Lecture 4: Tubular Transport Flashcards
why is the rate of reabsorption and secretion of substances variable throughout the tubular system
b/c of differences in permeability and transporters / channels
what facilitates transport across tubular epithelial cells
an enlarged surface area on the apical side (brush border) and basolateral side (micropedici)
epithelial cells are connected to eachother by _____ ?
tight junctions. they limit paracellular transport
what portion of the tubule system reabsorbs 80% of water
Proximal tubule and descending limb of the LoH
what part of the tubular system has thick ascending limbs and reabsorbs 30% of the NaCl filtered by the glomerulus
Ascending limb of the LoH and early distal tubule
how do NaCl and K enter the cell
via NKCC-2
what is the function of the distal convoluted tubule
reabsorbing NaCl via NCC
what does the collecting duct have a role in
adjusting the final urine composition
where does glucose and amino acid reabsorption occur
the proximal tubule
what transporters are present in the apical membrane of the PT
sodium dependent transporters (SGLT1 & SGLT2) for glucose and EAAT3, SIT1 for amino acids
what type of transport is utilized in the reabsorption of glucose from the lumen of the nephron back into the cells of the PT
secondary active transport, linked to sodium ions
Once back inside the PT cells how does glucose diffuse into the interstitial space
via facilitated diffusion proteins located in the basal membrane of PT cells
what drives small glucose molecules to diffuse into peritubular capillaries
protein oncotic pressure
what is tubular maximum (mg/min)
the max rate of reabsorption that is reached when all membrane transport proteins are saturated
below tubular maximum, what happens to the filtered load
it is all reabsorbed
what happens to the filtered load when TM is exceeded
it is excreted
where are tubular maximum primarily found
PT
where does the reabsorption of bicarbonate take place
PT
CO2 can move into epithelial cells via what methods
via transmembrane diffusion or AQP1
where are sodium dependent transporters (NHE3) that remove protons during the reabsorption of bicarbonate found
in the apical membrane of the proximal tubule
what transporters are present in the proximal tubule that help effectively reabsorb glucose?
SGLT1
what is the difference b/w carrier-mediated transport of glucose vs simple diffusion of glucose
carrier-mediated transport can max out (b/c it is sodium-dependent) whereas transport by simple diffusion doesn’t really max out, but is somewhat dependent on available SA
why is glucose usually not present in urine
b/c transport rate is not maxed out
– as long as max rate (TM) isn’t reached, glucose is able to be fully reabsorbed
what happens when tubular max is reached
glucose appears in urine
