Renal Transport Mechanisms Flashcards
What are the 5 barriers a substance must cross to be absorbed? (transepithelial transport)
leave tubular fluid –> through cell of tubular epithelium –> basolateral membrane –> interstitial fluid –> through capillary wall
Where are glucose and AAs reabsorbed?
How much?
100% reabsorbed in proximal convoluted tubule
What places is sodium reabsorbed?
65-70% in PCT 25% in thick ascending limb of LoH 5% DCT 3% collecting duct <1% bladder
What is reabsorbed in the PCT?
CHAMPSPUG calcium = 70% H2O = 70% Amino acids = 100% magnesium = 30% potassium = 70% sodium = 70% phosphate = 70% urea = 50% glucose = 100%
What things are absorbed 70% in the PCT?
SPPCH sodium phosphate potassium calcium H2O
How much Urea is reabsorbed in the PCT? What about other places?
50% reabsorbed in PCT
variable amts in DCT, collecting duct, and bladder
In general, what does the PCT reabsorb?
67% of filtered water and solutes
What is the key element in PCT reabsorption?
Na/K ATPase pump in the basolateral membrane
reabsorption of every substance is linked in some way to it
What is special about PCT reabsorption in regard to water?
PCT is freely permeable to water
solutes filtered –> osmosis occurs
isosmotic reabsorption = solutes and water absorbed at same rate = 300 mOsm
How does the Na/K ATPase pump help transport Na?
pump pushes 3 Na out to interstitial fluid –> lowers intracellular Na –> Na from tubule comes into cell down its gradient via sodium ion leak channels
overall = net movement from lumen to IF (and eventually back to blood)
What is transcellular transport?
What is paracellular route?
through the cells
in between cells (limited bc tight junctions
What does the sodium/hydrogen exchanger do?
found on apical membrane of PCT
pull in one sodium ion into the cell in exchange for moving one H+ out into the lumen
How is chloride reabsorbed?
more water than Cl- reabsorbed in first half of PCT –> Cl concentration rises in tubular fluid as it goes on –> provides concentration gradient –> passively goes back to blood via paracellular mvnt
What type of transport does chloride use?
paracellular mvnt is the main way
Where does water move paracellularly?
paracellular = thin descending LoH bc few tight jxns
in thick ascending LoH, way more tight jxns, so won’t go paracellularly
Where is aquaporin-1 present?
in the PCT
Where is aquaporin-2 present?
in collecting duct (under control of ADH)
Where is water absorbed throughout the nephron?
PCT = 67% (passive through aquaporins)
LoH - 15% (in descending only, via paracellular and aquaporins)
Early Distal tubule = 0%
late distal tubule and collecting duct = 8-17% (via aquaporins)
Where do ADH, ANP, and BNP regulate water reabsorption?
in late distal tubule and collecting duct via aquaporin II
What glucose transporters are located in the proximal PCT?
How much glucose do they reabsorb?
SGLT2 and GLUT2 = high capacity, low affinity
90%
What glucose transporters are located in the distal PCT?
how much glucose do they reabsorb?
SGLT1 and GLUT1 = low capacity, high affinity
10%
What is the “novel approach to type 2 diabetes treatment” mentioned?
inhibiting renal SGLT2 –> reduces blood glucose levels by decreasing glucose reabsorption –> just pee it out
What is TmG?
transport maximum for glucose = 375 mg/min
SGLT1 and 2 will absorb filtrate glucose until all receptor sites are full –> anything above Tm is not reabsorbed and escapes into the urine
What occurs in the thin descending loop of henle?
H2O permeable –> absorption of 25% of filtered H2O
impermeable to solutes
water leaves behind solutes –> increasing concentration in tubular fluid = increased osmolarity
What occurs in the thick ascending loop of henle?
35-40% of filtered NaCl resorbed here
in contrast to descending limb, solute reabsorbed w/out water
How does the countercurrent multiplier work in the LoH?
water only reabsorbed in descending –> super concentrated lumen fluid at the bottom –> solutes only reabsorbed in ascending –> gets less concentrated at the top
How do loop diuretics (furosemide) work?
inhibit sodium chloride reabsorption by competing for the Cl- binding site on the carrier
What is reabsorbed in theDCT?
sodium = 5%
calcium = 8%
magnesium = 8%
H2O and urea variable
Where is the NKCC transporter found?
What does it do?
found in thick ascending LoH
takes 1 Na, 1 K, and 2Cl from lumen into endothelial cells
What 5 transporters are utilized in the thick ascending LoH?
NKCC –> into lumen cells
apical K+ channel –> K back into lumen
Na/K ATPase –> 2K into cell from IF, 3Na to IF
Cl- channel –> into IF/blood
basal K+ channel –> K into IF down its gradient
What type of transporter is the Na/Cl cotransporter, and where is it?
symporter
located in DCT
pulls in both Na and Cl from the lumen into the DCT cells
When is aldosterone released, and what cells does it act upon?
released from adrenal cortex in response to angiotensin II or directly in response to increased plasma K+ –> acts on collecting ducts and DCT –> increased Na resorption and more secretion of excess K+
How does Na reabsorb in the late DCT and collecting duct?
Na channel (alpha, beta, etc)
How does sodium cross the apical membrane in the PCT?
Na-H andtiporter Na-glucose symporter (SLGT2) Na-symporter w/ AAs Cl/base antiporter paracellular
How does K+ sparing spironolactone work?
inhibits Na+/K+ exchange in distal tubule and collecting duct –> promotes K+ retention and Na+ and water loss through urine = hypotensive effect
How does ADH work?
makes distal and collecting tubules more permeable to water –> medullary osmotic gradient can act upon more dilute tubular fluid –> reabsorb more water
What happens if you drink a lot of water and need to lose the excess H2O without losing solutes?
Dont have any ADH secreted –> 20% of filtered fluid that reaches the distal tubule is not reabsorbed bc impermeable to water –> excretion of wastes remains constant –> dilute urine
