Control of Plasma pH

Question 1

State the normal plasma pH range

Answer 1

7.35-7.45

Question 2

Compare Acidaemia and Acidosis

Is Acidaemia or Alkalaemia more dangerous?

Answer 2

Acidaemia is low blood pH due to acidosis

Alkalaemia is more dangerous

Question 3

How does Alkalaemia affect Ca and nerves?

Answer 3

• Reduces solubility of Ca salts, so free Ca2+ leaves ECF and binds to bone and proteins-> Hypocalcaemia
• This makes nerves more excitable-> Parasthesia and Tetany (Uncontrolled muscle contractions)

Question 4

What is the mortality rate if pH is above;

• 7.55
• 7.65

Answer 4

• pH>7.55 mortality: 45%

- pH>7.65 mortality: 80%

Question 5

How does Acidaemia affect K+ movement?

When are effects of Acidaemia seen? When is it life threatening?

Answer 5

• Affects enzyme function, leading to K+ movement out of cells-> Hyperkalaemia
• Effects seen below pH 7.1
• Life threatening below pH 7.0

Question 6

The ECF [H+] is very low so small amounts of acid would change pH dramatically. This is buffered by H+ binding to various sites

What is the most important ECF buffer for H+?

Answer 6

The CO2/ HCO3 system

Question 7

Compare Respiratory Acidaemia and Alkalaemia

How are they compensated really?

Answer 7

• Acidaemia: Rises in pCO2
• Renal compensation: Increased HCO3
• Alkalmaemia: Falls in pCO2
• Renal compensation: Decreased HCO3

Question 8

Compare Metabolic Acidosis and Alkalosis

How are they compensated by the respiratory system?

Answer 8

• Acidosis: Decreased HCO3
• Respiratory compensation: Reduced pCO2 (hyperventilation)
• Alkalosis: Increased HCO3
• Respiratory compensation: Increased pCO2 (hypoventilation)

Question 9

Why is Respiratory compensation of metabolic alkalosis limited?

Answer 9

Hypoventialtion is limited as it can cause hypoxaemia

Question 10

If acid is produced metabolically, recovery of l filtered HCO3 will be insufficient to restore plasma [HCO3].

How does the kidney compensate?

Answer 10

• Kidney makes HCO3 (normally only made in RBCs)
• The byproduct of this, H+, must be excreted into urine
• To prevent a damaging urinary acidity, the H+ must be buffered by other filtered substances or by buffers made in the kidney

Question 11

How much HCO3 is reabsorbed in;

• PCT
• TAL
• DCT, via Intercalated cells

Answer 11

PCT: 80-90%
TAL: Up to 15%
DCT, Intercalated cells: Remaining

(Overall, 100% of HCO3 is reabsorbed)

Question 12

Describe HCO3 reabsorption in CD Intercalated cells

Answer 12

• H-ATPase on AM pumps H+ out (Na gradient too low to use NHE)
• CO2 enters cell and reacts with water-> H2CO3 which dissociates into H+ and HCO3-
• New HCO3- ions enter blood via HCO3-Cl Antiport on BM

Question 14

Compare H+ buffering systems in PCT and DCT

Answer 14

PCT;
- NH3 from Glutamine diffuses into lumen and combines with H+-> NH4+

DCT;
- H+ pumped out via H-ATPase into lumen and combines with Monobasic phosphate (HPO4 2-)-> H2PO4-

Question 15

Describe how HCO3 is created in PCT (and also some in DCT)

Answer 15

• Glutamine broken down into NH4+ and Alpha-ketoglutarate
• NH4+ dissociates into NH3 and H+, NH3 diffuses into lumen and binds to H+ to reform NH4+ (in lumen)
• Alpha-ketoglutate breaks down into 2 HCO3 ions, which enter blood via Na-HCO3 symport on BM

Question 16

How does ECF pH affect secretion of acid by kidney?

Why is this done?

Answer 16

Fall in pH-> Increased acid secretion into lumen

To prevent HCO3 depletion (Used to neutralise pH)

Question 17

Why does acidosis cause Hyperkalaemia?

Answer 17

• More H+ enters cell from ECF, so more K+ enters ECF from cell
• Less K+ excreted in distal nephron

(In alkalosis, less K+ enters ECF and more is excreted)

Question 18

Why does Hyperkalaemia cause acidosis?

Answer 18

• Tubular cells more alkaline, as H+ moves out of cells and K+ moves in
• To counter alkaline, HCO3 excreted in blood

Question 19

What are 4 causes of metabolic acidosis

Answer 19

• Excess acid production metabolically (Lactic)
• Acid ingestion
• Impaired renal acc excretion
• HCO3 loss

Question 20

If excess acid is produced metabolically, what replaces HCO3 in plasma?

Answer 20

The associated anion

This influences the anion gap

Question 21

What is the Anion gap? (Normally 8-14mM)

How does anion gap change when HCO3 is replaced by an anion not included in the calculation group?

Answer 21

Difference between combined sum of [K] and [Na] and combined sum of [Cl] and [HCO3]

Anion gap increases when HCO3 replaced (Renal acidosis doesn’t change anion gap as Cl replaces HCO3)

Question 22

What does the anion gap represent?

Answer 22

ECF level of unmeasured ions

Question 23

How do we differentiate between ;

• Renal compensation of respiratory alkalosis
• Respiratory compensation of metabolic acidosis

Answer 23

Anion gap will be increased in metabolic acidosis

Question 24

Explain the effect of Ethylene Glycol on the Anion Gap if ingested

What are the 2 substances this chemical is found in?

Answer 24

• Increases anion gap
• Due to acids produced by its metabolism
• Engine coolants
• Anti freeze

(Can be ingested as suicide attempt/ accident)

Question 25

List 4 signs/ symptoms of Ethylene Glycol ingestion

Answer 25

CNS related;

• Slurred speech
• Confusion
• Stupor
• May be misdiagnosed with alcohol intoxication

Question 26

What are 2 diagnostic clues of Ethylene Glycol ingestion

Answer 26

• Anion gap acidosis

- Oxalate crystals in urine with renal failure

Question 29

Monobasic phopshate (HPO4 2-) is a H+ buffer in urine.

When does it become more effective?

Answer 29

Gets more effective as urine pH falls (Urine [H+] increases)