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Respiratory > 5. Acid-Base Balance > Flashcards

5. Acid-Base Balance Flashcards

1
Q

What is the problem with alkalaemia?

A

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

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2
Q

What does pH>7.45 lead to?

A

Alkalaemia, leads to paraesthesia and tetany

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3
Q

What is the problem with acidaemia?

A

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

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4
Q

What does pH depend on?

A

Ratio of [HCO3-] to pCO2

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5
Q

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

A

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

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6
Q

How is HCO3- recovered in the kidney?

A 
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
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7
Q

How is HCO3- created in PCT?

A

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

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8
Q

What is the minimum pH of urine?

A

4.5

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9
Q

Why does hyperkalaemia happen in acidosis?

A

Potassium moves out of cells

Decreased potassium excretion in distal nephron

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10
Q

Why does alkalosis cause hypokalaemia?

A

Potassium ions move into cells

Enhanced exertion of potassium in distal nephron

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11
Q

Why can hyperkalaemia cause acid base disturbances?

A

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

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12
Q

Why does hypokalaemia cause acid base disturbances?

A

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

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13
Q

What can cause respiratory acidosis?

A

Hypoventilation -> hypercapnia

Hypercapnia -> fall in plasma pH

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14
Q

What is respiratory acidosis characterised by?

A

High pCO2
Normal HCO3-
Low pH

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15
Q

How can changes in pCO2 be compensated?

A

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

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16
Q

What is compensated respiratory acidosis characterised by?

A

High pCO2
Raised [HCO3-]
Relatively normal pH

17
Q

What is compensated respiratory alkalosis characterised by?

A

Low pCO2
Lowered [HCO3-]
Relatively normal pH

18
Q

What is metabolic acidosis?

A

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

19
Q

What is the anion gap?

A

Difference between measured cations and anions

Normally 10-18 mmol/L

20
Q

When is the anion gap increased?

A

If HCO3- is replaced by other anions

21
Q

Does the anion gap change in renal causes of acidosis?

A

No it remains unchanged

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

22
Q

What is metabolic acidosis initially characterised by?

A

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-

23
Q

How is metabolic acidosis compensated?

A

Peripheral chemoreceptors (carotid bodies) detect pH drop

  • stimulate ventilation
  • leading to decrease pCO2
24
Q

What is compensated metabolic acidosis characterised by?

A

Low HCO3-
Lowered pCO2
Nearer normal pH

25
Q

What happens in metabolic alkalosis?

A

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

26
Q

What are conditions that lead to respiratory acidosis?

A

Type 2 respiratory failure

27
Q

What happens in type 2 respiratory failure?

A

Low pO2 and high pCO2
Alveoli cannot be properly ventilated

Can be compensated for by increase in [HCO3-]

28
Q

What causes type 2 respiratory failure?

A

Severe COPD
Severe asthma
Drug overdose
Neuromuscular disease

29
Q

What conditions can lead to respiratory alkalosis?

A

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

30
Q

What happens if an anion gap is increased?

A

Indicated a metabolic production of acid

Could be ketoacidosis, lactic acidosis, uraemic acidosis

31
Q

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

A 
Renal tubularacidosis (rare) - problem with transport mechanisms in tubules, type 1 RTA, type 2 RTA
Severe persistent diarrhoea - due to loss of HCO3-
32
Q

What does non-renal causes of metabolic acidosis cause?

A

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+

33
Q

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

A

K+ moves out of cells due to acidosis and lack of insulin

But osmotic diuresis means K+ lost in urine

34
Q

What conditions can lead to metabolic alkalosis?

A 
Severe prolonged vomiting - loss of H+
Potassium depletion/mineralocorticoid excess
Certain diuretics (loop and thiazides)
35
Q

How is metabolic alkalosis corrected?

A

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

36
Q

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

A

Respiratory acidosis/alkalosis

37
Q

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

A

Metabolic acidosis/alkalosis

38
Q

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

A

Compensated respiratory acidosis

39
Q

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

A

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

Respiratory (23 decks)

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