Acid-Base Homeostasis

Question 1

How would you calculate blood pH?

Answer 1

• Henderson-Hasselbach equation

pH = pKa + log [HCO3-]/ [CO2]

Question 2

How would you calculate [CO2] in blood?

Answer 2

solubility constant x PCO2

Question 3

What does respiratory acid-base disturbances affect?

Answer 3

production and levels of CO2

Question 4

What does metabolic acid-base disturbances affect?

Answer 4

production and levels of H+ and HCO3-

Question 5

Classify acidosis and alkalosis

Answer 5

• acidosis: pH less than 7.35

- alkalosis: pH more than 7.45

Question 6

What are the causes of acid-base disturbances?

Answer 6

• increased/decreased CO2
• increased non-volatile acid/decreased base
• decreased non-volatile acid/increased base

Question 7

What change in chemical concentrations result in acidosis/alkalosis

Answer 7

acidosis:

• rise in CO2
• fall in HCO3-

alkalosis:

• fall in CO2
• rise in HCO3-

Question 8

Describe the mechanism for respiratory/metabolic acidosis

Answer 8

• metabolic acidosis:
• HCO3- = <24 Eq/L
• PCO2 = <40 mmHg
• respiratory compensation
• respiratory acidosis:
• HCO3- = >24 Eq/L
• PCO2 = >40 mmHg
• renal compensation

Question 9

Describe the mechanism for respiratory/metabolic alkalosis

Answer 9

• metabolic alkalosis:
• HCO3- = >24 Eq/L
• PCO2 = >40 mmHg
• respiratory compensation
• respiratory alkalosis:
• HCO3- = <24 Eq/L
• PCO2 = <40 mmHg
• renal compensation

Question 10

What can cause respiratory acidosis?

Answer 10

increased PO2 caused by:

• hypoventilation (less CO2 being blown off)
• ventilation-perfusion mis-match
• reduced lung diffusing capacity

causes:

• COPD
• blocked airway
• lung collapse
• injury to chest wall
• drugs reducing respiratory drive eg. morphine

Question 11

What is the compensatory mechanism for respiratory acidosis?

Answer 11

• renal compensation

- increases HCO3- reabsorption and increased HCO3- production to raise pH towards the normal

Question 12

What can cause respiratory alkalosis?

Answer 12

decreased PO2 caused by:
- alveolar hyperventilation (more CO2 being blown off)

causes:

• increased ventilation from:
• hypoxic drive
• diffuse interstitial lung diseases
• high altitude
• mechanical veniltation
• hyperventilation

Question 13

What is the compensatory mechanism for respiratory alkalosis?

Answer 13

• renal compensation
• reduces HCO3- reabsorption and decreased HCO3- production
• compensates for lower H+ levels bringing pH back to normal level

Question 14

What are the causes of metabolic acidosis?

Answer 14

• loss of HCO3- from gut in diarrhoea
• exogenous acid overloading (aspirin overdose)
• endogenous acid production (ketogenesis)
• failure to secrete H+ (renal failure)

Question 15

What is the compensatory mechanism for metabolic acidosis?

Answer 15

• respiratory compensation
• lower pH detected by peripheral chemoreceptor
• increases ventilatory drive lowering PCO2
• shifts bicarbonate equation to the left lowering H+ and HCO3- further moving pH towards normal
• doesn’t fully correct it

Question 16

What causes metabolic alkalosis?

Answer 16

• vomiting (loss of HCl from stomach)
• ingestion of alkali substances
• potassium depletion (diuretics)

Question 17

What is the compensatory mechanism for metabolic alkalosis?

Answer 17

• respiratory compensation
• increased pH detected by peripheral chemoreceptors
• decrease ventilation to raise PCO2
• shifts bicarbonate equation to the right increasing H+ and HCO3- levels moving pH towards normal
• small compensation
• renal response is to secrete less H+