5. Acid-Base Balance

Question 1

What is the problem with alkalaemia?

Answer 1

Lowers free calcium by causing Ca2+ ions to come out of solution
- increases neuronal excitability

Question 2

What does pH>7.45 lead to?

Answer 2

Alkalaemia, leads to paraesthesia and tetany

Question 3

What is the problem with acidaemia?

Answer 3

Increases plasma potassium ion concentration
- effects excitability, leads to arrhythmias
Increased [H+] can denature proteins
- effects muscle contractility, glycolysis, hepatic function

Question 4

What does pH depend on?

Answer 4

Ratio of [HCO3-] to pCO2

Question 5

Why doesn’t the acid produced due to metabolism not deplete HCO3-?

Answer 5

Kidneys recover all filtered HCO3-
PCT makes HCO3- from amino acids
DCT makes HCO3- from CO2 and H2O, H+ is buffered by phosphate and ammonia in urine

Question 6

How is HCO3- recovered in the kidney?

Answer 6

HCO3- filtered in glomerulus
Mostly recovered in PCT
H+ excretion linked to Na+ entry in PCT
H+ reacts with HCO3- in lumen to form CO2 which enters cell
Converted back to HCO3- which enters ECF

Question 7

How is HCO3- created in PCT?

Answer 7

Glutamine -> alpha-ketoglutarate + NH4+
NH4+ -> NH3 + H+
Alpha-keotglutarate -> 2HCO3-
NH3 enters lumen and forms NH4+

Question 8

What is the minimum pH of urine?

Answer 8

4.5

Question 9

Why does hyperkalaemia happen in acidosis?

Answer 9

Potassium moves out of cells

Decreased potassium excretion in distal nephron

Question 10

Why does alkalosis cause hypokalaemia?

Answer 10

Potassium ions move into cells

Enhanced exertion of potassium in distal nephron

Question 11

Why can hyperkalaemia cause acid base disturbances?

Answer 11

Makes intracellular pH of tubular cells more alkaline
- H+ ions move out of cells
- favours HCO3- exertion
Leads to metabolic acidosis

Question 12

Why does hypokalaemia cause acid base disturbances?

Answer 12

Makes intracellular pH of tubular cells more acidic
- H+ ions move into cells
- favours H+ excretion and HCO3- recovery
This leads to metabolic alkalosis

Question 13

What can cause respiratory acidosis?

Answer 13

Hypoventilation -> hypercapnia

Hypercapnia -> fall in plasma pH

Question 14

What is respiratory acidosis characterised by?

Answer 14

High pCO2
Normal HCO3-
Low pH

Question 15

How can changes in pCO2 be compensated?

Answer 15

By changes in [HCO3-]
Kidneys increase [HCO3-] to compensate for respiratory acidosis
Kidneys decrease [HCO3-] to compensate for respiratory alkalosis
Takes 2-3 days

Question 16

What is compensated respiratory acidosis characterised by?

Answer 16

High pCO2
Raised [HCO3-]
Relatively normal pH

Question 17

What is compensated respiratory alkalosis characterised by?

Answer 17

Low pCO2
Lowered [HCO3-]
Relatively normal pH

Question 18

What is metabolic acidosis?

Answer 18

If tissue produce acid, this reacts with and removes HCO3-
There is a fall in [HCO3-], leads to fall in pH
No increase in pCO2 as it is breathed out

Question 19

What is the anion gap?

Answer 19

Difference between measured cations and anions

Normally 10-18 mmol/L

Question 20

When is the anion gap increased?

Answer 20

If HCO3- is replaced by other anions

Question 21

Does the anion gap change in renal causes of acidosis?

Answer 21

No it remains unchanged

This is because not making enough HCO3- but this is replaced by Cl-

Question 22

What is metabolic acidosis initially characterised by?

Answer 22

Normal pCO2
Low HCO3-
Low pH
Increased anion gap if HCO3- is replaced by another organic anion from an acid
Normal anion gap if HCO3- replaced by Cl-

Question 23

How is metabolic acidosis compensated?

Answer 23

Peripheral chemoreceptors (carotid bodies) detect pH drop

• stimulate ventilation
• leading to decrease pCO2

Question 24

What is compensated metabolic acidosis characterised by?

Answer 24

Low HCO3-
Lowered pCO2
Nearer normal pH

Question 25

What happens in metabolic alkalosis?

Answer 25

[HCO3-] increases Normal pCO2 Increased pH Cannot normally be compensated to a great extent by reducing breathing - need to maintain pO2

Question 26

What are conditions that lead to respiratory acidosis?

Answer 26

Type 2 respiratory failure

Question 27

What happens in type 2 respiratory failure?

Answer 27

Low pO2 and high pCO2 Alveoli cannot be properly ventilated Can be compensated for by increase in [HCO3-]

Question 28

What causes type 2 respiratory failure?

Answer 28

Severe COPD Severe asthma Drug overdose Neuromuscular disease

Question 29

What conditions can lead to respiratory alkalosis?

Answer 29

Hyperventilation - low pCO2, rise in pH Hyperventilation in response to long-term hypoxia - type 1 respiratory failure -> low pCO2 with initial rise in pH, can be compensated by fall in [HCO3-], can restore pH to near normal

Question 30

What happens if an anion gap is increased?

Answer 30

Indicated a metabolic production of acid | Could be ketoacidosis, lactic acidosis, uraemic acidosis

Question 31

What conditions can lead to metabolic acidosis with a normal anion gap?

Answer 31

``` Renal tubularacidosis (rare) - problem with transport mechanisms in tubules, type 1 RTA, type 2 RTA Severe persistent diarrhoea - due to loss of HCO3- ```

Question 32

What does non-renal causes of metabolic acidosis cause?

Answer 32

Increased absorption of K+ by kidneys, and movement of K+ out of cells - leas to hyperkalaemia However, in diabetic ketoacidosis may be a total body depletion of K+

Question 33

Why can diabetic ketoacidosis cause total body depletion of K+?

Answer 33

K+ moves out of cells due to acidosis and lack of insulin | But osmotic diuresis means K+ lost in urine

Question 34

What conditions can lead to metabolic alkalosis?

Answer 34

``` Severe prolonged vomiting - loss of H+ Potassium depletion/mineralocorticoid excess Certain diuretics (loop and thiazides) ```

Question 35

How is metabolic alkalosis corrected?

Answer 35

Rise in pH of tubular cells leads to fall in H+ excretion and reduction in HCO3- recovery But, problem if there is also volume depletion

Question 36

If pCO2 is not normal, [HCO3-] is normal and pH has changed in opposite direction to pCO2, what is the likely cause?

Answer 36

Respiratory acidosis/alkalosis

Question 37

If [HCO3-] is not normal, pCO2 is normal and pH has changed in the same direction as [HCO3-], what is the most likely cause?

Answer 37

Metabolic acidosis/alkalosis

Question 38

If pCO2 is high, [HCO3-] is raised and pH is relatively normal, what has most likely occurred?

Answer 38

Compensated respiratory acidosis

Question 39

If [HCO3-] is low, pCO2 is low and pH is relatively normal, what has most likely occurred?

Answer 39

Could be either compensated respiratory alkalosis or compensated metabolic acidosis - if no respiratory disease or altitude exposure, unlikely to be respiratory - check anion gap, if increased is metabolic acidosis