7. Bicarbonate

Question 1

What is the principal buffer of the ECF and blood?

Answer 1

Bicarbonate

Question 2

pH is in proportion to…

Answer 2

HCO3-/pCO2

Question 3

What is the equation that regulates the acid/base balance in cells?

Which components of this equation are:

a. Regulated by the resp system
b. Added to the body by diet and metabolism
c. Regulated by the kidneys and lost in faeces

Answer 3

CO2 + H2O H2CO3 H+ + HCO3-

a. Regulated by the resp system = CO2
b. Added to the body by diet and metabolism = H+
c. Regulated by the kidneys and lost in faeces = HCO3-

Question 4

4 action of the kidney on bicarbonate?

Answer 4

Bicarbonate filtration
Bicarbonate reabsorption 
Bicarbonate regeneration by:
1. Titratable acid excretion
2. Ammonium excretion

Question 5

What happens to the pH of the tubule contents as it passes through nephron>?

Answer 5

Increases in acidity

i.e. pH drops

Question 6

Where are the points of bicarbonate reabsorption?

Answer 6

Proximal convoluted tubule (85-90%)

Remaining 10-15% in distal convoluted tubule and collecting tubule

Question 7

What is the process of HCO3- reabsorption in the distal and proximal convoluted tubule?

Answer 7

1. H+ is secreted into the tubular lumen.
   CO2+ H2O HCO3 + H+ catalysed by carbonic anhydrase
2. The HCO3 can then be absorbed into the blood.

Question 8

Difference between proximal and distal HCO3 reabsorption?

Answer 8

Proximal:
Secondary active transport of H+ out of cell and Na+ in at apical cell. Steep inward directed Na gradient.
When HCO3 leaves the cell into the blood it does so via Na co-transporter. 3 x HCO3 with 1 x NA

Distal:
H+ leaves cell into lumen via ATPase transporter (primary)
HCO3 leaves cell into blood via Cl- anti porter. 1 x HCO3 with 1 x Cl-

Question 9

Why must bicarbonate be regenerated?

Answer 9

As protons consume bicarbonate in the buffered system

Question 10

What is the process of titratable acid excretion?

Answer 10

The H+ leaving the cell into the tubular lumen is buffered by non-bicarbonate buffers.

Non-bicarbonate buffers:

• Phosphate **
• Urate
• Creatinine
• Beta-hydroxybutyrate

Relatively constant.

Question 11

Process of ammonium excretion?

Answer 11

Proximal tubule: Glutamine in the proximal tubule cells breaks down to ammonium and alpha-keto-glutarate.
The ammonium is excreted and the a-k-g is broken down the HCO3-.

Collecting tubule: H2O breaks down to H+ and OH-. The H+ is actively transported into lumen, combines with ammonium and forms ammonium for excretion. The OH- reacts with CO2, forming HCO3- for absorption

Whole process has potential to increase markedly

Question 12

What is the renal compensation for resp academia?

Answer 12

1. Rise in pCO2
2. Parallel change inside the renal tubule cells
3. Intracellular acidaemia
4. Increases uptake & use of glutamine & hence ammonium excretion
5. Increasing bicarbonate regeneration
6. Low intracellular pH increases tubular proton secretion and ensures optimum reabsorption of bicarbonate

Question 13

What is the renal compensation for resp alkalaemia?

Answer 13

1. Fall in pCO2
2. Parallel change inside the renal tubule cells
3. Intracellular rise in pH
4. Proton secretion falls
5. Bicarbonate reabsorption falls

Question 14

6 changes that result in increase in bicarb reabsorption and regeneration?

Answer 14

Increasing PCO2
Increased H+
Decreasing ECF volume
Increasing angiotensin II
Increasing aldosterone
Hypokalaemia

Question 15

6 changes that result in decrease in bicarb reabsorption and regeneration?

Answer 15

Decreasing PCO2
Decreased H+
Increasing ECF volume
Decreasing angiotensin II
Decreasing aldosterone
Hyperkalaemia

Question 16

Why does decreasing ECF volume and increasing angiotensin II result in increased bicarbonate reabsorption and regeneration?

Answer 16

• Stimulates sodium reabsorption
• Stimulates activity of Na / H exchange mechanism
• Ratio bicarb / H+ in tubular lumen falls
• Excess H+ in the tubules
• Bicarb fully reabsorbed & NEW bicarb formed

Question 17

What happens when kidney function is impaired?

Answer 17

Renal failure–>

• Damage to glomerulus and tubule
• Results in metabolic acidaemia
• Decrease in ammonium secretion by the tubule