Acid Base Balance

Question 1

State the normal range plasma pH

Answer 1

7.35 - 7.45

Question 2

Explain the effects of alkalaemia

Answer 2

• pH > 7.45
• Lowers free calcium by causing Ca2+ ions to come out of solution
  
  • Binds proteins
  • Increases neuronal excitability (calcium makes membrane less excitable)
• More serious than acidaemia

Question 3

Explain the effects of acidaemia

Answer 3

• pH < 7.35
• Increases plasma potassium ion concentration - hyperkalaemia
  
  • Affects excitability - particularly cardiac muscle
  • Arrhythmia
• Increasing [H+] denatures enzymes
  - Affects muscle contractility, glycolysis, hepatic function

Question 4

Interpret uncomplicated blood gas abnormalities and recognise respiratory acidosis, respiratory alkalosis, metabolic acidosis and metabolic alkalosis

Answer 4

1. Look whether pH has increased or decreased
2. Loos at pCO2 and [HCO3] and see if increased or decreased
3. If pH change and pCO2 change are in the opposite direction, then respiratory cause
4. If pH change and [HCO3] change are in the same direction, then metabolic cause

Question 5

How do the kidneys compensate for respiratory acidosis/alkalosis

Answer 5

• Kidneys increase [HCO3] to compensate for respiratory acidosis
• Kidneys decrease [HCO3] to compensate for respiratory alkalosis
• Takes time, 2-3 days

Question 6

Describe how reabsorption of HCO3 in proximal tubule works

Answer 6

• HCO3 filtered at the glomerulus and most recovered in PCT
• Basolateral membrane has Na/K ATPase pumping sodium out of the cell
• Na/H exchanger allows Na entry into the cell which drives H+ secretion into the nephron lumen
• H+ reacts with HCO3- in the lumen to form CO2 which enters cell freely
• Converted back to HCO3, which enters ECF through Na/HCO3 cotransporter

Question 7

Outline how HCO3 creation occurs in proximal tubule

Answer 7

• Glutamine is converted to α-ketoglutarate and NH4
• NH4 dissociates into NH3 and H+
• NH3 uncharged so can cross membrane and enter lumen where it forms NH4 which exits through urine
• α-ketoglutarate forms 2 HCO3 which enters ECF

Question 8

Define titratable acid

Answer 8

Any acid that has the ability to lose a proton in an acid base reaction

Question 9

Outline the role of NH4 in buffering H+

Answer 9

• Ammonium generation from glutamine in proximal tubule can be increased in response to low pH
• NH4+ -> NH3 + H+
• NH3 freely moves into lumen and through interstitium
  
  • Able to move from PCT (where it is formed) to the DCT and CT to buffer H+
• H+ actively pumped into lumen in DCT and CT
• H+ combines with NH3 to form NH4+
  
  • Trapped in lumen and excreted in urine
• NH4+ can also be taken up in ascending limb and transported to interstitium and dissociates to H+ and NH3
  - NH3 then enters lumen of collecting duct

Question 10

What titratable acids can buffer H+

Answer 10

NH4, H2PO4

Question 11

Explain the effect of H+ levels on K+ concentration

Answer 11

• Acidosis leads to hyperkalaemia
  
  • H+ moves into cells as high concentration of [H+] out of cell
  • Causes potassium ions to move out of cells
  • Decreased potassium excretion in distal nephron
• Alkalosis leads to hypokalaemia
  
  • H+ moves out of cells
  • Potassium ions move into cells
  • Enhanced excretion of potassium in distal nephron

Question 12

State some causes of respiratory acidosis

Answer 12

• Type 2 respiratory failure
  
  • Low pO2 and high pCO2
  • Alveoli cannot be properly ventilated
  • Severe COPD, severe asthma, drug overdose, neuromuscular disease
• Can be compensated for by increase in [HCO3]
• Chronic conditions can be well compensated such that pH near normal
  - However, higher level of ‘normal’ pCO2 will be established

Question 13

State some causes of respiratory alkalosis

Answer 13

• Hyperventilation
  
  • Anxiety/panic attacks - acute setting
  • High altitude
  • Low pCO2, rise in pH
• Hyperventilation in response to long-term hypoxia - type 1 respiratory failure
  
  • Low pCO2 with initial rise in pH
  • Chronic hyperventilation can be compensated for by fall in [HCO3]
    
    • Can restore pH to near normal

Question 14

Describe the common causes of metabolic alkalosis

Answer 14

• Stomach is a major site of HCO3 production
  
  • By-product of H+ secretion
  • Severe prolonged vomiting - loss of H+
  • Mechanical drainage of stomach
• Hypokalaemia makes the intracellular pH of tubular cells more acidic
  
  • H+ ions move into tubular cells as K+ exits cells
  • Favours H+ excretion and HCO3 recovery
  • Metabolic alkalosis
• Certain diuretics - loops and thiazide

Question 15

How can metabolic acidosis occur

Answer 15

• If the tissues produce acid, this reacts with and removes HCO3
• Leads to a fall in [HCO3] and fall in pH
• Extra CO2 produced is breathed off at the lungs so there is no increase in arterial pCO2

Question 16

Describe the anion gap

Answer 16

• Difference between measured cations and anions

- ([Na] + [K]) - ([Cl] + [HCO3])

Question 17

How can anion gap change

Answer 17

• Gap is increased if HCO3 is replaced by other anions
  
  • If a metabolic acid (such as lactic acid) reacts with HCO3, the anion of the acid replaces HCO3
• In renal causes of acidosis anion gap will be unchanged
  - Not making enough HCO3, but this is replaced by Cl

Question 18

Give examples of metabolic acids which can increase anion gap

Answer 18

• Keto acidosis - diabetes
• Lactic acidosis
• Uraemic acidosis - advanced renal failure

Question 19

Gives examples when anion gap has not changed in metabolic acidosis

Answer 19

• Renal tubular acidosis (RTA) - rare
  
  • Problems with transport mechanisms in the tubules
  • Type 1 (distal) RTA - inability to pump out H+
  • Type 2 (proximal) RTA - problems with HCO3 reabsorption
• Severe persistent diarrhoea
  - Loss of HCO3 replaced by Cl