5. Acid-Base Balance Flashcards
What is the problem with alkalaemia?
Lowers free calcium by causing Ca2+ ions to come out of solution
- increases neuronal excitability
What does pH>7.45 lead to?
Alkalaemia, leads to paraesthesia and tetany
What is the problem with acidaemia?
Increases plasma potassium ion concentration
- effects excitability, leads to arrhythmias
Increased [H+] can denature proteins
- effects muscle contractility, glycolysis, hepatic function
What does pH depend on?
Ratio of [HCO3-] to pCO2
Why doesn’t the acid produced due to metabolism not deplete HCO3-?
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
How is HCO3- recovered in the kidney?
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
How is HCO3- created in PCT?
Glutamine -> alpha-ketoglutarate + NH4+
NH4+ -> NH3 + H+
Alpha-keotglutarate -> 2HCO3-
NH3 enters lumen and forms NH4+
What is the minimum pH of urine?
4.5
Why does hyperkalaemia happen in acidosis?
Potassium moves out of cells
Decreased potassium excretion in distal nephron
Why does alkalosis cause hypokalaemia?
Potassium ions move into cells
Enhanced exertion of potassium in distal nephron
Why can hyperkalaemia cause acid base disturbances?
Makes intracellular pH of tubular cells more alkaline
- H+ ions move out of cells
- favours HCO3- exertion
Leads to metabolic acidosis
Why does hypokalaemia cause acid base disturbances?
Makes intracellular pH of tubular cells more acidic
- H+ ions move into cells
- favours H+ excretion and HCO3- recovery
This leads to metabolic alkalosis
What can cause respiratory acidosis?
Hypoventilation -> hypercapnia
Hypercapnia -> fall in plasma pH
What is respiratory acidosis characterised by?
High pCO2
Normal HCO3-
Low pH
How can changes in pCO2 be compensated?
By changes in [HCO3-]
Kidneys increase [HCO3-] to compensate for respiratory acidosis
Kidneys decrease [HCO3-] to compensate for respiratory alkalosis
Takes 2-3 days
What is compensated respiratory acidosis characterised by?
High pCO2
Raised [HCO3-]
Relatively normal pH
What is compensated respiratory alkalosis characterised by?
Low pCO2
Lowered [HCO3-]
Relatively normal pH
What is metabolic acidosis?
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
What is the anion gap?
Difference between measured cations and anions
Normally 10-18 mmol/L
When is the anion gap increased?
If HCO3- is replaced by other anions
Does the anion gap change in renal causes of acidosis?
No it remains unchanged
This is because not making enough HCO3- but this is replaced by Cl-
What is metabolic acidosis initially characterised by?
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-
How is metabolic acidosis compensated?
Peripheral chemoreceptors (carotid bodies) detect pH drop
- stimulate ventilation
- leading to decrease pCO2
What is compensated metabolic acidosis characterised by?
Low HCO3-
Lowered pCO2
Nearer normal pH
What happens in metabolic alkalosis?
[HCO3-] increases
Normal pCO2
Increased pH
Cannot normally be compensated to a great extent by reducing breathing - need to maintain pO2
What are conditions that lead to respiratory acidosis?
Type 2 respiratory failure
What happens in type 2 respiratory failure?
Low pO2 and high pCO2
Alveoli cannot be properly ventilated
Can be compensated for by increase in [HCO3-]
What causes type 2 respiratory failure?
Severe COPD
Severe asthma
Drug overdose
Neuromuscular disease
What conditions can lead to respiratory alkalosis?
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
What happens if an anion gap is increased?
Indicated a metabolic production of acid
Could be ketoacidosis, lactic acidosis, uraemic acidosis
What conditions can lead to metabolic acidosis with a normal anion gap?
Renal tubularacidosis (rare) - problem with transport mechanisms in tubules, type 1 RTA, type 2 RTA Severe persistent diarrhoea - due to loss of HCO3-
What does non-renal causes of metabolic acidosis cause?
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+
Why can diabetic ketoacidosis cause total body depletion of K+?
K+ moves out of cells due to acidosis and lack of insulin
But osmotic diuresis means K+ lost in urine
What conditions can lead to metabolic alkalosis?
Severe prolonged vomiting - loss of H+ Potassium depletion/mineralocorticoid excess Certain diuretics (loop and thiazides)
How is metabolic alkalosis corrected?
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
If pCO2 is not normal, [HCO3-] is normal and pH has changed in opposite direction to pCO2, what is the likely cause?
Respiratory acidosis/alkalosis
If [HCO3-] is not normal, pCO2 is normal and pH has changed in the same direction as [HCO3-], what is the most likely cause?
Metabolic acidosis/alkalosis
If pCO2 is high, [HCO3-] is raised and pH is relatively normal, what has most likely occurred?
Compensated respiratory acidosis
If [HCO3-] is low, pCO2 is low and pH is relatively normal, what has most likely occurred?
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
