ChemPath: Acid-Base Handling

Question 1

What is the normal range for H+ concentration? What is normal pH

Answer 1

35-45 nmol/L in ECF

pH 7.35-7.45

Question 2

What equation links H+ concentration to pH?

Answer 2

pH = log1/[H+]

Question 3

What are the three main physiological buffers?

Answer 3

• Bicarbonate (ECF, renal tubular fluid)
• Haemoglobin (RBC)
• Phosphate (renal tubular fluid/intracellular)

NOTE: also protein and bone

Question 4

What is the rate of production of H+ ions per day?

Answer 4

50 - 100 mmol/day

Question 5

Describe how the kidneys excrete H+ ions. Where does this occur?

Answer 5

Via bicarbonate through the production of carbonic acid
Occurs in proximal tubules

Question 6

Describe how H+ ions pass through the renal epithelial membrane.

Answer 6

H+ ions cannot pass through the membrane itself so a transport system is necessary (Na+/H+ transporter)

Question 7

What is the rate of production of carbon dioxide per day?

Answer 7

20,000-25,000 mmol/day

Question 8

Describe the relationship between CO2 and repiratory rate

Answer 8

• Respiration is controlled by chemoreceptors in the hypothalamic respiratory centre
• An increase in CO2 will stimulate an increase in ventilation which then brings down CO2 concentration

Question 9

What information is provided by ABGs?

Answer 9

• pO2
• pCO2
• pH

Question 10

What are 3 mechanism of metabolic acidosis?

Answer 10

Caused by:

• Increased H+ production (e.g. DKA)
• Decreased H+ excretion (e.g. renal tubular acidosis)
• Bicarbonate loss (e.g. diarrhoea)

Question 11

Describe the ABG results in a metabolic acidosis

Answer 11

• Low pH / increased H+
• Low pCO2
• Low HCO3-

Question 12

What are 3 mechanisms of respiratory acidosis?

Answer 12

Caused by:

• Decreased ventilation
• Poor lung perfusion
• Impaired gas exchange

E.g. chronic lung disease (COPD), opioids, sedatives, neuromuscular weakness

NOTE: metabolic compensation is slower than respiratory compensation

Question 13

Describe the ABG results in a respiratory acidosis

Answer 13

• Low pH / increased H+
• High pCO2
• High HCO3-

Question 14

What happens in chronic respiratory acidosis?

Answer 14

Metabolic compensation will drive H+ down to near normal range, however pCO2 and bicarbonate will remain elevated

Question 15

What are 3 mechanisms of metabolic alkalosis?

Answer 15

Caused by:

• H+ loss (e.g. pyloric stenosis)
• Hypokalaemia
• Ingestion of bicarbonate

Question 16

Describe the ABG results in a metabolic alkalosis

Answer 16

• High pH / low H+
• High pCO2
• High HCO3-

Question 17

What is the main mechanism behind respiratory alkalosis, and what are 3 causes?

Answer 17

Main mechanism is hyperventilation, which can be due to:

• Voluntary
• Artificial ventilation
• Stimulation of the respiratory centre

Question 18

Describe the ABG results in a respiratory alkalosis

Answer 18

• High pH / low H+
• Low pCO2
• Low HCO3-

Question 19

What derangement of acid-base balance would be caused by pyloric stenosis?

Answer 19

Metabolic alkalosis due to loss of H+ from profuse vomiting

Question 20

Which condition classically causes a mixed respiratory alkalosis and metabolic acidosis?

Answer 20

• Aspirin overdose
• Aspirin causes high anion gap metabolic acidosis whilst simulataneously causing respiratory alkalosis by stimualting the respiratory centre

Question 21

What is the cause of an elevated anion gap in metabolic acidosis?

Answer 21

• Ketoacidosis
• Lactic acidosis
• Uraemia
• Toxins (ethylene glycol, methanol, salicylates)