Acid-Base Balance Flashcards
What is the normal pH range of plasma?
pH 7.35 - 7.45
Anything greater 7.45 = alkalemia
Anything less 7.35 = acidaemia
What is the consequence of alkalemia?
- Alkalemia lowers free calcium as it causes Ca2+ ions to come out of solution
- Decreased Ca2+ concentration makes neuronal cells more excitable
- Can lead to paraethesia and tetany
What are some of the consequence of acidaemia?
- Acidaemia increases plasma K+ concentration
- affects excitability of cardiac cells leading to arrhythmia
- Acidaemia can denature proteins affecting muscle contractility, glycolysis, hepatic function
What is the equation that controls plasma pH?
How do the kindneys control plasma pH?
By adjusting the recovery of HCO3- and actively secreting H+ ions
How do the lungs control plasma pH?
Alveolar ventilation determines pCO2 and pO2
Rate of ventilation controlled by chemoreceptors
In normal circumstances, why does increased acid production from metabolism not deplete HCO3-?
- The kidneys recover all filtered HCO3-
- Proximal tubules makes HCO3- from amino acids, putting NH4+ into urine
- H+ ions excreted to lumen but reacts with HCO3- to give H2O + CO2 which can diffuse into the tubular cell
- HCO3- enters ECF
Explain how the proximal tubule creates HCO3-
- Inside proximal tubule cells Glutamine → alpha- ketogluturate and NH4 +
- alpha- ketogluturate breaks down to HCO3- which is transported to ECF along with Na+ ions
- NH4+ breaks fown to NH3 (ammonia) and H+ ions
- Ammonia freely diffused into lumen of tubule and combines with H+ → NH4+ acting as an ion buffer
What is the function of ammonia throughout the kidney?
Ammonia acts as a buffer by binding with H+ ions
What is the minimum pH of urine?
pH 4.5
How do acidosis and alkalosis affect K+ ions in the nephon?
H+ and K+ are both cations, therefore movement of one will cause the reciprocal movement of th other
Acidosis: causes K+ to move out cell causing hyperkalaemia and decreased K+ excretion at the distal nephron
Alkalosis: causes K+ to move into cells causing hypokalaemia and increased K+ excretion at the distal nephron
How do changes in K+ ions lead to metabolic acidosis?
Metabolic acidosis caused by hyperkalaemia
Hyperkalemia causes H+ ions to move out of tubular cells which favours HCO3- excretion → metabolic acidosis
How do changes in K+ ions lead to metabolic alkalosis?
Hypokalaemia makes intracellular pH of tubular cells more acidic → causes H+ to move into tubular cells
Favour H+ excretion → metabolic alkalosis
What is respiratory acidosis and what would you expect to see in the blood results to confirm this?
Hypoventilation causing hypercapnia (increased pCO2) → equilibrium shifts right → fall in plasma pH
Blood results:
- High pCO2
- Normal HCO3-
- Low pH
What is respiratory alkalosis and what would you expect to see in the blood results to confirm this?
Hyperventilation → hypocapnia (low CO2) → equilibrium shifts left → pH rises
Blood results:
- pCO2 low
- HCO3- normal
- pH high
In respiratory acidosis and alkalosis, what is compensation? What will you see in the bloods if fully compensated?
To compensate for changes in pH the kidneys can alter [HCO3-]
acidosis: kidneys increase [HCO3-]
alkalosis: kidneys decrease [HCO3-]
If fully compensated pH will be normal
How long does it take for the kidneys to compensate from respiratory acid/ alkalosis?
2-3 days
What is metabolic acidosis? What will you see in the blood results to confirm this?
Metabolically active tissues produce H+ which reacts the HCO3- to deplete stores → pH drops
Bloods:
- pCO2 normal
- low [HCO3-]
- pH low
What is the anion gap?
The difference between measurable cations and anions, mainly:
([Na+] + [K+] ) - ([Cl-] + [HCO3-])
What is the normal anion gap? In what circumstances would the gap change?
Normally 10-18 mmol.l-1
Gap increases if HCO3- replaced by other anions (not measurable) e.g. when lactic acid reacts with HCO3-
Why is the anion gap unchanged in renal causes of acidosis?
Although less HCO3- is made, it is replaced by Cl- ions which are measurable
How does the body compensate for metabolic acidosis and what will you see in the blood results?
Drop in pH detected by peripheral chemoreceptors → signal to respiratory centre to increase ventilation → decreases pCO2
Bloods:
- normal pH
- low pCO2
- low HCO3-
What happens in metabolic alkalosis?
[HCO3-] increases
Bloods:
- pH high
- pCO2 normal
- [HCO3-] raised
Can metabolic alkalosis be compensated?
Yes but only to a small extent as you cannot reduce breathing too much as pO2 needs to be maintained
What conditions can lead to respiratory acidosis and how?
Type 2 respiratory failure
alveoli are not properly ventilated meaning low pO2 and high pCO2
Includes conditions: COPD, severe asthma, drug overdose and neuromuscular disease
What can cause respiratory alkalosis?
- Hyperventilation - in anxiety / panic attacks
- Type 1 respiratory failure
What conditions can lead to metabolic acidosis?
- Diabeteic ketoacidosis
- Lactic acidosis
- Uremic acidosis in advanced renal failure
- Severe, persisent diarrhoea
What can lead to metabolic alkalosis?
- severe, prolonged vomiting (loss of H+)
- K+ depletion
- Certain diuretics
How do non-renal causes of metabolic acidosis cause hyperkalaemia?
Acidosis = increased H+ ions
H+ moves into cell and K+ moves out of cells cause hyperkalaemia
How can diabetic ketoacidosis lead to total body depletion of K+?
K+ moves out of cells due to acidosis and osmotic diuresis means the ions are lost in urine
How does metabolic acidosis lead to hypokalaemia?
- Less H+ excretion in nephron causes more K+ to be excreted
- K+ also moves into cells
- Leads to hypokalaemia
