Renal Transport

Question 1

Describe the polarity of epithelial cells.

Answer 1

(functionally distinct sides)

• apical membrane
• basolateral membrane

Question 2

Where are tight junctions, and what do they do?

Answer 2

near apical surface

segregate membrane surfaces (and associated proteins) from one another – this determines function of epithelium (transport directionality)

Question 3

Where are there transepithelial potential differences?

Answer 3

across peritubular/interstitial space (0 mv) and tubular lumen

• interstitial space is always used as the references (0)
• tubular lumen can be electronegative, neutral, or electropositive – has implications for ECs in terms of what types of transport is being favoured

Question 4

What is the brush border?

Answer 4

membrane folding and villi on renal epithelial cells that amplify/enhance the apical surface area

• greater SA for transport to occur across
• especially important in proximal tubule, where bulk of transport occurs

Question 5

Na+ Reabsorption

Where does the bulk of Na+ reabsorption occur?

Answer 5

proximal tubule

~2/3 reabsorbed by the time we reach the end of PT

Question 6

Na+ Reabsorption

Where does regulation of Na+ reabsorption occur?

Answer 6

collecting duct

Question 7

Na+ Reabsorption

What is the filtered load?

Answer 7

amount of Na+ in ultrafiltrate per day

GFR x [Na+]plasma

Question 8

Na+ Reabsorption

What is fractional reabsorption?

Answer 8

fraction of Ma+ that has been successfully reabsorbed

1 - [Na+]tubular/[Na+]ultrafiltrate

where [Na+]tubular is the % of Na+ remaining along nephron
where [Na+]ultrafiltrate is the % of Na+ remaining in Bowman’s space (100%)

Question 9

Na+ Reabsorption

What is fractional excretion?

Answer 9

how much Na+ being lost vs. how much has been filtered – amount of excretion is quite low compared to how much is filtered

excretory rate of Na+ / filtered load of Na+
= [Na+]urinary x V / GFR x [Na+]ultrafilitrate

Question 10

Na+ Reabsorption

What is Na+ transport dependent on?

Answer 10

• available transporters
• regional permeability (regulated by tight junctions)
• composition of the delivered tubular fluid (changes based on flow rate through nephron)
• transepithelial voltage gradient (helps favour come types of transport)

Question 11

Na+ Reabsorption

How much Na+ is reabsorbed (beginning from proximal tubule to collecting duct)?

Answer 11

almost 100% of the Na+ that gets filtered from glomerulus into the tubule

Question 12

Na+ Reabsorption

Proximal Tubule – Mechanism

Answer 12

see notes

Question 13

Na+ Reabsorption

Thin Descending Limb – Is the thin descending limb permeable to NaCl?

Answer 13

NO

Question 14

Na+ Reabsorption

Thin Ascending Limb –Is the thin ascending limb permeable to NaCl?

Answer 14

YES – some/minimal passive Na+ reabsorption occurs (not much transport proteins contributing here)

Question 15

Na+ Reabsorption

Thick Ascending Limb – Mechanism

Answer 15

see notes

Question 16

Na+ Reabsorption

Distal Tubule – Mechanism

Answer 16

see notes

Question 17

Na+ Reabsorption

Distal Tubule – Does the composition of proteins change along the tubule?

Answer 17

YES – composition of proteins (especially found on apical membrane) varies depending on part of distal tubule

Question 18

Na+ Reabsorption

Distal Tubule – How can hypertension be reduced?

Answer 18

reduced amount of Na+ transport, paired with reduced H2O absorption

Question 19

Na+ Reabsorption

Distal Tubule – What are thiazides?

Answer 19

diuretics – help get rid of excess H2O, and slightly reduces blood presure

Question 20

Na+ Reabsorption

Distal Tubule – What are the principal cells?

Answer 20

DCT1 and DCT2

Question 21

Na+ Reabsorption

Collecting Duct – Mechanism

Answer 21

see notes

Question 22

What is the last portion of the nephron where fluid in the tubule is still modifiable?

Answer 22

collecting duct

Question 23

Na+ Reabsorption

Collecting Duct – Why is this part of the tubule so important?

Answer 23

regulates Na+ reabsorption

more fine tuning and regulation of the final composition of fluid

Question 24

Na+ Reabsorption

Collecting Duct – Mechanism

Answer 24

see notes

Question 25

Where does bulk K+ reabsorption occur?

Answer 25

proximal tubule

Question 26

Where is K+ reabsorption the same for all K+ levels? Where is it different?

Answer 26

PT and TAL reabsorption is same for all K+ levels CD reabsorption is different for normal/elevated vs. low K+ levels

Question 27

Where does regulation of K+ reabsorption/secretion occur?

Answer 27

collecting duct

Question 28

What happens if there is normal/elevated K+ levels in the collecting duct?

Answer 28

secretion predominates – K+ leaves blood, and going to be excreted

Question 29

What happens if there is low K+ levels in the collecting duct?

Answer 29

reabsorption predominates – excrete < 1% of K+ filtered

Question 30

What does K+ concentration determine in the collecting duct?

Answer 30

direction of transport – secretion vs. reabsorption

Question 31

K+ Reabsorption Proximal Tubule – Mechanism

Answer 31

see notes

Question 32

What does transport do?

Answer 32

alters tubular fluid composition and decreases flow rate

Question 33

What is the TF/P ratio?

Answer 33

ratio of concentration of solute (x) in tubular fluid to solute (x) in plasma at a given distance along PT - describes how composition of tubular fluid is changing compared to fluid in plasma

Question 34

What is the filtrate in the glomerulus very similar to?

Answer 34

composition to plasma – EXCEPT there is no protein in filtrate

Question 35

What is used as the reference point when observing TF/P ratios? Why?

Answer 35

inulin - substance is freely filtered at glomerulus, and is NOT reabsorbed/secreted – therefore, amount of inulin in tubular fluid is constant - use as reference to observe how other things in PT fluid are changing

Question 36

What decreases TF/P?

Answer 36

reabsorption of valuable substances – such as glucose, amino acids, HCO3-

Question 37

What increases TF/P?

Answer 37

reabsorption of Na+ (less) and Cl- (more)

Question 38

From beginning to end of proximal tubule, how does fluid composition resemble?

Answer 38

goes from closely resembling plasma (without protein) to becoming essentially NaCl

Question 39

In the proximal tubule why is transepithelial potential (PD) inegative at beginning, but then becomes positive?

Answer 39

initially negative because there is very little Cl- movement later movement of Cl- makes it positive

Question 40

How does proximal tubular fluid flow rate change along the tubule?

Answer 40

decreases progressively – result of bulk H2O reabsorption into bloodstream

Question 41

K+ Reabsorption Thick Ascending Limb – Mechanism

Answer 41

see notes

Question 42

K+ Secretion Distal Convoluted Tubule – Mechanism

Answer 42

see notes

Question 43

K+ Handling Collecting Duct – Mechanism

Answer 43

see notes

Question 44

Where does bulk K+ reabsorption occur?

Answer 44

- PT | - TAL – paired with Na+ reabsorption