B5.020 Prework 1 Water and Electrolyte Absorption and Secretion Flashcards
when does diarrhea manifest
both the small intestine and colon have a large reserve capacity for reabsorption, diarrhea manifests when this is exceeded
intracellular tight junctions
restrict passive flow across the intestinal epithelium in response to established osmotic gradients
function of transcellular proteins
workhorse of intestinal luminal activity
move molecules and water through cells
may work against electrochemical gradients (require energy)
subject to transcriptional and post-transcriptional regulation
active transport
requires ATP for movement of molecules across the membrane
primary active transport
the ATP driven Na+/K+ stores energy by creating a steep concentration gradient for Na+ entry into the cell
secondary active transport
as Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell
paracellular trans-epithelial transport
movement of solutes and water through tight junctions
dictated primarily by electrochemical gradient
3 key points surrounding the process of trans epithelial transport
- membrane transport proteins drive transcellular transport of ions
- transport of ions sets up electrochemical gradients
- electrochemical gradients allow paracellular transport of fluid through tight junctions
differences between small intestine and colon absorption functions
small intestine: tight junctions more permeable, larger passive flux of fluids, absorption of nutrients, vitamins, minerals, salt and water
colon: tight junctions less permeable, only absorption of sodium and water favored
effect of motility on absorption/secretion
motility/transit time increased: less time for absorption across epithelium
motility/transit time decreased: more time for absorption across epithelium
function of villi
absorption
fluid absorption primarily depends on Na+ transport
Na+ may be coupled with Cl-, nutrients, bile acids, and other solutes
function of crypts
secretion
primarily follows Cl- and bicarb
small intestine ion transport mechanisms
bicarb secretion
electroneutral NaCl absorption
chloride secretion
small intestine nutrient/mineral/other transport mechanisms
Na+ coupled nutrient absorption H+ coupled nutrient absorption N+ coupled bile acid absorption Ca2+ absorption Fe absorption
colon ion transport mechanisms
Na+ absorption
K+ secretion and absorption
Cl- secretion
short chain fatty acid absorption
mediators that regulate transport mechanisms in diarrhea
infection inflammation gut hormones chemical mediators enteric nervous system
action of Na+/K+ ATPase
on basolateral side of enterocytes
actively drives sodium out of cell and potassium in at a ratio of 3:2
creates a Na+ electrochemical gradient between enterocyte and lumen
glucose/AA transport
fostered by the electrochemical gradient established by Na+/K+ ATPase
proteins along apical aspect of enterocyte utilize Na+ transport to facilitate absorption of glucose and AAs
SGLT1 transporter in context of diarrhea
preserved function
allows oral rehydration using glucose coupled sodium absorption to promote fluid absorption by coupling sodium with glucose in solution
NaCl transport
NaCl is absorbed in conjunction with export of H+ and bicarb
relies on Na+/K+ ATPase to establish gradient
Na+/H+ cation exchanger works in conjunction with Cl-/HCO3- exchanger allowing NaCl reabsorption
KCC1
co transport of Cl- and K+
CFTR function
coupled import of Na, K, and Cl in basolateral membrane
as intracellular Cl increases, Cl is secreted via apical CFTR
CFTR expression highest in ileum and colon
relevant to secretory diarrhea
osmotic diarrhea
solute driven water losses, more prominent in colon
non absorbed solutes create intraluminal concentration gradients that draw in fluid into the lumen and promote fluid loss
carb malabsorption common example
secretory diarrhea
crypt secretion leads to more prominent small intestine losses
due to dysregulation of intestinal ion transport which leads to imbalance of ion secretion/absorption
favors secretion, water follows