Lec 21 Digestion and Absorption I Flashcards
What is surface area of intestine?
200 m squared
What are amplification factors of folds of kerkring, villi, microvilli respectively?
folds of kerkring: 3
villi: 10
microvilli: 20
Where are epithelial cells born? where do they migrate up [crypt vs villus]? How long does differentiation/migration take?
- cells born near bottom of crypt
- differentiate and mature as migrate up villus
- process = 5-6 days
What is function of villus?
absorption
What is function of crypt?
secretion
Difference betweeen villi/crypts in intestine?
small intestine: villi and crypts
colon: just crypts
What is total oral intake h2o per day?
2 L/day
What is total salivary secretion per day?
1 L/day
What is total gastric secretion per day?
2 L/day
What is total bile secretion per day?
1 L/day
What is total pancreatic secretion per day?
2 L/day
What is total fluid presented to SI?
8 L/day
What is total jejunal secretion per day?
4 L/day
What is total ileal secretion per day?
2 L/day
What is total H2O presented to colon per day?
1.5 L/day
What is total H20 in fecal excretion per day?
0.1-0.2 L/day
Is daily net effect secretion or absorption h2o?
absorption
What is total fluid presented to small intestine per day? absorption efficiency? max absorption capacity?
total fluid presented: 7-9 L/day
Absorption Efficiency: 75-80%
Max absorption: 12 L/day
What is total fluid presented to colon per day? absorption efficiency? max absorption capacity?
total fluid presented: 1.5-2 L/day
Absorption Efficiency: 90%
Max absorption: 5 L/day
How does space between cells change in fasting vs absorptive state?
fasting: cells packed closer together
absorptive/when eating: more space between cells so water can transport in
What is mech of H2O absorption?
- solute [Na] transported into cell or between cells
- creates osmotic gradient
- water flows into space to equalized osmolality
- hydrostatic pressure develops relative to capillary
- water flows into capillary to equalize hydrostatic
How does passive permeability change along intestine?
- passive permeability decreases from proximal to distal
- jejunum > Ileum > colon
- means more volume H2O absorbed in proximal
Does small bowel or large bowel disease produce higher volume of diarrhea? why?
- small bowel disease produces higher volume diarrhea
- due to higher volume h2o absorption and higher passive permeability in small intestine
Which parts of intestine have highest net h2o movement over time?
proximal > distal
duodenum > jejunum > ileum > colon
What is major ion that drives fluid absorption?
sodium
What 3 ways sodium transported?
- solute-coupled Na transport
- Na hydrogen exchanger
- electrogenic Na transport
What is mech solute-coupled Na tranpsort?
Na transported from lumen along with glucose or amino acids
What type of Na exchange is basis of oral rehydration therapy in diarrhea?
solute coupled Na transport
What is mech of Na hydrogen exchanger?
allows Na and Cl entry, HCO3 secretion
What is mech of electrogenic Na transport?
- Na exits basolateral membrane via Na-K ATPase
- luminal Na enters cell via apical membrane channel, down electrochemical gradient
What is effect of Na-K atpase? what does it set up?
- sets up Na gradient for Na-H antiporter and electrogenic Na absorption via Na channel
Mech of Cl absorption?
- follows electrochecmical gradient generated by Na transport and via Cl-HCO3 exchange
What is major ion that drives fluid secretion?
Cl
Where does Cl enter [and how]? Where does it exit??
- enters basolaterally [Na:K:2Cl transporter]
- exceeds electrochemical equilibrium
- exits apical membrane [CFTR Cl channel]
When is Cl secreted? What increases Cl secretion?
- Cl is secreted always at basal rate
- secretion increased by: hormonal, neural, inflammatory factors that activate
- – cAMP, cGMP, intracellular Ca
What is basis of secretory diarrhea?
increased Cl secretion
What are 7 hormones that promote intestinal secretion?
- VIP
- secretin
- PGE1
- bradykinin
- ACh
- serotonin
- histamine
How does VIP affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via cAMP
How does secretin affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via cAMP
How does PGE1 affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via cAMP
How does bradykinin affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via cAMP
How does ACh affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via Ca
How does seretonin affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via Ca
How does histamine affect intestinal secretion? via what secondary messenger?
promotes intestinal secretion via Ca
What are 4 neurotransmitters than promote intestinal secretion via cAMP?
VIP
Secretin
PGE1
Bradykinin
What are 3 neurotransmitters than promote intestinal secretion via Ca?
ACh
Serotonin
Histamine
What are 5 bacterial toxins that promote intestinal secretion via cAMP/GMP?
- cholera [V. cholera] [cAMP]
- heat labile toxin [E. coli] [cAMP/GMP]
- campylobacter [cAMP]
- salmonella [cholera-like] [cAMP]
- yersinia enterocolitica [cGMP]
What do laxatives do to intestinal secretion?
promote intestinal secretion
What do dihydroxy bile acids and long chain fatty acids do to intestinal secretion? via what secondary messengers?
- promote intestinal secretion
- via cAMP and Ca
What does ricinoleic acid [castor oil] do to intestinal secretion? via what secondary messenger?
promotes intestinal secretion via cAMP
What does senokot do to intestinal secretion? via what secondary messenger?
promotes intestinal secretion via Ca
What is mech of osmotic diarrhea?
- non-absorbable solute in bowel lumen
- water enters lumen
- solute and water load exceeds colonic absorptive capacity
- mucosal transport processes usually intact – no underlying problem with transporters
2 examples of things that commonly can cause osmotic diarrhea?
failure to absorb dietary solute
- lactose in lactase deficiency
ingestion non-absorbable solute
- sorbitol, aspartame, etc artificial sugar
2 consequences of osmotic diarrhea? is it life threatening?
- water depletion [not Na depletion] –> not life-threatening
- stool volume decreases with fasting
Mech of secretory diarrhea?
- stimulation of normal secretory process
- absorptive process intact but overwhelmed
2 consequences of secretory diarrhea? is it life threatening?
- salt and water depletion –> may be life-threatening
- stool volume persists despite fasting = very large volume
What examples of things that cause secretory diarrhea?
secretagogues that increase cAMP/cGMP/Ca
- bacterial toxins [cholera]
- hormones [VIP]
- bile acids [ileal resection, bacterial overgrowth]
- drugs
- inflammatory mediators [histamine, bradykinin, serotonin, IL-1]
Mech of cholera induced diarrhea?
- secretory diarrhea
- cholera toxin increases cAMP
- — increases Cl channel into lumen
- — inhibits Na/H exchanger [which reabsorbs Na]
What are two types of motility diarrhea?
hypermotility, hypomotility
What is cause of hypermotility diarrhea? 4 examples?
- insufficient contact time for absorption
- hyperthyroidism, cholinergics, laxatives, anxiety
What is cause of hypomotility diarrhea?
- altered peristalsis, stasis, bacterial overgrowth
- impaired innervation [diabetes, IBS, vagotomy], any cause of stasis or bacterial overgrowth