chronic diarrhea

Answer 11

sx lasting more than one month

Question 12

etiologies of diarrhea

Answer 12

1. infectious/inflammatory
2. osmotic
3. dysmotility
4. secretory

Question 13

infectious/inflammatory etiology of diarrhea

Answer 13

disruption of epithelial tight junctions allows electrolytes and water into lumen

Question 14

osmotic etiology of diarrhea

Answer 14

presence of malabsorbed or non-absorbable osmoles keeps water in lumen

Question 15

dysmotility etiology of diarrhea

Answer 15

decreased transit time through the gut lumen impairs effective absorption

Question 16

secretory etiology of diarrhea

Answer 16

increased H2O entry into lumen due to increased Cl- or HCO3- secretion or decreased Na+ absorption

Question 17

general history questions for diarrhea

Answer 17

1. onset (acute/sudden vs. gradual)
2. character
   -consistency (soft, mushy, liquid)
   -average # of stools/day
   -volume
3. pattern
   -continuous vs. intermittent
   -postprandial
   -nocturnal

Question 18

examples of causes of infectious diarrhea

Answer 18

*invasive bacterial infections (salmonella, shigella, campylobacter, yersinia)
*bacterial toxins (C. diff, S. Aureus, toxigenic E. coli)
*enteropathic viral infections (rotavirus; norwark virus)
*parasites (giardia, cryptosporidium, etc)

Question 19

history clues for infectious diarrhea

Answer 19

*recent travel or camping trips
*onset after meal at restaurant, outing, communal meal
*handling or consumption of raw or undercooked meats, poultry, seafood
*recent use of antibiotics (risk for C. diff)
*children in daycare
*use of well water

Question 20

examples of causes of osmotic diarrhea

Answer 20

*ingestion of non-absorbable osmoles (Mg++ containing antacids; sugar alcohols)
*disaccharide malabsorption (lactase deficiency, fructose intolerance)
*selective fat malabsorption (gastric resection/bypass, pancreatic insufficiency, bile salt deficiency)
*global malabsorption (short bowel syndrome, mucosal diseases)

Question 21

history clues for osmotic diarrhea

Answer 21

*use of Mg++ containing antacids
*ingestion of sugar alcohols (sugar free syrups, candies, gums)
*ingestion of dairy products
*GI surgery (RYGB, cholecystectomy)
*pancreatic disease

Question 22

examples of causes of secretory diarrhea

Answer 22

*toxins (E. coli, CHOLERA, other vibrios)
*drugs (antroquinone laxatives, misoprostol, etc)
*bile salts (post-cholecystectomy, s/p ileal resection, SIBO)
*GI endocrine tumors

Question 23

cholera & secretory diarrhea

Answer 23

*cholera toxin A irreversibly activates adenylate cyclase, increasing cAMP levels
*activates Cl- secretion via CFTR; Cl- pours out of enterocytes
*Na+ passively follows Cl-
*water passively follows the increased flux of Cl- and Na+
*cholera patients may produce up to 20 L/day of watery stool
*cholera toxin also inhibits non-nutrient Na+ and Cl- absorption by villus cells

note - solute-coupled Na+ absorption remains intact!

Question 24

history clues for secretory diarrhea

Answer 24

*high volume, watery diarrhea
*NOCTURNAL DIARRHEA
*syncope, hypotension
*ER visits for rehydration
*hypokalemia
*hx of longstanding diabetes

Question 25

examples of causes of dysmotility diarrhea

Answer 25

*increased motility (gastric bypass surgery, post-vagotomy, ileal or IC valve resection, hyperthyroidism)
*decreased motility (diabetic enteropathy, scleroderma, etc)

Question 26

approach for diagnosis of chronic diarrhea

Answer 26

*stool analysis
*celiac & IBD serology
*endoscopy & biopsy
*radiology
*hydrogen breath tests

Question 27

MOA for oral rehydration therapy (in tx of cholera, etc)

Answer 27

solute-coupled Na absorption with glucose