S5) Acid Base Balance

Question 1

What is alkalaemia?

Answer 1

Alkalaemia is a plasma pH greater than 7.45

Question 2

What is acidaemia?

Answer 2

Acidaemia is a plasma pH less than 7.35

Question 3

Which conditions can arise from alkalaemia?

Answer 3

• Paraesthesia
• Tetany

Question 4

Why does alkalaemia lead to paraesthesia and tetany?

Answer 4

• Free calcium is lowered by causing Ca²⁺ to come out of solution
• This increases neuronal excitability

Question 5

Describe the mortality associated with alkalaemia

Answer 5

• 45% mortality - pH rises to 7.55
• 80% mortality - pH rises to 7.65

Question 6

Describe the effect of acidaemia on excitability in the body

Answer 6

• Increases plasma [K⁺]
• Affects excitability (particularly cardiac muscle) which causes arrhythmias

Question 7

Describe the effect of acidaemia on enzymes

Answer 7

• Increasing [H⁺] affects many enzymes and denatures proteins
• This affects muscle contractility, glycolysis, hepatic function

Question 8

Describe the mortality associated with acidaemia

Answer 8

• Severe effects – below pH 7.1
• Life threatening – below pH 7.0

Question 9

pCO₂ is determined by respiration.

Identify the factors which disturb and control pCO₂

Answer 9

• Controlled by chemoreceptors
• Disturbed by respiratory disease

Question 10

[HCO₃^-] is determined by the kidneys.

Identify the factors which disturb and control [HCO₃^-]

Answer 10

• Controlled by the kidney
• Disturbed by metabolic and renal disease

Question 11

Why does metabolic acid not deplete hydrogen carbonate?

Answer 11

• The kidneys recover all filtered HCO₃^-
• PCT makes HCO₃^- from AA, putting NH₄⁺ into urine
• DCT makes HCO₃^- from CO₂ and H₂O (H⁺ is buffered by phosphate and ammonia in the urine)

Question 12

Describe the renal control of hydrogen carbonate

Answer 12

• HCO₃^- filtered at the glomerulus & recovered in PCT
• H+ excretion linked to Na⁺ entry in PCT
• H⁺ reacts with HCO₃^- (lumen) to form CO₂ which enters cell and is converted back to HCO₃^- which enters ECF

Question 13

Describe the role of glutamine in the creation of hydrogen carbonate in th PCT

Answer 13

• Glutamine breaks down and forms α-ketoglutarate
• HCO₃^- and ammonium are produced
• HCO₃^- enters ECF and NH₄⁺ enters lumen (urine)

Question 14

Describe the renal control of hydrogen carbonate in the distal tubule and collecting duct

Answer 14

• DCT and CD also actively secrete H⁺
• H⁺ buffered by ammonia and phosphate to produce NH₄⁺ and H₂PO₄^- which are excreted
• HCO₃^- now can enter plasma

Question 15

Excretion of ammonium is the major adaptive response to an increased acid load.

Describe the role of ammonium in the renal control of HCO₃^-

Answer 15

NH4⁺ ⇒ NH₃ + H⁺

• NH₃ freely moves into lumen and throughout interstitium
• H⁺ actively pumped into lumen in DCT and CT
• H⁺ combines with NH₃ to form NH₄⁺ (trapped in lumen)

Question 16

What is the pH of urine?

Answer 16

The minimum pH of urine is 4.5

Question 17

How much acid is secreted per day and why?

Answer 17

• Total acid excretion = 50 – 100mmol H⁺ per day
• Needed to keep normal [HCO₃^-]

Question 18

Explain how acidosis leads to hyperkalaemia

Answer 18

• K⁺ move out of cells
• Decreased K⁺ excretion in distal nephron

Question 19

Explain how alkalosis leads to hypokalaemia

Answer 19

• K⁺ move into cells
• Enhanced excretion of K⁺ in distal nephron

Question 20

Explain how hyperkalaemia can lead to metabolic acidosis

Answer 20

• Hyperkalaemia makes pH_i of tubular cells more alkaline
• H⁺ move out of the cells which favours HCO₃^- excretion

Question 21

Explain how hypokalaemia can lead to metabolic alkalosis

Answer 21

• Hypokalaemia makes pH_i of tubular cells more acidic
• H⁺ move into the cells which favours H⁺ excretion and HCO₃^- recovery

Question 22

How does hypoventilation lead to respiratory acidosis (acidaemia)?

