PHYS Handling of Bicarbonate - Week 10

Question 1

pH plasma

Answer 1

7.35-7.45

Question 2

Where are bicarbonate ions filtered?

Answer 2

Glomerulus.

Question 3

Where is bicarbonate reabsorbed/recovered at the kidney?

Answer 3

Mainly PCT & also intercalated cells of the distal tubules & collecting ducts.

Question 4

Daily filtered substance ‘a’ =

Answer 4

Daily filtered substance ‘a’ = GFR x [plasma]’a’.

Question 5

Describe how HCO3- is reabsorbed into PCT cells?

Answer 5

Apical/luminal side of renal tubular cells are impermeable to HCO3-.
1. HCO3- instead combines with the secreted H+ @ brush border -> CO2 + H2O via carbonic anhydrase.
2. Lipid soluble CO2 diffuses into PCT cell cytoplasm + OH- -> HCO3-.
3. Non-electrogenic Na+/H+ anti-porter secretes H+ into the lumen (via active transport).
Basolateral membrane of renal tubular cells
1. HCO3- crosses the basolateral membrane via Na+/HCO3- symporter (via passive transport) at ratio of 1:3.
2. Active Na+/K+ pump moves ions at 3:2.

Question 6

Describe how HCO3- is regenerated at PCT cells?

Answer 6

Glutamine enters cells via peritubular capillaries (80%) & urinary filtrate (20%) -> ammonium via glutaminase OR alpha-ketoglutarate (triggered by acid intracellular pH) via metabolism w by-products -> NH4- secreted into tubular lumen via Na+/H+ OR NH4+ exchanger -> by-products form HCO3- ->HCO3- exits via basolateral Na+/HCO3- co-transporter -> bloodstream.
For each NH4+ excreted, one HCO3- is regenerated and released into bloodstream.

Question 7

2 x cell types @ Collecting Duct & State pumps present in each cell type.

Answer 7

• Alpha-intercalated cells – contain apical H+/ATPase pump
• Beta-intercalated cells – contains apical chloride/bicarbonate exchanger.

Question 8

What change is triggered in the Collecting Duct in response to metabolic acidosis?

Answer 8

Inducing metabolic acidosis results in the conversion of beta-intercalated cells to alpha-intercalated cells -> give kidneys a greater ability to secrete protons & return pH to the normal range.

Question 9

What is inhibited in the Collecting Duct if pH < 4?

Answer 9

Proton pumps - restricting ability of kidneys to excrete more H+.

Question 10

At the distal & collecting tubules - H+ secretion -> ____ & HCO3- reabsorption -> ____

Answer 10

K+ reabsorption.
Cl- secretion.

Question 11

What chemical is the most predominant buffer against pH changes in the urine?

Answer 11

Phosphate.

Question 12

What is the usual pH of urine?

Answer 12

6.2.