Fluid and Electrolyte Absorption DSA Flashcards
How much fluid do the small and large intestines absorb per day?
9 L/day
What causes diarrhea?
disturbances in the absorptive mechanisms that lead to excess fluid loss
What determines whether the fluid will move via paracellular or cellular route?
permeability of tight junctions
What is the resistance in the small intestine tight junctions and large intestine tight junctions?
small intestines: “leaky” tight junctions, low resistance permits PARACELLULAR movement
large intestines: “tight” tight junctions, high resistance permits CELLULAR movement
What is the major site for Na+ absorption?
jejunum
What transporters are on the apical membrane of the jejunum?
Na+ -monosaccharide cotransporters
Na+-amino acid cotransporters
Na+-H+ exchange
What is the source of H+ in the jejunum?
CO2+H2O H2CO3 H+ + HCO3-
How does Na+ leave the basolateral membrane in the jejunum?
Na+-K+ ATPase
What is the net electrolyte movement in the jejunum?
Na+ and HCO3- absorbed into blood
H+ excretion into lumen
What electrolytes are absorbed in the ileum?
Na+ and Cl-
What are the transport mechanisms on the apical membrane of the ileum?
Na+ -monosaccharide cotransporters (in)
Na+-amino acid cotransporters (in)
Na+(in)-H+(out) exchange
**HCO3- (out)/Cl- (in) exchange
What are the transport mechanisms on the basolateral membrane of the ileum?
Na+(to blood)/K+ (in) ATPase
Cl- transporter (to blood)
What is the net electrolyte movement of the ileum?
H+ and HCO3- secreted to lumen
Na+ and Cl- absorbed to blood
What channels are on the apical membrane of the colon?
Na+ channel (in) = absorption
K+ channel (out) = secretion
What induces the synthesis of Na+ channels?
aldosterone
What is the effect of aldosterone on colon epithelial cells?
increased synthesis of Na+ channels –> increased Na+ absorption –> increased K+ secretion due to increased Na+ pumped by the Na+ K+ ATPase
What is the effect of the high flow rate of intestinal fluid in the colon during diarrhea?
causes increased colonic K+ secretion, resulting in increased K+ loss in feces and HYPOkalemia.
What are the differences in function of the intestinal epithelial cells in the villi vs crypts?
villia - absorb fluid and electrolytes
crypts - secrete fluid and electrolytes
What transport mechanisms are on the apical membrane of epithelial cells in the crypts of the intestine?
Cl- channels
What are the transport mechanisms on the basolateral membrane of the epithelial cells in the crypts of the intestines?
Na+/K+ ATPase
Na+/K+/2Cl- cotransporter (in)
What is the net electrolyte movement in the crypts?
Cl- secreted cellularly through Cl- channel
Na+ and H2O secreted paracellularly following Cl- transport
Are the Cl- channels of the apical membrane of the crypts usually open or closed?
closed, except when responding to hormones and neurotransmitters from receptors on the basolateral membrane
What hormones or neurotransmitters affect the Cl- channels on the apical membrane in the crypts?
-mechanism
- ACh
- VIP
through adenylyl cyclase generating cAMP –> open Cl- channels
What happens in cholera?
- adenylyl cyclase is maximally stimulated
- Cl- channels in crypts are continuously open –> continuous secretion of NaCl and H2O
- fluid secretion by crypt cells overwhelms the absorptive capacity of the intestinal villar cells
- causes severe life-threatening diarrhea
What does diarrhea cause?
LO3
Circulatory collapse
- decreased ECF volume
- decreased intravascular volume
- decreased arterial pressure
Electrolyte (HCO3- and K+) loss
- hyperchloremic metabolic acidosis with normal anion gap (greater loss of HCO3- compared to Cl-)
- hypokalemia
What system attempts to restore BP?
renin-angiotensin II-aldosterone system
-attempts will be futile if the volume of fluid lost from GI tract is too great or too rapid
What are the main causes of diarrhea?
LO4
- decreased surface area for absorption: infection and inflammation of SI
- osmotic diarrhea: presence of nonabsorbable solutes in the lumen of intestins (lactase deficiency)
- secretory diarrhea: excessive secretion of fluid by crypt cells (E coli, cholera toxin)
How does lactase deficiency cause osmotic diarrhea?
LO 5
- lactose is not digested to glucose and galactose (absorbable forms)
- undigested lactose is not absobed and remains in lumen of intestine
- retains water by remaining in lumen
- bacteria in intestine may degrade lactose to more osmotically active solutes (compounding the problem)
How does cholera toxin cause secretory diarrhea?
LO6
- cholera toxin enters intestinal crypt cells by crossing apical membrane
- A subunit of cholera toxin moves to basolateral membrane where it catalyzes ADP ribosylation of alpha s of Gs protein coupled to adenylyl cyclase
- ADP ribosylation of alpha s inhibits GTPase activity (GTP cannot be converted to GDP), so adenylyl cyclase is permanently activated
- cAMP levels remain high. Cl- channels in apical membrane are kept open.
Cl- secretion followed by Na+ secretion followed by H2O secretion, overwhelms absorptive villi of the SI and colon.
What is a major advancement in treatment of diarrheal disease worldwide?
oral rehydration solution
-solution contains Na+, Cl-, and HCO3- which stimulate intestinal absorption of fluid and electrolytes
What is the absoption/secretion function of the duodenum?
bring chyme to osmotic equilibrium with plasma
- water absorbed from hypotonic solutions
- water secreted/Na+ and Cl- absorbed from hypertonic solution