Answer 22

• Hypoventilation → hypercapnia (pCO₂ rises)
• Hypercapnia → fall in plasma pH

Question 23

Identify 3 factors which characterise respiratory acidosis

Answer 23

• High pCO₂
• Normal HCO₃^-
• Low pH

Question 24

How does hyperventilation lead to respiratory alkalosis (alkalaemia)?

Answer 24

• Hyperventilation → hypocapnia (fall in pCO₂)
• Hypocapnia → rise in pH

Question 25

Identify 3 factors which characterise respiratory alkalosis

Answer 25

- Low pCO₂ - Normal HCO₃^- - Raised pH

Question 26

How do kidneys compensate for respiratory acidosis?

Answer 26

The kidneys **increase [HCO₃^-]** to compensate for respiratory acidosis

Question 27

How do kidneys compensate for respiratory alkalosis?

Answer 27

The kidneys **decrease [HCO₃^-]** to compensate for respiratory alkalosis

Question 28

Identify 3 factors which characterise compensated respiratory acidosis

Answer 28

- High pCO₂ - Raised [HCO₃^-] - Relatively normal pH

Question 29

Identify 3 factors which characterise compensated respiratory alkalosis

Answer 29

- Low pCO₂ - Lowered [HCO₃^-] - Relatively normal pH

Question 30

What is the anion gap?

Answer 30

The **anion gap** is the difference between measured cations and anions ([Na⁺] + [K⁺]) – ([Cl^-] + [HCO₃^-])

Question 31

What is the normal value for the anion gap and why?

Answer 31

Normally **10 – 18 mmol.l^-1** due to other anions that are not measured

Question 32

How can the anion gap be increased?

Answer 32

Increased if HCO₃^- is replaced by other anions: ## Footnote *E.g. If a metabolic acid (such as lactic acid) reacts with HCO₃^- the anion of the acid replaces HCO₃^-*

Question 33

In renal causes of acidosis, the anion gap will be unchanged. Why?

Answer 33

Not making enough HCO₃^- but this is replaced by Cl^-

Question 34

In terms of the anion gap, identify 2 factors which characterise metabolic acidosis

Answer 34

- **Increased anion gap** if HCO₃^- is replaced by another organic anion from an acid - **Normal anion gap** if HCO₃^- replaced by Cl^-

Question 35

Explain how carotid bodies compensate for metabolic acidosis

Answer 35

- Peripheral chemoreceptor (carotid bodies) detect pH drop - Ventilation is stimulated which leads to a decrease in pCO₂

Question 36

Identify 3 factors which characterise compensated metabolic acidosis

Answer 36

- Low HCO₃^- - Lowered pCO₂ - Nearer normal pH

Question 37

What is observed in Type II Respiratory Failure?

Answer 37

- Low pO₂ and high pCO₂ - Alveoli cannot be properly ventilated

Question 38

Identify 4 clinical conditions in Type II respiratory failure which lead to respiratory acidosis

Answer 38

- Severe COPD - Severe asthma - Drug overdose - Neuromuscular disease

Question 39

How can respiratory acidosis be compensated for?

Answer 39

Increase [HCO₃^-]

Question 40

What is observed in Type I Respiratory Failure (hyperventilation in response to long term hypoxia)?

Answer 40

Low pCO₂ with initial rise in pH

Question 41

Identify 2 clinical conditions in Type I respiratory failure which cause respiratory alkalosis

Answer 41

- Chronic hyperventilation - Anxiety/panic attacks

Question 42

How can respiratory alkalosis be compensated for?

