Physiology 6 - Acid Base

Question 1

What are the major sources of H+?

Answer 1

Respiratory Acid (CO2+H2O -> Carbonic acid)

Metabolic Acid (Inorganic e.g. sulphuric acid from Amino acids or organic e.g. fatty acids/lactic acid)

Question 2

What is the normal arterial pH?

Answer 2

7.4

Question 3

What is the normal concentration of bicarbonate/

Answer 3

24mmoles/l

Question 4

What are the major H+ buffer systems of the body?

Answer 4

1) Bicarbonate
2) Plasma proteins
3) Dibasic -> Monobasic phosphate (HPO4 {2-} + H{+} -> H2PO4{-} )
4) Intracellular buffers
5) Bone carbonate

Question 5

Whats the consequence of using intracellular buffers?

Answer 5

H+ ions moved into the cells must either come with Cl- or be exchanged with K+ to maintain electrical equilibrium.
In acidosis this can cause Hyperkalemia –> Vfib & death

Question 6

Whats the consequence of using bone carbonate as a buffer?

Answer 6

Occurs mainly in chronic renal failure when H+ can’t be excreted.
Causes bone wasting due to the chronic acid load

Question 7

How much H+ do you take in a day?

Answer 7

50-100mmoles/day

Question 8

BY what mechanisms do the kidneys regulate acid/base balance?

Answer 8

1) Reabsorption of Bicarbonate

Generate new Bicarbonate

2) Excretion of H+ as titratable acids
3) Excretion of H+ with ammonium

Question 9

Explain the process of HCO3- reabsorption?

Answer 9

1) H+ ions actively secreted into proximal tubule (coupled to passive Na+ Reabsorption)
2) H+ & filtered bicarbonate form carbonic acid
3) dissociates to CO2/H2O which are then reabsorped
4) forms carbonic acid again in proximal tubule cell
5) dissociates to H+ & bicarbonate
6) bicarbonate is reabsorped and H+ secreted again for the same purpose

Question 10

How is H+ excreted as a titrable acid?

Answer 10

Excess (Exceeding Tm) dibasic PO4{2-} ions reach distal tubule.
H+ secreted into distal tubule (coupled to passive Na+ reabsorption) and binds to dibasic phosphate
Making monobasic phosphate (HPO4{-})
Which is then excreted

This process is dependant on blood PaCO2

Also works with uric acid and creatinine

Question 11

What else is produced when H+ ions are excreted as titratable acids?

Answer 11

New bicarbonate.
Blood CO2 is absorbed into distal tubule cells
+water –> Carbonic acid
Then dissociates to H+ (for secretion) and HCO3- (absorped into blood)

Question 12

Whats different about ammonium excretion compared to other methods of regulating Acidity?

Answer 12

It is variably active.
Normally it excretes 30-50mmoles H+/day but during a chronic acid load the kidneys can synthesize new proteins over 4–5 days and up that to 250mmoles/day

Question 13

How does ammonium excretion work in the distal tubule?

Answer 13

Ammonium (NH3) is lipid soluble but ammonia (NH4+) is not.
Distal Tubule:
1) Renal glutaminase deaminates amino acids producing NH3
2) NH3 moves into lumen, combines with H+ –> NH4+ and is excreted

The H+ ions are secreted from the distal tubule cells after being produced from blood CO2 (So this process is also reliant on PaCO2)

Question 14

How does ammonium excretion work in the proximal tubule?

Answer 14

Almost the same as in the distal.

But H+ and NH3+ combine in the cell and are actively excreted using a NH4+/Na+ exchanger

Question 15

What else is produced during ammonium excretion?

Answer 15

HCO3- is produced when you make H+ from blood CO2 to secrete. The bicarbonate is then reabsorped into the blood

Question 16

Summary of renal regulation:

Answer 16

1) HCO3- reabsorption # No new HCO3- # No net excretion of H+ # Proximal tubule
2) H+ excretion as titrable acids # New HCO3- produced # Net loss of H+ as monobasic phosphate # Distal tubule
3) Ammonium excretion # New HCO3- # Net loss of H+ as NH4+ # Proximal and distal tubule

Question 17

Describe the blood gasses of Respiratory Acidosis?

Answer 17

High PaCO2 = Directly
High HCO3- = Kidney’s regulating pH
Slightly acidic pH

Question 18

What could cause respiratory alkalosis?

