nephron summary

Question 1

What occurs at the PT

Answer 1

Reabsorption

• 66% water and inorganic ions (Na, Cl, K, Ca2+, PO4)
• 100% glucose and amino acids (Really want to keep)
• 90% bicarbonate

Question 2

What transport mechanisms are present in the PT

Answer 2

A) Transcellular (across epi cells)
1. Primary active (ATP)
2. Secondary active (another gradient driven)
- cotransporter/symporter
- counter transport/antiporter
B) Paracellular (b/w cells = passive)

Question 3

What is the predominant mechanism in PT

Answer 3

Na coupled transporters
Therefore function of NaKATPase is critical
- NaKATPase consume 90% of ATP –> therefore PT is high in mitochondria rich)

Question 4

What is the relative amount of mitochondria in PT

Answer 4

High

• as contains alot of NaKATPase
• due to predominant Na driven transport in PT

Question 5

NaKATPase relationship with Naglucose tranporter

Answer 5

1. NaKATPase actively creates Na gradient
   - drives secondary transport/reabsorption of glucose AGAINST its concentration gradient
2. H2O follows Na and reabsorbed via paracellular osmosis
3. Solutes (K+) are reabsorbed/ move with this water –> “Solvent drag”

Question 6

What happens to the osmolality of lumen in PT

Answer 6

constant

- as both NaCl and H2O are be reabsorbed

Question 7

Can bircarbonate diffuse across cell membranes in PT?

Answer 7

No
it is charged, therefore not freely diffusible
- needs to be broken down by apical CA to CO2 in lumen –> CO2 freely diffuses into cell –> CA in cell converts CO2 into H2cO3 and then H+ and HCO3 –> a) H+ is pumped out into lumen via NaH+ exchanger
b) 90% HCO3- reabsorbed out through channel in cell
Note: acidified lumen drives this reaction

Question 8

How can Bicarbonate be reabsorbed

Answer 8

CA (carbonic anhydrase)

- located on brush borer + cytoplasm

Question 9

What is the explanation for the acidification which is occurring in the PT

Answer 9

Bicarbonate is being reabsorbed + increasing H+ transport into lumen –> acidification/decreased pH
-DRIVEN BY Na UPTAKE

Question 10

What percentage of bicarbonate is filtered at the PT

Answer 10

90%

Question 11

PT tubular acidosis

Answer 11

When PT dysfunctions –> PT acidosis (not generating bicarbonate)

Question 12

What two methods generate bicarbonate in the PT?

Answer 12

1. NaH exchanger driven bicarb production
2. Glutamine breakdown into bicarb + NH4
   Note: NH4 secreted out and Na consequentially secrete back in via exchanged

Question 13

Glutamine stimulation

Answer 13

Increase glutamine metabolism if increased ECF H+ concentration

Question 14

What is the function of H+ ions in the lumen of PT

Answer 14

Secretion of H+ in lumen in order to DRIVE bicarbonate reabsorption

Question 15

Fanconi syndrome

Answer 15

PT cannot reabsorb bicarbonate, phosphate, aa, glucose, –> increased excretion of these