Answer 42

Decrease [HCO₃^-]

Question 43

Identify and describe 3 clinical conditions which can cause metabolic acidosis

Answer 43

- **Keto-acidosis:** seen in diabetes - **Lactic acidosis:** exercising to exhaustion leading to poor tissue perfusion - **Uraemic acidosis:** advanced renal failure leading to reduced acid secretion & build up of plasma phosphate, sulphate, urate

Question 44

Identify 2 clinical conditions which lead to metabolic acidosis with a normal anion gap

Answer 44

- Renal tubular acidosis - Severe persistent diarrhoea

Question 45

Renal tubular acidosis involves problems with transport mechanisms in the tubules. Identify and describe its 2 forms

Answer 45

- **Type 1 RTA (distal)** involves the inability to pump out H⁺ - **Type 2 RTA (proximal)** involves problems with HCO₃^- reabsorption

Question 46

Identify 3 clinical states which lead to metabolic alkalosis

Answer 46

- Severe prolonged vomiting (loss of H⁺ from stomach) - Potassium depletion - Mineralocorticoid excess

Question 47

Certain diuretics can also cause metabolic alkalosis. Identify 2

Answer 47

- Loop diuretic - Thiazide diuretic

Question 48

What process is occuring in the following: Abnormal pCO₂, normal [HCO₃^-] and pH has changed in opposite direction to pCO₂

Answer 48

Respiratory acidosis / alkalosis

Question 49

What process is occurring in the following: Abnormal [HCO₃^-], normal pCO₂ and pH has changed in the same direction as [HCO₃^-]

Answer 49

Metabolic acidosis / alkalosis

Question 50

What process is occurring in the following: High pCO₂, raised [HCO₃^-] and pH is relatively normal

Answer 50

Compensated respiratory acidosis

Question 51

What process is occurring in the following: Low [HCO₃^-], low pCO₂ and pH is relatively normal

Answer 51

Compensated respiratory alkalosis / compensated metabolic acidosis (check anion gap)

Question 52

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.22 (7.35 - 7.45) - pCO₂ = 8.1 (4.7 - 6.0) - HCO₃^- = 24 (22 - 26) - pO₂ = 8.9 (9.3 - 13.3)

Answer 52

Uncompensated **respiratory acidosis**

Question 53

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.22 (7.35 - 7.45) - pCO₂ = 4.5 (4.7 - 6.0) - HCO₃^- = 13 (22 - 26) - pO₂ = 12.4 (9.3 - 13.3)

Answer 53

Partially compensated **metabolic acidosis**

Question 54

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.46 (7.35 - 7.45) - pCO₂ = 3.3 (4.7 - 6.0) - HCO₃^- = 17 (22 - 26) - pO₂ = 13.3 (9.3 - 13.3)

Answer 54

Partially compensated **respiratory alkalosis**

Question 55

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.22 (7.35 - 7.45) - pCO₂ = 4.8 (4.7 - 6.0) - HCO₃^- = 14 (22 - 26) - pO₂ = 11.8 (9.3 - 13.3)

Answer 55

Uncompensated **metabolic acidosis**

Question 56

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.28 (7.35 - 7.45) - pCO₂ = 8.6 (4.7 - 6.0) - HCO₃^- = 30 (22 - 26) - pO₂ = 8.6 (9.3 - 13.3)

Answer 56

Partially compensated **respiratory acidosis**

Question 57

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.46 (7.35 - 7.45) - pCO₂ = 5.6 (4.7 - 6.0) - HCO₃^- = 29 (22 - 26) - pO₂ = 10.1 (9.3 - 13.3)

Answer 57

Uncompensated **metabolic alkalosis**

Question 58

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.35 (7.35 - 7.45) - pCO₂ = 7 (4.7 - 6.0) - HCO₃^- = 28 (22 - 26) - pO₂ = 8.6 (9.3 - 13.3)

Answer 58

Fully compensated **respiratory acidosis**

Question 59

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.35 (7.35 - 7.45) - pCO₂ = 4.1 (4.7 - 6.0) - HCO₃^- = 17 (22 - 26) - pO₂ = 13.0 (9.3 - 13.3)

Answer 59

Fully compensated **metabolic acidosis**

Question 60

Describe the form of acidosis/alkalosis happening in the patient: - pH = 7.5 (7.35 - 7.45) - pCO₂ = 4.5 (4.7 - 6.0) - HCO₃^- = 26 (22 - 26) - pO₂ = 13.0 (9.3 - 13.3)

Answer 60

Uncompensated **respiratory alkalosis**