Answer 18

Acute- aspirin or high altitude

Chronic - Low PaO2 or high altitude

Question 19

Describe the bloods of respiratory alkalosis?

Answer 19

Low PaCO2 = directly
Low HCO3- = because less H+ means less secretion which means less HCO3- reabsorption/production
Slightly raised pH

Question 20

What can cause metabolic acidosis?

Answer 20

Excess H+ or loss of HCO3-:

1) Increased H+ production e.g. DKA
2) Decreasd H+ excretion e.g. renal failure
3) Decreased intestinal HCO3- reabsorption e.g. Diarrhoea

Question 21

Describe the bloods of metabolic acidosis?

Answer 21

Low PaCO2 = kussmauls respiration blows off CO2 to lower H+
Low HCO3- = Directly (either lost or used up buffering extra H+)
pH slightly low

Question 22

What can cause metabolic alkalosis?

Answer 22

H+ ion loss in vomit
Excess aldosterone –> Na+ reabsorption in exchange for H+ –> More H+ excretion and also less H+ available for HCO3- reasborption.
Excess HCO3- administration in the renally impaired
Massive blood transfusions (contain citrate for anticoagulation)

Question 23

Describe the bloods of metabolic alkalosis?

Answer 23

High PaCO2 = to raise acidity
High HCO3- = Directly
Slightly high pH

Question 24

What is the anion gap?

Answer 24

A measure of the difference between the principle cations (Na+/K+) and anions (Cl-/HCO3-).
Usually about 14-18mmoles/l more cations than anions.

Question 25

When is the anion gap a useful measurment?

Answer 25

In acidosis
It increases when bicarbonate is used up by lactic acidosis/DKA

It stays the same when HCO3- is lost in the gut as its compensated by extra Cl-

Question 26

Patient with pH = 7.32, [HCO-3]= 15 mM, PCO2 = 30mmHg (4kPa)
What Acid/base disturbance is this?

Answer 26

Metabolic acidosis

pH low = Acidosis
HCO3- & PaCO2 are low = metabolic acidosis

Question 27

Patient with pH = 7.32, [HCO-3]= 33 mM, PCO2 = 60mmHg (8kPa)

What acid/base disturbance is this?

Answer 27

pH low = Acidosis
PaCO2 high = Respiratory acidosis
High HCO3- = Chronic

Crhonic because more H+ means more HCO3- production and reabsorption.
In the acute form HCO3- would not be elevated

Question 28

Patient with pH = 7.45, [HCO-3] = 42 mM, PCO2 = 50mmHg (6.7kPa)

What they got?

Answer 28

high pH = alkalosis

High HCO3- & PaCO2 = metabolic

Question 29

pH = 7.45, [HCO-3]= 21 mM, PCO2 = 30mmHg (4kPa)

What they got?

Answer 29

high pH = alkalosis
Low PaCO2 = Respiratory
Normal HCO3- = acute (chronically it would adjust downward)

Question 30

Patient with pH = 7.31, PCO2 = 7.7.kPa, (58mmHg), [HCO3-] =36mmoles/l.
Which of the following is true:
1. It is likely that he has renal disease.
2. He may have an acute respiratory infection.
3. It is possible that he may have chronic bronchitis.
4. There will be a decrease in his excretion of ammonium ions.
5. His plasma potassium will be reduced.

Answer 30

3!!

1) He’s acidotic but his HCO3- is still raised so hes not losing to renal disease
2) Hes in respiratory acidosis, we know its not acute due to the high HCO3- so it not a resp infection
3) His High HCO3- indicates it a chronic respiratory acidosis as its compensating with more HCO3-
4) False it will increase
5) False it will go up as H+ is exchanged into cells for K+ in order to be buffered

Question 31

The following acid/base values were obtained:
pH = 7.25, [HCO3-] = 12mmoles/l, PCO2 = 3.3kPa (25mmHg)

Which of the following are true?

1) They are indicative of a respiratory acidosis
2) The reduction in Pco2 is a result of under-breathing
3) The subject has probably been taking bicarbonate of soda
4) It could be related to impaired renal function
5) The subject may have been vomiting very badly

Answer 31

4!! They’re in metabolic acidosis

1) Low HCO3- and PaCO2 indicates its metabolic
2) False, you hyperventilate in response to acidosis
3) Why would their bicarbonate be so low
4) True, thats where the HCO3- might be going
5) that would cause alkalosis (So a high pH)