4- Na regulation in the nephron

Question 1

how much Na and water is reabsorbed at the PCT

Answer 1

67% Na and 65% H2O

Question 2

is ion reabsorption at the PCT isosmotic, hyperosmotic or hyposmotic

Answer 2

isosmotic, i.e. H2O follows Na so fluid being added to plasma from tubule does not change conc.

Question 3

what drives Na and H2O transport into epithelial cells at the PCT

Answer 3

basolateral Na/K ATPase - lowers epithelial cell [Na] by excreting 3Na for 2K

Question 4

which is Na and H2O transported across the epithelial barrier in PCT

Answer 4

1. Apical Na transporters
   - Na/H antiporter
   - Na/glucose symporter (SGLT2)
   - Na/AA co-transporter
2. Paracellular Na and H2O diffusion (through TJs) - high permeability PCT
3. AQP1 water channel

Question 5

what drives Na and H2O transport into peritubular capillaries at the PCT

Answer 5

1. osmotic gradient established by solute reabsoption, ie osmolarity in interstitial spaces increases
2. increased hydrostatic force in interstitium between tubular epithelial cell and peritubular capillary (from water mov from tubule)
3. increased oncotic force in peritubuar capillary due to loss of 20% filtrate at glomerulus but cells and proteins left in blood

Question 6

describe Na and water mov. at the descending limb of loop of Henle

Answer 6

• No tight junctions and many aquaporin channels (AQP1) so passive mov. of water (10-15%) via osmosis. Driven by increasing medullary interstitial [Na] caused by counter current system.
• No Na transporters so Na and Cl ions are concentrated in lumen, ready for active transport in ascending.

Question 7

is filtrate at the end of the descending limb of loop of Henle isosmotic, hyperosmotic or hyposmotic

Answer 7

hyperosmotic due to diffusion of water out by osmosis along descending limb

Question 8

describe Naand water mov. at the thin ascending limb of loop of Henle

Answer 8

• passive reabsorption of Na driven by conc. gradient

- no water mov.

Question 9

describe Na mov. at the thick ascending limb of loop of Henle

Answer 9

active Na transport via apical NKCC2 and basolatera Na/K ATPase

Question 10

what happens to the K and Cl transported into epithelial cells by NKCC2 at the TAL

Answer 10

• K diffuses back into tubule via ROMK (important to drive NKCC2 as low `[K] in filtrate here)
• Cl moves into interstitium/blood via Cl channel

Question 11

is filtrate at the end of the ascending limb of loop of Henle isosmotic, hyperosmotic or hyposmotic

Answer 11

hyposmotic due to active transport of Na (25%) and Cl into blood by NKCC2

Question 12

describe Na mov at the DCT

Answer 12

Reabsorption of 5-8% Na+ :
- DCT1 - via apical electroneutral NCC and basolateral Na/K ATPase

• DCT2 - via apical NCC and ENaC, and basolateral Na/K ATPase

Question 13

describe Na mov at the CT

Answer 13

via apical ENaC and basolateral Na/K ATPase of principal cells (intercalated cells don’t transport Na)