Lec 5 Small Intestine Diarrhea Flashcards
How is water absorbed in small intestine?
passively following active absorption of solute
What is main ion of absorption?
Na
What is main ion of secretion?
Cl
What percent of water in gut lumen is normally absorbed?
95%
What are folds of kerkring?
folds in small intestine
Where are stem cells located in small bowel epithelium?
crypts contain stem cell compartment
What is role of paneth cells?
secrete lysosymes and defensins to protect stem cell compartment
located at base of crypts
Where are paneth cells located?
only in small intestine – not stomach or colon
What 3 factors of small intestine increase its surface area?
- folds of kerkring [3x]
- villi [10x]
- microvilli [20x]
What is surface area of small intestine?
200 m^2
How much water do we intake every day?
2 L
How much fluid presented to small intestine each day?
8 L (7-9) to small intestine
How much fluid presented to colon each day?
1.5 L (1.5-2)
How much feces excreted each day?
100-200 mL
What is absorption efficiency of small intestine?
75-80%
What is absorption efficiency of colon?
> 90%
What is max absorption capacity of small intestine?
12 L/day
What is max absorption of colon?
5 L/day
Is paracellular transport active or passive?
always passive = due to electrochemical gradient
Which part of bowel has highest passive paracellular permeability?
highest permeability in jejunum = very leaky
lower in ileum, lowest in colon
more distal = pores are smaller and tighter
What are the 3 mechs by which Na is transported?
- Na channel: passive transport into cell down electrochemical gradient
- Glucose or AA coupled Na co-transport into cell
- Na-H exchanger: Na into cell in exchange for H
Na once in cell exits basolaterally by Na-K ATPase
How is electric neutrality kept in the Na-H exchange path?
Na into cell in exchange for H out of cell
driving force for HCO3-Cl exhcnager: Cl into cell in exchange for HCO3 secretion to maintain neutrality
Is potassium secreted or absorbed in gut?
secretion more than absorption
How is K secreted in small intestine?
secreted passively secondary to lumen-negative potential created by Na absoprtion
How is Cl primarily absorbed in gut?
follows electrochemical gradient created by Na transport
What are 2 main ways to stimulate absorption?
- enhance Na absorption directly
- slow intestinal transit
What are 4 things that increase Na absorption?
- mineralocorticoirds (colon more than small intestine)
- glucocorticoids
- somatostain [octreotide]
- adrenergic agonists [epinephrine, clonidine]
What is effect of somatostatin on gut?
can act as hormone
- enhances Na absorption
- slows intestinal transit
–> decrease diarrhea
What is effect of clonidine on gut?
a2 agonist –> increases Na absorption –> decrease diarrhea
What are 2 drugs [or types] that slow intestinal transit?
- opiates
- somatostatin [octreotide]
Mech of Cl secretion?
- enters basolaterally via Na:K:2Cl transporter and exits apical membrane via Cl channel {CFTR]
theres is a basal rate of Cl secretion
What things increase Cl secretion?
anything that activates cAMP, cGMP, or intracellular Ca
What is mech by which yersinia, Campylobacter, some types of E Coli, and cholera cause diarrhea?
by increasing cAMP –> increase secretions
What is effect of bile acids in colon?
increase cAMP and Ca and cause secretory diarrhea
What is normal transit time through small bowel?
2-6 hours
What are some factors that increase colonic transit?
- cholinergics
- anxiety
- feeding
- laxative
- distension
What are some factors that slow colonic transit?
- anti cholinergics
- depression
- opiates
What is stool weight/water for definitive diarrhea?
> 150-200 g stool/24 hr
> 150-200 mL stool/24 hr
What defines acute vs chronic diarrhea?
acute < 2-3 wks; usually self limited infectious cause
chronic > 3 wks
What type of motility in small bowel in fed state?
2 types:
segmental non-propulsive contractions: mix food, retard passage
peristalsis: short waves with proximal contraction and distal relaxation
What type of motility in small bowel in fasting state?
cyclic stripping waves to clear contents and prevent bacterial overgrowth = MMC
What are characteristics of small bowel diarrhea?
- high volume [b/c more volume overwhelming colon]
- moderate increase in number of bowel movements
- minimal urgency
- no tenesmus
- little mucus
What are characteristics of colonic diarrhea?
- low volume [b/c normal volume entering colon]
- frequency
- urgency
- tenesmus
- mucus
- blood
A problem with which part of bowel will cause bile acid induced diarrhea?
ileal dysfunction – b/c ileum is where bile acids normally absorbed
What is pathogenesis behind osmotic diarrhea?
- non-absorbable solute in bowel lumen –> draws water in
- mucosal transport processes usually intact
What are some solutes that commonly cause osmotic diarrhea?
- lactose [if lactase deficient]
- sorbitol [in chewing gum]
- minerals like magnesium
What are charateristics of osmotic diarrhea?
- water (not Na) depletion –> not life threatening
- stool volume decreases with fasting
- osmotic gap in fecal fluid
- acidic stool pH [b/c bacteria ferment the solute]
What is mech of secretory diarrhea?
excess stimulation of normal secretory processes
What are some examples of things that cause secretory diarrhea?
- bacterial toxins [cholera]
- hormones [VIP
- bile acids
- drugs [caffeine]
- inflammatory mediators [histamine, bradykinin, eiconsanoids, 5HT]
What is mech of cholera?
cholera toxin enters enterocyte and causes increase in cAMP –> drives Cl out of cell –> secretory diarrhea
What are characteristics of secretory diarrhea?
- salt and water depletion –> may be life threatening
- stool volume persists despite fasting
- no osmotic gap
- stool pH neutral
What causes hypermotility diarrhea?
due to insufficient absorption time
- hyperthyroidism, cholinergics, laxatives, anxiety
What causes hypomotility diarrhea?
altered peristalsis and stasis leading to bacterial overgrowth
due to impaired innervation as in DM or any other cause of stasis or bacterial overgrowth
