Acid-Base Balance 2

Question 1

Values of pH, [HCO3-] in the plasma and arterial PCO2 when their is normal acid-base balance?

Answer 1

Plasma pH - 7.35 - 7.45

[HCO3-] in the plasma - 23 - 27 mmol/l

Arterial PCO2 - 35 - 45 mmHg

Question 2

What happens when acid-base balance is disrupted?

Answer 2

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

Question 3

Classifications of disturbances to acid-base balance?

Answer 3

Disturbance of respiratory origin:
• Respiratory acidosis
• Respiratory alkalosis

Disturbances of non-respiratory origin:
• Metabolic acidosis
• Metabolic alkalosis

Question 4

How does immediate buffering of a pH change occur?

Answer 4

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

Question 5

How can the values be measured and calculated?

Answer 5

A blood-gas analyser can measure pH and PCO2

[HCO3-] in the plasma can then be calculated using the Henderson-Hasselbalch equation

Question 6

Cause of respiratory acidosis and examples of when this may occur?

Answer 6

Retention of CO2 by the body:
• Chronic bronchitis
• Chronic emphysema
• Airway restriction, e.g: bronchial asthma, tumour
• Chest injuries
• Respiratory depression

Question 7

How do illnesses that cause CO2 retention in the body cause an acidosis?

Answer 7

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

Question 8

Indications of uncompensated respiratory acidosis?

Answer 8

pH < 7.35

PCO2 > 45 mmHg

Question 9

How does compensation for a respiratory acidosis occur?

Answer 9

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

Question 10

In summary, why does [HCO3-] in the plasma rise?

Answer 10

1. As a result of the disorder, i.e: CO2 retention drives equilibrium to the right
2. As a result of renal compensation

Question 11

How does correction for a respiratory acidosis occur?

Answer 11

Requires lowering PCO2 by restoration of normal ventilation

Question 12

Cause of respiratory alkalosis and examples of when this may occur?

Answer 12

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

Question 13

How does excessive removal of CO2 from the body cause a respiratory alkalosis?

Answer 13

Excess CO2 removal drives equilibrium to the left so:
• Both [H+] and [HCO3-] in the plasma fall

Decreased [H+] in the plasma causes alklaosis

Question 14

When is uncompensated respiratory alkalosis indicated?

Answer 14

pH >7.45

PCO2 < 35 mmHg

Question 15

How does compensation for a respiratory alkalosis occur?

Answer 15

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

Question 16

How does correction for a respiratory alkalosis occur?

Answer 16

Restoration of normal ventilation

Question 17

Cause of metabolic acidosis and examples of when this may occur?

Answer 17

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-)

Question 18

How do H+ (from a source other than CO2) cause a metabolic acidosis?

Answer 18

[HCO3-] in the plasma is depleted as a result of buffering excess H+ OR due to loss of HCO3- from the body

Question 19

When is uncompensated metabolic acidosis indicated?

Answer 19

pH < 7.35

[HCO3-] in the plasma is low

Question 20

How does compensation for a metabolic acidosis occur?

Answer 20

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)

Question 21

How does correction for a metabolic acidosis occur?

Answer 21

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

Question 22

Why is respiratory compensation essential in a metabolic acidosis?

Answer 22

Acid load cannot be excreted immediately; thus, respiratory compensation is essential

Question 23

Cause of metabolic alkalosis and examples of when this may occur?

Answer 23

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

Question 24

How does metabolic alkalosis occur when there is loss of H+ from the body?

Answer 24

As a result of loss of H+, or addition of base, [HCO3-] in the plasma rises

Question 25

When is uncompensated metabolic alkalosis indicated?

Answer 25

pH >7.45

[HCO3-] is very high

Question 26

How does compensation for a respiratory alkalosis occur?

Answer 26

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

Question 27

How does correction for a metabolic alkalosis occur?

Answer 27

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

Question 28

Summary of the compensation and correction occurring in resp acidosis/alkalosis?

Answer 28

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

Question 29

Summary of the compensation and correction occurring in metabolic acidosis/alkalosis?

Answer 29

Resp system is NOT the cause of the disturbance thus CAN contribute to compensation (increased/decreased ventilation)

Correction is mediated by the renal system

Question 30

Summarise the response to an acid load?

Answer 30

1. 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
2. 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)