solute handeling

Question 1

explain the routes of solute transport

Answer 1

1. paracellular transport
   
   1. solutes and fluid move between tubular epithelial cells
2. transcellular transport
   
   1. solutes and fluid move through tubular epithelial cells by first crossing the apical and then basolateral membrane

Question 2

explain the movement that governs paracellular transport

Answer 2

1. solutes and fluid movmeent governed by
   
   1. permeability of the tight junctions
      
      1. “leakiness” decreases from proximal tubule to collecting duct due to differences in claudin isotypes.
      2. this pretains ions and water
   2. transepithelial chemical gradient
   3. transepithelial electrical gradient
   4. solvent drag
      
      1. water flow sweeps dissolved soluttes
      2. it tcan be againstt electromottive force

Question 3

what are the driving forces for flow in the nephron?

Answer 3

1. concentration gradient between the nephron an. interstitium
   
   1. we care about the difference between transepithelial (between. tight junctions)
2. electrical gradient
   
   1. contributtes to the driving force
3. solvent drag
   
   1. movement of water, bulk movement

Question 4

draw and diagram the following and explain:

1. PAH, creatinine, insulin, urea, Cl-, K+, Na+, HCO3, glucose, amino acids

Answer 4

1. the degree of concentration in the nephron fora. given substance is not uniform across the nephron
2. the degree of concentration in the nephron between solutes is not the same

Question 5

how much of the following is filtered and secreted per day?

H2o, Na+, Cl-

Answer 5

• water
  
  • 180L/day
  • 1.5lters
• Na+
  
  • 25200mEq/day
  • 150
• Cl-
  
  • 18000mEq/day
  • 150

Question 6

How can the two types of Na+ transport generate a gradient across the cell membrane from the lumen-cell-interstitial?

Answer 6

Transcellular and paracellular sodium transport allow for sodium to move into the cell passively, by pumping it into the interstitium. this gradient/flow is used to transport other compounds and ions back into the cells.

1. transcellular
   
   1. passive Na+ transport in apical border (near lumen)
   2. driven by chemical gradient and transmembran potential
   3. Na/K pump onbasolateral side pumps the Na out into the interstitum
2. paracellular
   
   1. driven by electrochemical gradient
      
      1. also moved by solvent drag, water moves
   2. tight junction differes between segments
   3. generally, decrease conductance with distance along nephron
      
      1. leaky in some locations

Question 7

percentage of H2O uptake in the following components

1. PT
2. Loop
3. DCT
4. CCT

Answer 7

1. no reabsorption in the tDLH
2. once fluid passes throug hduct of bellini it is no longer modified = urine

Question 8

discriminate the lumenal charge difference between the early PT and late PT

Answer 8

1. early PT
   
   1. -3mV
   2. Na leaks back in to the lumen
   3. paracellular
2. late PT
   
   1. +3mV
   2. Na favors movment into the cells
   3. transcellular

Question 9

diagram the PT absorption fraction

Answer 9

1. most Na absorption occurs in PT
2. ultrafiltrate is isoosmotic through out the entire PT length. However, chemical composition varies from plasma as we move away from the corpuscle
3. luminal sodium increases slightly

Question 10

what are two important apical membrane transporters in the early PT?

Answer 10

NHE3- symporter

SGLT2 - symporter

Question 11

what is an important Na transporter on the basolateral membrane of the early PT?

Answer 11

Na/K ATPase establishes gradient

NBC1 pumps HCO3 into the interstiuim

Question 12

describe the paracellular Na transport in the early PT.

Why does the Na+ in the lumen remain unchanged?

Answer 12

1/3 of Na leaks back into the lumen paracellularly

1. important to transport solutes and glucose accross

Because water moves along with Na+, [Na]lumen remains unchanged (absolute amount through decreases)

Question 13

describe Na movement in the late PT

Answer 13

1. transcellular
   
   1. movement via NHE3
2. paracellular
   
   1. route into the interstitum
   2. electromotive force
      
      1. the lumen is positive

Question 14

describe cloride transport in the early PT

Answer 14

1. The lumen is negative
2. paracellular into interstitum
   
   1. EMF
   2. solvent drag
3. however movment is limited and anion reabsorption is dominated by HCO3

Question 15

describe Cl movement in the late PT

Answer 15

1. transcellular
   
   1. dominant route
   2. CFEX
      
      1. base/Cl- exchanger
2. paracellular
   
   1. favorable gradient but is countered by EMF
   2. Cl-channel
   3. Cl/K - symporter

Question 16

how much of the water is absorbed in the PT?

Answer 16

67% of filtered load absorbed in PT

driven by solute absorption

1. paracellular
   
   1. leaky tight junctions
2. trancellular
   
   1. aquaporins

Question 17

describe the movement of solutes and water in the tTDL

Answer 17

1. NO MOVEMENT OF Na or Cl in this section of the nephron
2. lack of permeability is critical for concentrating urine
3. no movement on the descending

Question 18

describe the flow of solutes and water in the tALH

Answer 18

1. in the tDLH, NaCl is concentrated since water has moved out
2. the high luminal [NaCl] opwers reabsorption in tALH PARACELLULAR passive movement

Question 19

describe the movement of solutes and water in the TAL

Answer 19

transcellular NaCl transport in TAL

1. NKCC2
   
   1. secondary active electro neutral symporter
   2. critical in establishing the medullary gradient created by Na and Cl
2. NHE3
   
   1. par of acid handeling
3. Cl- channel
4. Cl/K symporter

Question 20

critical in establishing the medullary gradient created by Na and Cl

Answer 20

NKCC2

secondary active electro neutral symporter that is critical in establishing the medullary gradient created by Na and Cl

Question 21

identify the following

Answer 21

Question 22

what affect does the NKCC2 have on the lumen?

how does this affect paracellular movment?

Answer 22

TAL

NKCC2 is electro neutral, but when combined with the action of K, an electrogenic system results = generating a positive charge in the lumen.

paracellular movement is driven by the EMF despite the chemical gradient that favors movement into the lumen- drawing in Na, Ca, Mg

Question 23

describe solute and water transport in the TAL

Answer 23

Question 24

describe solute and water movment in the DCT

Answer 24

1. no paracellular movement
2. transcellular is via Na/Cl cotransporter
3. Cl export is via Cl channel

Question 25

Special cells found in the collecting tubule/duct

Answer 25

intercalated cells -alph/beta

principal cells- aldosterone mediated, ENaC

Question 26

describe solute and water movement through the collecting duct

Answer 26

1. Principal cells
   
   1. ENaC- SODIUM
      
      1. Na channel
   2. generates negative lumen charge, offset by K+ channel ROMK
   3. Na/K ATPase generating Na gradient
2. intercalated cells
   
   1. pendrin(Cl/HCO3 exchanger)
      
      1. Beta-apical side
      2. alpha/principal = not involved in Cl transport
   2. Cl channel
      
      1. basolateral side

Question 27

Cl movment in the Collecting duct is accomplished by two routes. explain

Answer 27

1. paracellular
   
   1. driven by tranepithelial large electrical gradient
      
      1. lumen is negative
2. trancellular -ONLY ON BETA INTERCALATED CELLS
   
   1. apical side
      
      1. pendrin- Cl/HCO3 exchanger
   2. basolateral side
      
      1. Cl channel