Session 5 - Acid-Base Balance Flashcards
What pH range should blood plasma normally be maintained at?
7.35 - 7.45
What defines acidaemia and alkalaemia?
Acidaemia - Plasma pH less than 7.35
Alkalaemia - Plasma pH greater than 7.45
What are the effects to the plasma and cells of alkalaemia?
Alkalaemia lowers free calcium, by causing Ca2+ ions to come out of solution.
This can increase neuronal excitability and cause tetany.
How does alkalaemia cause Ca2+ to come out of solutin / blood Ca2+ to lower?
- pH Increases, so H+ <
- Albumin and other blood proteins become anions (lose H+).
- Ca2+ binds to these proteins
(In severe alkalaemia)
Which is more dangerous? Alkalaemia or Acidosis?
Alkalosis can be more serious.
What are the mortalities of a plasma pH 7.55, and 7.65?
- 55 = 45% mortality
7. 65 = 80% mortality
What are the effects of acidaemia on the body?
- Increase plasma potassium ion concentration.
(effects excitability, particularly cardiac muscle, can cause arrythmia) - Increased H+ can affect enzymes (denature)
(effects muscle contractility, glycolysis, liver etc)
When are the effects of acidaemia a) severe, and b) life threatening?
Severe = pH <7.1
Life threatening = pH <7.0
Which factors in the body control pH?
- Ratio of [HCO3] to pCO2.
- pCO2 defined by respiration.
- [HCO3-] controlled by kidneys
Why does metabolic acid production not deplete HCO3-?
- Kidneys recover all filtered HCO3-
- Proximal tubule makes HCO3- from amino acids, putting NH4+ in urine.
- Distal tubule makes bicarbonate from CO2 and water, H+ is buffered by phosphate and ammonia in urine.
How are bicarbonate ions produced in the proximal tubule?
Glutamin > a-ketoglutarate
which: produces bicarbonate and ammonium.
Bicarbonate enters ECF, ammonium enters lumen (urine).
How are hydrogen ions removed from the body, without being reformed into water and moving back into tubule cells?
- H+ ions actively secreted
- H+ buffered by ammonia and phosphate, producing NH4+ and H2PO4- in urine.
- This prevents CO2 from being reformed from bicarbonate, and bicarbonate re-enters the plasma.
Why is buffering of the H+ in urine important?
Stops urine becoming too acidic.
What is the major adaptive response to increased acid load in healthy people?
Excretion of ammonium.
- Ammonium generation from glutamine can be increased.
- NH3 moves freely into lumen and through interstitium.
- H+ actively pumped into lumen in DCT + CT.
- H+ combines with NH3, making NH4+
this is trapped in lumen, and excreted.
What is the minimum pH of urine?
4.5
What is the total acid excretion in urine per day?
50-100mmol H+ per day
How does acidosis and alkalosis affect potassium?
Acidosis = >H+
- Potassium ions move OUT of cells, H+ moves in
- Decreased potassium excretion in distal nephron
Alkalosis =
How does hypokalaemia affect pH?
- Hypokalaemia makes H+ ions move into cells (tubular cell pH more acidic)
- Favours H+ excretion and HCO3- recovery
Metabolic Alkalosis
How does hyperkalaemia affect tubule pH, and plasma pH?
- Hyperkalaemia - H+ move out of cells. (makes tubular cell pH more alkaline)
- Favours HCO3- excretion
Metabolic acidosis
What is uncompensated respiratory acidosis?
Hypoventilation (hypercapnia)
Fall in pH, increases pCO2.
Causes acidosis (acidaemia)
NORMAL HCO3-
What is uncompensated respiratory alkalosis?
Hyperventilation (hypocapnia)
RISE in pH, decreased pCO2 (fewer H+ ions)
Alkalosis (alkalaemia)
Nomal HCO3-
How can the kidneys compensate against respiratory acidosis or alkalosis?
Acidosis = Increase HCO3- Alkalosis = Decrease HCO3-
What is metabolic acidosis?
When tissues produce acid, which reacts with, and removes HCO3-.
Fall in HCO3- and thus pH.
Why is there no change in pCO2 with metabolic acidosis?
The extra CO2 produced is breathed off at lungs.
What is the anion gap?
The measured difference between the main cations and anions.
([Na+] + [K+]) - ([Cl-] + [HCO3-]
What is the normal anion gap? When does it change?
10-18mmol/L
Change if bicarbonate is replaced by other anions.
In renal causes of acidosis, why is the anion gap unchanged?
- Not making enough HCO3-, but this will be replaced by Cl-
How is metabolic acidosis compensated?
Peripheral chemoreceptors detect stimulation of ventilation
Low HCO3-
Lowered pCO2
Nearer normal pH
Why can metabolic alkalosis not be corrected for by breathing?
Reducing ventilation would impair pO2, which needs to be maintained for perfusion of tissues.
Which conditions can lead to respiratory acidosis?
Type 2 Respiratory failure
- Low PO2, high pCO2
- alveoli not properly ventilated
COPD, Asthma, drug overdose, neuromuscular disease
Which conditions can lead to respiratory alkalosis?
Hyperventilation
- Anxiety/ panic attacks
- Low pCO2, >pH
Hyperventilation in response to long term hypoxia
- Low pCO2, initial rise in pH which is compensated for by fall in HCO3-
Which conditions can lead to metabolic acidosis?
If anion gap is INCREASED - must be metabolic.
-Keto-acidosis (diabetes)
- Lactic acidosis (exercise exhaustion, low tissue perfusion)
- Uraemic acidosis (Advanced renal failure)
What causes uraemic acidosis in advanced renal failure?
Reduced acid secretion in kidneys.
Build up of phosphate, sulphate and urate in blood.
Which conditions lead to metabolic acidosis with a normal anion gap?
- Renal tubular acidosis (rare)
problems with transport mechanisms in tubules.
- Type 1 RTA - inability to pump out H+
- Type 2 RTA - problems with HCO3- reabsorption.
- Severe persistent diarrhoea, through loss of HCO3-
(replaced by Cl- so gap unaltered)
What happens to K+ in non renal acidosis?
Increased reabsorption of K+.
Movement of K+ out of cells.
> HYPERKALAEMIA
(in diabetic ketoacidosis, may be total body depletion of K+)
Which conditions can lead to metabolic alkalosis?
- Severe prolonged vomiting (stomach makes lots of HCO3-)
- Potassium depletion
- Some diuretics (loop/ thiazide)
How is metabolic alkalosis corrected?
> pH causes fall in H+ excretion. Reduced HCO3- recovery.
In volume depletion, what happens in metabolic alkalosis?
Capacity to lose HCO3- is reduced, due to high rate of Na+ recovery.
Recovering Na+ favours H+ excretion and HCO3- recovery.
How can metabolic alkalosis cause hypokalaemia?
Less H+ excreted at nephrons, causes more K+ to be excreted.
Alkalosis causes K+ to move INTO cells.
causes hypokalaemia
