Acid-base homeostasis

Question 1

How do you calculate plasma pH (equation)?

Answer 1

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

Bicarbonate is in mmol/L and [CO2] is calculated from PCO2 and a solubility constant

Question 2

What is pKa defined as?

Answer 2

The pH at which 50% is ionised and 50% is unionised in the reaction

Question 3

What is the pKa of carbonic acid/bicarbonate?

Answer 3

6.1

Question 4

What is the normal pH of blood?

Answer 4

7.4

Question 5

How much more bicarbonate is there in the blood compared with carbonic acid?

Answer 5

20 x more bicarbonate than there is carbonic acid

Question 6

How can the absolute levels of bicarbonate be altered?

Answer 6

• By changing breathing

- Increased CO2 leads to more H2CO3 and vice versa

Question 7

Changes to CO2 or H2O would be deemed as what kind of acid-base disturbance?

Answer 7

Respiratory

Question 8

Changes to H+ or HCO3- would be deemed as what kind of acid-base disturbance?

Answer 8

Metabolic

Question 9

What are the typical causes of acid-base disturbances?

Answer 9

• Increased CO2
• Decreased CO2
• Increased non-volatile acid / decreased base
• Increased base / decreased non-volatile acid

Question 10

What can an acidosis be caused by?

Answer 10

• Rise in PCO2

- Fall in HCO3-

Question 11

What can an alkalosis be caused by?

Answer 11

• Fall in PCO2

- Rise in HCO3-

Question 12

What are the two methods of compensating for acid-base disturbances?

Answer 12

• Resipiratory system altering ventilation - quick

- Kidneys altering excretion of bicarbonate - 2-3 days

Question 13

What would the plasma HCO3- and the plasma pH be in patients with COPD?

Answer 13

• HCO3- would be increased

- pH would be decreased

Question 14

What can cause a metabolic alkalosis?

Answer 14

Vomitting - loss of HCl from stomach

• Ingestion of alkali substances
• Potassium depletion (e.g diuretics)

Question 15

What condition can cause a decrease in plasma pH and a decrease in HCO3-?

Answer 15

Diabetic ketogenisis (metabolic acidosis)

Question 16

What does hyperventilation cause?

Answer 16

REspiratory alkalosis

• HCO3- decreased
• pH increased

Question 17

How is respiratory acidosis compensated?

Answer 17

Renal compensation - increased HCO3- reabsorption and increased HCO3- production - raises pH towards normal

Question 18

What can cause respiratory acidosis?

Answer 18

• COPD
• Blocked airway - foreign body or tumour
• Lung collapse
• Injury to chest wall
• Drugs reducing respiratory drive e.g morphine, barbituates, General anaesthetics

Question 19

How is respiratory alkalosis compensated?

Answer 19

REduced HCO3- reabsorption and reduced HCO3- production

Question 20

What are the causes of respiratory alkalosis?

Answer 20

• Increased ventilation from hyoxic drive in pneumonia
• Diffuse interstitial lung diseases
• High altitude
• Mechanical ventilation

Question 21

What can cause Metabolic acidosis?

Answer 21

• Loss of HCO3- e.g from gut in diarrhoea
• Exogenous acid overloading (apirin overdose), endogenous acid production (ketogenesis)
• Failure to secrete H+, eg in renal failre

Question 22

How is metabolic acidosis compensated?

Answer 22

• The lower pH is detected by peripheral chemoreceptors, causes an increase in ventilaion which lowers PCO2
• Bicarbonate equation is also driven further to the left, lowering H+ and HCO3- concentration further
• Decrease in H+ conc. moves pH towards normal
• Resp compensation cannot fully correct imalance so excess H+ needs to be removed or HCO3- restored

Question 23

What is the compensatory mechanism for metabolic alkalosis?

Answer 23

• Decreased ventilation
• Equation also driven further to the right, increasing H+ and HCO3-
• Increase in H+ moves pH towards normal
• Resp compensation often too small
• Renal response to secrete less H+