renal 2

Question 1

tubular reabsorption: describe where water reabsorption occurs in nephron

Answer 1

• nothing (water of na) reabsorbed in Bowman’s capsule
• by end of proximal tubule, 75% of filtered water gets reabsorbed
• loop of Henle, distal tubule, and collecting duct reabsorbs some water but not a lot

Question 2

what is the primary location of water reabsorption of the filtrate in nephron

Answer 2

proximal convoluted tubule

Question 3

where does sodium reabsorption occur in nephron

Answer 3

bulk is in proximal tubule (mechanisms that take place in proximal tubule run at their max efficiency, like a sponge for water and solutes, soaks up immediately)

2nd is in loop of Henle (very very small amts in distal tubule and collecting duct)

Question 4

in an electron micrograph of a proximal tubule cell, inside of tubule system called ___, fluid that passes thru is ___

bottom of cell comes into contact with ___, called ___ part of cell

Answer 4

lumen, luminal fluid
paratubular capillaries, basal

Question 5

proximal tubule cells are well adapted for transport b/c they have lots of ____ called ___ that project into luminal fluid, they are so abundant, apical surface also called ___

what function do these have

Answer 5

finger-like projections, microvilli
brush border
they increase the surface area of the apical area and function to make reabsorption across apical membrane more efficient

Question 6

proximal tubule cells are packed with lots of ___ which provide energy needed to run transport processes

Answer 6

mitochondria

Question 7

proximal tubule cells that are adjacent to each other are connected by a ___ b/w them, typically these are not permeable to water and solutes, however…

Answer 7

tight junction
there are exceptions

Question 8

there are many mechanisms for the reabsorption of sodium in the nephron, the specific mechanism in operation depends on ____

Answer 8

location in the tubular system

Question 9

primary driver of sodium reabsorption is the ___ located in the ___

Answer 9

sodium pump (Na/K ATPase)
basal membrane

Question 10

the sodium pump that functions in sodium reabsorption makes the intracellular concentration of sodium very ___…

Answer 10

low, high sodium in luminal fluid makes a favorable gradient for reabsorption of sodium across apical brush border membrane

Question 11

in early section of proximal tubule, first step in sodium reabsorption is ____ in the brush border

Answer 11

sodium/proton exchange protein
sodium diffusing down conc gradient in exchange for protons being pushed out - sodium grabbed by sodium pump, diffuses into capillaries and is carried away by blood of cap system

Question 12

in addition to the sodium pump being the primary driver of sodium reabsorption in the early proximal tubule, what are 2 other mechanisms?

Answer 12

sodium/glucose cotransport across apical membrane

sodium/amino acid cotransporter

Question 13

in the late/back half of proximal tubule, in addition to the sodium/proton exchanger and the sodium pump, now another mechanism…

Answer 13

sodium/chloride diffusing thru tight junctions b/w cells and being passively reabsorbed paracellularly

Cl ion can be reabsorbed by following the passive movement and electrical gradient of sodium (since Cl is neg charged, follows across cell & into blood)
- water also follows the movement of these ions passively (reabsorbed by osmosis by following osmotic gradient created by movement of Na and Cl)

Question 14

where in the loop of Henle does sodium reabsorption occur?

Answer 14

no Na reabsorption in descending limb (not permeable to Na)

ascending limb has thick & thin section

Question 15

Na reabsorption in the thin section of loop of Henle is ___…

Answer 15

passive, moves down conc gradient thru an epithelial Na channel, this channel is basically just a hole in the membrane, non-gated

Question 16

Na reabsorption in the thick section of loop of Henle involves…

Answer 16

Na & Cl reabsorption involves a combination of transport mechanisms

• at basal membrane, a sodium pump (active component that favors movement from lumen and into cell)
• in apical membrane, a Na/K/2Cl cotransport (large transmembrane protein, has 4 binding sites- 1 for Na, 1 for K, 2 for Cl), all bind to outside of transporter, transporter changes shape and pushed all 4 of these ions thru the membrane –> Cl follows passively thru a passive Cl channel

Question 17

sodium reabsorption in the distal tubule and collecting duct:

in early part of distal tubule: ___ (gradient established from lumen to cell by ____)

later part of distal tubule: still have sodium pump, now have _____ also

in various parts of distal tubule and collecting duct, ___

Answer 17

Na/Cl cotransport (basal sodium ATPase)

epithelial sodium channel (EAC channel)

sodium/proton exchanger

Question 18

what is the common feature of sodium reabsorption in the nephron?

