149 - Acid-Base Balance

Question 1

Plasma buffer systems
1
2

Answer 1

HCO3- + H+ H2CO3 (carbonic acid)

H2CO3 CO2 + H2O

Question 2

Features of HCO3- + H+ H2CO3 / H2CO3 CO2 + H2O

Answer 2

Equilibrium is far in favour of CO2.

Very slow reaction, that is fast in the presence of carbonic anhydrase.

Question 3

Location of carbonic anhydrase

Answer 3

In the cytoplasm of all cells.

Particularly high in RBCs

Question 4

Ratio of HCO3- + H+ and H2CO3

Answer 4

H2CO2- and H+ = 26mM

H2CO3 = 3 micromolar (H2CO3 is mostly in the form of CO2 + H2O, rapidly converted by carbonic anhydrase)

Question 5

From the action of carbonic anhydrase, what form a buffer system in the blood?

Answer 5

CO2, bicarbonate.

These are effectively an acid-base pair.

Question 6

pK of CO2 + H2O HCO3- + H+

Answer 6

6.1 (when 50% of reaction is on one side)

Question 7

Henderson-Hasselbach equation

Answer 7

For any acid-base pair, pH = pKa + log([base]/[acid])

Question 8

Example of Henderson Hasselbach equation for bicarbonate/carbon dioxide system

Answer 8

pH = pKa + log([base]/[acid])

6. 1 + log([HCO3-]/[CO2])
7. 1 + log([HCO3-]/[0.03 x pCO2])

[HCO3-] is often 24mM
pCO2 is often 40mmHg

With these values, pH ~= 7.4

Question 9

Example of plasma buffering systems, other than CO2/HCO3-
1
2
3

Answer 9

1) Plasma proteins (~10mEq).
2) Phosphate (~2mEq), as H PO42- or H2PO4- or H3PO4
3) Intracellular haemoglobin (Hb + H+ HbH+)

Question 10

CO2 conversion to HCO3-

Answer 10

CO2 formed in mitochondria diffuses into blood and into RBC. High carbonic anhydrase in RBC converts it to bicarbonate.

Question 11

Organs that can alter pH
1 a
2 a, b

Answer 11

• Lungs can
– alter pCO2 through changes in ventilation
• Kidneys can
– alter HCO3- by changes in production & excretion
– alter pH by changes in H+ excretion

Question 12

Acids produced in the body, dealt with by the kidneys
1
2
3
4

Answer 12

• Sulphuric & phosphoric acids from proteins & lipids
• Lactic acid anaerobic metabolism
• Keto acids from fatty acids
• 70 mmol of strong acid per day

Question 13

How are acids initially buffered?

Answer 13

By HCO3-.

Some buffering also provided by Hb

Question 14

Example of a state where keto acids can be very high

Answer 14

Diabetes

Question 15

First thing to show up when there is an increase in non-volatile acids

Answer 15

Decreased HCO3-

Question 16

Non-volatile acids

Answer 16

Acids that aren’t H2CO3. Not breathed out by the lungs

Question 17

Acid/base Gi secretions

Answer 17

Acid in stomach acid, HCO3- in pancreatic secretions

Question 18

Anion gap

Answer 18

Anions and cations in body should all add together for a net charge of 0.

All ions can’t be realistically measured

When commonly-measured anions and cations (Na+, K+, HCO3-, Cl-) are measured and added together, unmeasured anions leave a gap of 12mmol/L.

Question 19

What contributes most to anion gap?

Answer 19

Albumin contributes ~80%

Question 20

Acidoses associated with a high anion gap
1
2
3

Answer 20

1) Lactic acidosis (from ischaemia, anaerobic exercise)
2) Diabetic ketoacidosis
3) Renal injury

Question 21

Effect of hypoventilation on pH

Answer 21

Decrease in pH from retention of CO2.

Bicarbonate will increase in response, but not so much as to negate decreased pH.

Question 22

How does HCO3- increase when there is a respiratory acidosis?

Answer 22

Increased production, reabsorption of HCO3- in the kidney.

Equilibrium will favour HCO3-

Question 23

Renal response to respiratory acidosis

Answer 23

Increased production and reabsorption of HCO3-.
Increased H+ excretion.
Renal response is quite limited.

Question 24

Example of a metabolic alkalosis, and body response
1
2
3

Answer 24

1) Vomiting leads to loss of H+.
2) Slightly decreased ventilation (but can’t do this much, as you need to breathe).
3) Kidneys increase HCO3- filtration, reduces the amount of H+ excretion. This is a stronger compensation than that of the lungs.

Question 25

What is equal to bicarbonate creation?

Answer 25

Acid excretion

Question 26

Bicarbonate creation and acid excretion 
1
2
3
4
5
6
7
8

Answer 26

1) Na/H+ antiport secretes H+
2) H+ in filtrate combines with filtered HCO3- to form CO2
3) CO2 diffuses into cell and combines with water to form H+ and CO3- (under influence of carbonic anhydrase)
4) H+ is secreted again and excreted
5) HCO3- is reabsorbed
6) Glutamine is metabolised to NH4+ and HCO3-
7) NH4+ is immediately excreted
8) HCO3- is reabsorbed

Question 27

What happens to HCO3- in the kidneys if concentrations of it exceed those of H+?

Answer 27

It remains in the tubule, is excreted

Question 28

What happens if H+ in the kidneys exceeds concentrations of HCO3-?

Answer 28

Binds other buffers (EG phosphate and creatinine).

Question 29

Number of days of acidosis before renal tubule cells begin making more ammonia?

Answer 29

About 2 days of acidosis before more ammonia is made to buffer

Question 30

How is acidic urine formed?

Answer 30

When H+ secretion is in excess of bicarbonate.

Leads to acid excretion.

Question 31

Event that accompanies passing of acidic urine

Answer 31

HCO3- production.

Bicarbonate reabsorption into capillary is associated with formation of bicarbonate within cell

Question 32

Cell type in the kidneys associated with control of acidosis

Answer 32

Intercalated cell type A

Question 33

How do intercalated cell type A respond to acidosis?

Answer 33

H+/K+ ATP ase secrete H+, take in K+.