6 - Ion Absorption

Question 1

What is the blood buffer system?

Answer 1

CO2+H2o H+ + HCO3-

Question 2

What mechanisms enable HCO3- to be reabsorbed in the PCT?

Answer 2

H+ exchanged out of cell for Na+
Out of cell, H+ reacts with HCO3- to form CO2 and H2O
H20 and CO2 diffuse into the cell
Inside cell, CO2 and H2O is converted back to H+ and HCO3-
H+ is recycled back into Na/H exchanger
HCO3- is excreted basolaterally via Na/HCO3 symporter

Question 3

Why are buffer molecules needed in the kidneys?

Answer 3

Minimum urine pH is 4.5 but a higher conc of H+ is produced so need buffer molecules to be able to safely excrete the H+

Question 4

Where does ammonium buffering occur?

Answer 4

Proximal tubule

Question 5

Describe how ammonium acts a buffer in the kidneys

Answer 5

Inside the cell, glutamine is converted into NH4+ and alpha-ketogluterate.
A-ketogluterate is converted in HCO3, which is symported out with Na.
NH4+ is broken down to NH3 and H+. The NH3+ is excreted into the lumen where it reacts with excess H+ to form NH4+, which is excreted into the urine

Question 6

Where does H2PO4- buffering occur?

Answer 6

Alpha intercalated cells of the collecting duct

Question 7

Describe how H2PO4- acts a buffer in the kidneys

Answer 7

H2O and CO2 in the cell react to form H+ and HCO3-
HCO3- is exchanged for Cl- basolaterally
H+ secreted apically by H+ ATPase
In the lumen reacts with HPO4 (2-) to form H2PO4- which is excreted in urine