5.5. Acid/Base Balance - Reabsorption of Bicarbonate

Question 1

Where does the bulk of bicarbonate reabsorption occur?

Answer 1

In the proximal tubule

Question 2

What initiates the mechanism by which bicarbonate is reabsorbed?

Answer 2

Active H+ ion secretion from the tubule cells (into the tubule lumen), coupled with passive Na+ reabsorption

Question 3

1. What does the H+ ions, secreted from the tubule cells (into the tubule lumen), react with?
2. What does this form?

Answer 3

1. Filtered Bicarbonate (HCO3-)

2. Carbonic Acid (H2CO3)

Question 4

Where does the reaction in the tubule, between H+ ions and the filtered bicarbonate (HCO3-), occur?

Answer 4

On the tubule luminal membrane

Question 5

1. What does the reaction between H+ ions and the filtered bicarbonate (HCO3-) in the tubule lumen happen in the presence of?
2. What does this cause?

Answer 5

1. Carbonic Anhydrase
2. The splitting of the formed Carbonic Acid, into:
   a) H2O
   b) CO2

Question 6

What happens to the H2O, formed by the Carbonic Anhydrase within the Tubule Lumen?

Answer 6

It is excreted

Question 7

What happens to the CO2, formed by the Carbonic Anhydrase within the Tubule Lumen?

Answer 7

It travels into the Tubule Luminal Cell

Question 8

What happens to the CO2 which enters into the Tubule Luminal Cell?

Answer 8

It reacts with the H2O present in the cell to form intracellular Carbonic Acid

Question 9

What happens to the newly-formed intracellular Carbonic Acid ?

Answer 9

It is in the presence of Carbonic Anhydrase and so splits into:
a) H+ ions
b) HCO3-
within the tubule cell

Question 10

What happens to the H+ ions which are formed due to the Carbonic Anhydrase in the Tubule Luminal Cell?

Answer 10

They are actively secreted, re-starting the cycle

Question 11

What happens to the HCO3-

ions which are formed due to the Carbonic Anhydrase in the Tubule Luminal Cell?

Answer 11

These pass into the peritubular capillaries and then out into circulation

Question 12

What is significant about this ion being reabsorbed?

Answer 12

Although it is not the same ion, the net effect is the same

Question 13

What is the Glomerular Filtration Rate?

Answer 13

125ml/minute = 180L/day

Question 14

What is the plasma bicarbonate concentration?

Answer 14

24mmol/L

Question 15

How much bicarbonate is filtered per day?

Answer 15

Glomerular Filtration Rate = 180L/day
Plasma Bicarbonate concentration = 24mmol/L
180 x 24mmol/L = 4320mmol filtered per day

Question 16

What would happen if there was no reabosorption of Bicarbonate?

Answer 16

A failure to reabsorb bicarbonate would result in a loss of 4320mmol per day which would be the same as adding the same number of H+ ions