Kidney Function 2

Question 1

ADH/vasopressin

Answer 1

• increases water reabsorption in the collecting duct
  
  • increases permeability of CD and DCT by inducing aquaporin channels into the plasma membrane
• increases peripheral vascular resistance which increases arterial BP

Question 2

Aldosterone

Answer 2

• acts on DT and CD
• increases reabsorption of ions and water
  
  • conservation of Na
  • secretion of K
  • water retention
  • increased BP

Question 3

What hormones control Na and water reabsorption?

Answer 3

• ADH/vasopressin (water)
• aldosterone (Na)

Question 4

What hormones control Ca2+ reabsorption?

Answer 4

• parathyroid hormone (PT)
• vitamin D3 (DT)
• both favour retention

Question 5

What percent of water is excreted?

Answer 5

1%

Question 6

What percent of Na is excreted?

Answer 6

0.5%

Question 7

What percent of K is excreted?

Answer 7

10%

Question 8

What percent of Ca2+ is excreted?

Answer 8

2%

Question 9

What percent of phosphate is excreted?

Answer 9

20%

Question 10

What percent of glucose is excreted?

Answer 10

0% (if plasma concentration is <15mmol)

Question 11

What percent of creatinine is excreted?

Answer 11

100%

(filtered, not reabsorbed, secreted to a small extent)

Question 12

What percent of urea is excreted?

Answer 12

50%

Question 13

Na is reabsorbed

Answer 13

actively, through the cell into ECF via Na/K-ATPase on basolateral (ECF) side

reabsorption on the apical membrane varies with region of nephron

Question 14

In the proximal tubule, Na is reabsorbed on the apical membrane via

Answer 14

• Na/glucose cotransporter
• Na/AA cotransporter
• Na/PO4 cotransporter
• etc.

Question 15

Reabsorption of Cl and HCO3 is driven by

Answer 15

• Na reabsorption leaving a -ve charge in the lumen, forming an electrochemical gradient (~3mV)
• negatively charged ions Cl and HCO3 tf drawn across membrane:
  
  • HCO3 transcellularly
  • Cl via paracellular space
• water follows both transcellularly and paracellularly via osmotic gradient created by Cl movement

Question 16

What is solvent drag?

Answer 16

• applies only at PT
• as water is reabsorbed, solutes are concentrated in the lumen
• movement of H2O via paracellular pathway carrying solutes dissolved in it:
  
  • K, Ca, urea

Question 17

How are Cl and HCO3 reabsorbed in the proximal tubule?

Answer 17

early PT:

• HCO3 transcellularly via electrochemical gradient
• tf increased [Cl] in early PT
  
  • increased -ve charge of lumen draws Na+ back via paracellular spaces

late PT:

• increased [Cl] from HCO3 reabsorption establishes a concentration gradient for Cl
• Cl moves out of the lumen, reabsorbed via paracellular spaces
  
  • occurs faster than Na+ leaves; est +3mV in lumen
  • Na+ moves transcellularly and paracellularly w/Cl,

Question 18

How does Na reabsorption occur in the tALH?

Answer 18

• passively via paracellular spaces due to concentration gradient:
  
  • Na is high in lumen (1200mOs predominantly NaCl)
  • Na is relatively low in ECF (1200mOs 2/3 NaCl, 1/3 urea)
  • tf Na moves passively from lumen to ECF

Question 19

How is Na reabsorbed in the TALH?

Answer 19

• actively, driven by Na/K ATP-ase on basolateral (ECF) side
• Na enters apical membrane via Na/K/Cl co-transporter (target for frusemide)
  
  • uses gradient generated by Na/K ATP-ase
• some Na flows through paracellular pathway but junctions are much tighter in TALH to prevent water movement

Question 20

How is Na reabsorbed in the DT?

Answer 20

• enters apical membrane and proceeds transcellularly via Na/Cl transporter (target of thiazide diuretics)
• actively reabsorbed on basolateral membrane via Na/K-ATPase

Question 21

How is Na reabsorbed in the CD?

Answer 21

• passively on apical membrane via ENaC (epithelial Na channeL0
• actively on basolateral surface via Na/K-ATPase
• K+ is passively secreted into lumen

Question 22

How is urine concentrated in the LOH?

Answer 22

• active transport of Na, Cl, K into the medullary interstitium increases osmolarity
• water is drawn out of the descending LOH across osmotic gradient to equilibriate with interstitium
• more fluid enters loop and the cycle repeats
  
  • concentration of urine increases down descending LOH
  • concentration of urine decreases up ascending LOH