Answer 18

basal sodium pump all across nephron (provides energy that creates Na gradient that allows reabsorption to occur in the first place)

Question 19

regulation of sodium reabsorption is by this hormone

what does it target

Answer 19

aldosterone

when BP goes down & blood sodium goes up –> sensed by kidney and triggers synthesis and release of aldosterone by adrenal cortex –> aldosterone then acts on the kidney, specifically, on the thick ascending limb of loop of Henle, distal tubule, and collecting duct (proximal tubule already running at its max)

Question 20

aldosterone does 2 things:

1- increases expression of ___ in the basal membrane, which increases the ___ across apical membrane and stimulates reabsorption

2- increases insertion of ___ in apical membrane, which increases ___ –> big increase in Na reabsorption

Answer 20

sodium pump, gradient
EAC (epithelial sodium channels), apical permeability

the more sodium reabsorbed, the more water follows by osmosis –> increases blood volume –> raises BP back to normal

Question 21

100% of the glucose that gets reabsorbed is done in the ___, zero glucose excreted in urine of healthy kidney

Answer 21

proximal convoluted tubule

Question 22

glucose gets reabsorbed across apical membrane in proximal tubule by this large transmembrane protein, ___

2 isoforms:

Answer 22

Na/glucose (co)transporter
SGLT type I (located in back/late end proximal tubule)
SGLT type II (located in front/early end of tubule)

both isoforms work the same way

Question 23

Na/glucose (co)transporter 2 isoforms that reabsorb glucose in proximal tubule:

how does it work

Answer 23

glucose is moved across membrane by sodium cotransport (conc of glucose inside cell is higher than conc of glucose in lumen) –> glucose being transported against conc gradient, active transport (required expenditure of energy on part of the cell), however, sodium is moving down a very large conc gradient which was set up by sodium pump –> so it’s the sodium pump that directly splits ATP and sets up gradient –> huge amt of potential energy stored in gradient that is dissipated as sodium moves downhill across apical membrane –> as PE dissipated, it pulls glucose in against its gradient

it’s an indirect form of active transport called secondary active transport

Question 24

glucose transport displays what kind of kinetics?

Answer 24

just like how enzymes run faster the more substrate there is up t a point where all active sites are saturated, then runs at its max velocity….glucose transport works same way – instead of active site, has binding site for glucose, transport runs faster the more glucose there is until all binding sites are saturated - called the Tmax or transport maximum of glucose

from 0-20 mmol, fairly linear increase (when hit 20mmol, hit transport maximum, glucose reabsorption stays at its max rate, excess glucose is lost in urine)

Question 25

where and how are amino acids reabsorbed?

Answer 25

in proximal tubule
sodium/amino acid cotransporter

AA conc inside cell is higher than in lumen, amino acids moved against gradient –> again, it’s downward movement of soium that powers the AA transport – AA absorption across apical membrane is also by secondary active transport (AA transporters are specific groups of AAs, have about 4 or 5 diff ones, they work on similar structured AAs)

Question 26

how is bicarbonate reabsorbed?

Answer 26

bicarbonate helps adjust pH of blood, it is relatively big and neg charged, so will not diffuse (not that many bicarbonate transporters)

sodium/proton exchangers (plenty of protons in luminal fluid), bicarbonate and protons can chemically react to form CO2 and water (this rxn is slow), so there is an isoform carbonic anhydrase 4 on the burhs border microvilli exposed to luminal fluid –> catalyzes rxn very rapidly –> CO2 is very lipid soluble and diffuses across brush border into cell (first step is conversation into CO2 and diffusion) –> once across cell, soluble form of carbonic anhydrase (converts CO2 back into bicarbonate and protons) –> diffuse across basal membrane thru large channels, diffuse back into blood