ChemPath: Acid-Base Handling

Question 1

What is the normal range for H+ concentration?

Answer 1

35-45 mmol/L in ECF

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
• Haemoglobin
• Phosphate

NOTE: also protein and bone

Question 4

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

Describe where H+ is derived from

Answer 4

50 - 100 mmol/day

Metabolism of proteins, carbohydrates and fats produce H+ (alongside: CO2, H2O)

Question 5

Describe how the kidneys excrete H+ ions.

Answer 5

1. In the filtrate, bicarbonate combines with H+ (which moves from the tubular cell into the filtrate via a Na+/H+ channel) to form carbonic acid.
2. Carbonic acid is catalysed into H20 and CO2 which diffuse into the tubular cells (as are not polar and can move across the membrane freely)
3. H20 and CO2 are catalysed back into Carbonic Acid which breaks down into HCO3- and H+
4. HC03+ is reabsorbed into the blood via Na+/HCO3- or HCO3-/Cl- channels.

Overall kidneys excrete H+ and regenerate HCO3-. (regenerating HCO3- is crucial as in the blood when HC03- buffers H+ it gets used up)

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+ exchange)

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 respirtaory control over carbon dioxide.

Answer 8

• Respiratory 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 is the primary abnormality in metabolic acidosis? List three causes with examples.

Answer 10

Primary abnormality is increased H+ (with decreased bicarbonate)

Caused by:

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

Question 11

What is the primary abnormality in respiratory acidosis? List three causes with examples.

Answer 11

Primary abnormality is increased CO2 (therefore, increased H+) and a slight increase in bicarbonate

Caused by:

• Decreased ventilation
• Poor lung perfusion
• Impaired gas exchange

NOTE: metabolic compensation is slower than respiratory compensation

Question 12

What is the primary abnormality in metabolic alkalosis? List three causes.

Answer 12

Primary abnormality is decreased H+ (with increased bicarbonate)

Caused by:

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

Question 13

What is the primary abnormality in respiratory alkalosis? List three causes.

Answer 13

Primary abnormality is reduced CO2

If prolonged, this can lead to reduced renal H+ excretion and reduced bicarbonate generation

Can be caused by hyperventilation due to:

• Voluntary
• Artificial ventilation
• Stimulation of the respiratory centre

Question 14

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

Answer 14

Metabolic alkalosis due to loss of H+ from profuse vomiting

Question 15

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

Answer 15

• Aspirin overdose
• Aspirin stimulates ventilation and reduces renal excretion of H+