Acid-Base Balance 2 Flashcards
Values of pH, [HCO3-] in the plasma and arterial PCO2 when their is normal acid-base balance?
Plasma pH - 7.35 - 7.45
[HCO3-] in the plasma - 23 - 27 mmol/l
Arterial PCO2 - 35 - 45 mmHg
What happens when acid-base balance is disrupted?
1st step is compensation - restoration of pH as fast as possible, irrespective of what happens to the [HCO3-] in the plasma and PCO2
2nd step is correction - restoration of pH AND [HCO3-] in the plasma AND PCO2 to normal
Classifications of disturbances to acid-base balance?
Disturbance of respiratory origin:
• Respiratory acidosis
• Respiratory alkalosis
Disturbances of non-respiratory origin:
• Metabolic acidosis
• Metabolic alkalosis
How does immediate buffering of a pH change occur?
Immediate dilution of the acid/base in ECF, with:
• Buffers in the blood, e.g: Hb, HCO3-
• Buffers in the ECF
NOTE: acidosis reduces [HCO3-] in the plasma
However, buffer stores are quickly depleted so kidney must rectify stores
How can the values be measured and calculated?
A blood-gas analyser can measure pH and PCO2
[HCO3-] in the plasma can then be calculated using the Henderson-Hasselbalch equation
Cause of respiratory acidosis and examples of when this may occur?
Retention of CO2 by the body: • Chronic bronchitis • Chronic emphysema • Airway restriction, e.g: bronchial asthma, tumour • Chest injuries • Respiratory depression
How do illnesses that cause CO2 retention in the body cause an acidosis?
CO2 retention drives the equilibrium of the buffer system to the right:
• [H+] and [HCO3-] in the plasma both increase
Increased [H+] in the plasma results in a acidosis
Indications of uncompensated respiratory acidosis?
pH < 7.35
PCO2 > 45 mmHg
How does compensation for a respiratory acidosis occur?
Resp system is the cause so renal system must cooperate
Blood PCO2 drives H+ secretion by the kidney, i.e: CO2 retention stimulates H+ secretion into the filtrate
H+ secretion:
• Drives HCO3- reabsorption
• Generates titratable acid (TA) and NH4+
• Acid is excreted and “new” HCO3- is added to the blood
In summary, why does [HCO3-] in the plasma rise?
- As a result of the disorder, i.e: CO2 retention drives equilibrium to the right
- As a result of renal compensation
How does correction for a respiratory acidosis occur?
Requires lowering PCO2 by restoration of normal ventilation
Cause of respiratory alkalosis and examples of when this may occur?
Excessive removal of CO2 by the body:
• Low inspired PO2 at altitude, e.g: hypoxia stimulates peripheral chemoreceptors and hyperventilation lowers PCO2
• Hyperventilation (due to fever, brain stem damage)
• Hysterical over-breathing
How does excessive removal of CO2 from the body cause a respiratory alkalosis?
Excess CO2 removal drives equilibrium to the left so:
• Both [H+] and [HCO3-] in the plasma fall
Decreased [H+] in the plasma causes alklaosis
When is uncompensated respiratory alkalosis indicated?
pH >7.45
PCO2 < 35 mmHg
How does compensation for a respiratory alkalosis occur?
As the resp system is the cause, the renal system must compensate
Blood PCO2 drives H+ secretion by the kidney; thus, excessive removal of CO2 reduces H+ secretion into the tubule; this means that HCO3- is no longer reabsorbed and urine is alkaline
No TA and NH4+ is formed, so no “new” HCO3- is generated
How does correction for a respiratory alkalosis occur?
Restoration of normal ventilation
Cause of metabolic acidosis and examples of when this may occur?
Excess H+ from any source OTHER THAN CO2:
• Ingestion of acids or acid-producing foodstuffs
• Excessive metabolic production of H+, e.g: lactic acid during exercise or ketoacidosis
• Excessive loss of base from the body, e.g diarrhoea (loss of HCO3-)
How do H+ (from a source other than CO2) cause a metabolic acidosis?
[HCO3-] in the plasma is depleted as a result of buffering excess H+ OR due to loss of HCO3- from the body
When is uncompensated metabolic acidosis indicated?
pH < 7.35
[HCO3-] in the plasma is low
How does compensation for a metabolic acidosis occur?
Resp system participates in compensation (as it is not the cause)
Decreased plasma pH stimulates peripheral chemoreceptors and ventilation is quickly increased (more CO2 is blown off):
• [H+] in the plasma is lowered, raising the pH towards normal
• [HCO3-] in the plasma is also lowered (as equilibrium is driven to the left)
How does correction for a metabolic acidosis occur?
Filtered HCO3- is very low and readily absorbed
H+ secretion continues and produces TA and NH4+ to generate more “new” HCO3-; the acid load is excreted (acidic urine) and [HCO3-] in the plasma is restored
Ventilation can then be normalised
Why is respiratory compensation essential in a metabolic acidosis?
Acid load cannot be excreted immediately; thus, respiratory compensation is essential
Cause of metabolic alkalosis and examples of when this may occur?
Excessive loss of H+ from the body:
• Loss of HCl from the stomach, e.g: vomiting
• Ingestion of alkali/alkali-producing foods, e.g: NaHCO3 (historical antacid)
• Aldosterone hypersecretion causes stimulation of Na+/H+ exchange at the apical membrane of the tubule, so there is acid secretion)
This is LESS COMMON than metabolic acidosis
How does metabolic alkalosis occur when there is loss of H+ from the body?
As a result of loss of H+, or addition of base, [HCO3-] in the plasma rises
When is uncompensated metabolic alkalosis indicated?
pH >7.45
[HCO3-] is very high
How does compensation for a respiratory alkalosis occur?
Increased pH slows ventilation (peripheral chemoreceptors)
CO2 is retained and PCO2 rises, shifting equilibrium to the right
[H+] in the plasma rises, lowering pH; however, [HCO3-] in the plasma also rises further
How does correction for a metabolic alkalosis occur?
Filtered HCO3- load is so large compared to normal that not all of the filtered HCO3- is reabsorbed
No TA or NH4+ are generated and HCO3- is excreted (alkaline urine)
[HCO3-] in the plasma falls to normal level
Summary of the compensation and correction occurring in resp acidosis/alkalosis?
Resp system is the cause of the disturbance thus cannot contribute to compensation; the renal system compensates
Correction requires restoration of normal resp system function
Summary of the compensation and correction occurring in metabolic acidosis/alkalosis?
Resp system is NOT the cause of the disturbance thus CAN contribute to compensation (increased/decreased ventilation)
Correction is mediated by the renal system
Summarise the response to an acid load?
- Induced by an increase in PCO2, which increases H+:
• Intracellular buffering (relatively ineffective)
• Little extracellular buffering
• Increased renal excretion of H+ (hours-days)
• Correction requires restoration of normal ventilation - Induced by non-CO2 sources (H+ load);
• Extracellular buffering by HCO3- (this is immediate)
• Respiratory compensation (within minutes)
• Intracellular buffering by proteins and organic anions (2-4 hours)
• Renal excretion of H+ (days-weeks)
