Week 4 Volume Control

Question 1

what percentage of Na and H2O is reabsorbed in the descending limb of the loop on henle?

Answer 1

Na- 0%
H2O- 10-15%
creating hyperosmotic ultra filtrate at the bottom of the loop
- countercurrent multiplication

Question 2

what is counter current multiplication?

Answer 2

Descending limb contains aquaporin channels but no Na/K/Cl transport
thick ascending limb contains No aquaporin channels but Na/K/Cl cotransporters

Na, Cl-,K leave lumen in thick aascending limb creating an interstitial fluid that is hyperosmotic = this causes H2O to leave into the interstitial space from the descending limb as this is the only place where aquaporin channels are present.
once equilibrium met between descending and interstitial space creates a hyperosmotic fluid in lumen which passes through tip of loop which is increasingly concentrated and then into the thin ascending limb where salts passively moving down conc grad back out and then into the thick ascending limb has Na/K/Cl channels which actely pump ions out due to H2O not being able to enter as no aquaporin channels, = increases osmolarity in interstitium and full circle

Question 3

what percentage of Na and H2O are reabsorbed in the Ascending limp of the loop of henle?

Answer 3

Na- 25%

H2O- 0%

Question 4

what percentage of Na and H2O is reabsorbed in the DCT?

Answer 4

Na- 5%

H2O- 0%

Question 5

what percentage of Na and H2O is reabsorbed in the collecting duct?

Answer 5

Na- 3%

H2O-5% dependant on intake of fluids

Question 6

explain the reabsorption and distribution of channels in S1 of the PCT?

Answer 6

Na/K ATPase on the basolateral membrane sets up a grad for Na to move from the lumen into the cell dragging glucose with it via a symporter
the glucose will continue down its conc gradient into the interstitial space and then into peritubular capillaries
the 3Na that has been moved into the interstitial space also moves along conc gradient to capillary
K leaks back out of cell
- also a Na/H exchanger which pumps H out of cell therefore setting up gradient to allow organic cation antiporter
- AA also lost
- Conc of urea and Cl increases compensating for loss of glucose and AAs- provides conc gradient for Cl reabsorption in S2

Question 7

what happens in the PCT at S2/3?

Answer 7

basolateral 3Na/2K ATPase which pull chloride through to capillary from lumen because of Na,
Cl also moves paracellularly due to conc gradient
- no SGLUT symporter so Na/H exchanger moves Na out of lumen for ATPase pump
- H2O wants to move into capillary due to high oncotic pressure in peritubular capillaries

Question 8

what is the function of the glomerulotubular balance system?

Answer 8

second line of defence- blunts Na excretion response to any GFR changes that occur despite autoregulation
- always reabsorb 67% of whatever GFR brings in rather than a set conc.

Question 9

along with NaKCC2 channel in thick ascending limb what other channels are important on both apical and basolateral sides?

Answer 9

on apical- 
- NaKCC2 which drives 2Cl, 1K and 1Na into cell 
- ROMK allowing K back into lumen
on basolateral-
- Cl channels into capillary 
- Na/K ATPase which drives all

Question 10

why is the location of ROMK in the thick ascending limb important?

Answer 10

drives K out of cell into lumen creating K gradient for NaKCC2
- if didnt NaKCC2 would not work

Question 11

which part of nephron uses most energy?

Answer 11

thick ascending limb of loop of henle

- most sensitive to hypoxia

Question 12

The fluid leaving the loop of henle is what?

Answer 12

hypo-osmotic compared to plasma because all of the solutes are pumped out in thick ascending limb

Question 13

explain the different transporters in the DCT?

Answer 13

Na/K ATPase drives Na/Cl (NCC) transporter on apical membrane - Cl- channel on basolateral membrane
- important site for Ca reabsorption- NCX- Na Ca exchanger brings Na into cell and Ca into capillary coupled to a proposed Ca channel on apical membrane bringing Ca into cell - affected by PTH

filtrate therefore further diluted- more hypo-osmotic

Question 14

explain what happens in late DCT and CD?

Answer 14

fine tuning of filtrate
2 cell types
- principle- 70%, responsible for reabsorption of Na via ENaC
- type B intercalated cells- reabsorb Cl- actively and secrete H and HCO3-

also a K channel on apical membrane which moves K into Lumen

again all driven by Na/K ATPase

variable H2O uptake through aquaporin dependant on ADH

Question 15

what percentage of Na and H2O is reabsorbed in the PCT?

Answer 15

67% NA
65% H20
= isosmotic reabsorption