Acid-base homeostasis - respiratory

Question 1

An acid is defined as

Answer 1

any chemical substance that can donate a proton, H+

Question 2

A base (alkali) is defined as

Answer 2

any chemical substance that can accept a proton, H+

Question 3

pH =

Answer 3

-log10 [H+]

Question 4

pKa is defined as

Answer 4

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

Question 5

The pKa for carbonic acid/bicarbonate is

Answer 5

6.1

Question 6

Normal pH is

Answer 6

7.4

Question 7

The absolute levels of bicarbonate can be changed by changes to breathing - Increased CO2 leads to

Answer 7

more H2CO3 and vice versa

Question 8

acidosis

Answer 8

pH < 7.35

Question 9

alkalosis

Answer 9

pH >7.45

Question 10

Causes of Acid-Base Disturbances

Answer 10

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

Question 11

respiratory disorders

Answer 11

Where primary change is to the CO2 levels

Question 12

metabolic disorders

Answer 12

Where primary change is to bicarbonate levels

Question 13

• An acidosis can be caused by:

Answer 13

– Rise in PCO2

– Fall in HCO3-

Question 14

• An alkalosis can be caused by:

Answer 14

– Fall in PCO2

– Rise in HCO3-

Question 15

Respiratory Acidosis, Results from an increase in PCO2 caused by:

Answer 15

– Hypoventilation (less CO2 being blown off)
– Ventilation-perfusion mismatch
– Reduced lung diffusing capacity

Question 16

• From Henderson-Hasselbalch equation, an increase in PCO2 causes

Answer 16

an increase in H+, so a lowering of pH, Thus, plasma HCO - levels increase to compensate for increased H+ concentration 3

Question 17

• Renal compensation in respiratory acidosis

Answer 17

increased HCO - reabsorption and increased HCO - 33

production – raises pH towards normal

Question 18

• Causes of respiratory acidosis

Answer 18

– COPD
– Blocked airway 
– foreign body or tumour
– Lung collapse
– Injury to chest wall
– Drugs reducing respiratory drive, eg morphine, barbiturates, general anaesthetics

Question 19

respiratory alkalosis

Answer 19

• Results from a decrease in PCO2 generally caused by
– alveolar hyperventilation (more CO2 being blown off)
This causes a decrease in H+ concentration and thus a rise in pH

Question 20

renal compensation in respiratory alkalosis

Answer 20

– reduced HCO - reabsorption, and reduced HCO - production 33
– Thus plasma HCO - levels fall, compensating for lower H+, moving pH back 3
towards normal

Question 21

• Causes of respiratory alkalosis

Answer 21

– Increased ventilation, from hypoxic drive in pneumonia, diffuse interstitial lung diseases, high
altitude, mechanical ventilation
– Hyperventilation – brainstem damage, infection driving fever

Question 22

Metabolic Acidosis

Answer 22

• Results from an excess of H+ in the body,
• This reduces HCO3 levels (shifts equation to the left)
• Addition of acid decreases pH, ventilation unaffected so PCO2 initially normal

Question 23

respiratory compensation in metabolic acidosis

Answer 23

– the lower pH is detected by peripheral chemoreceptors, causes an increase in ventilation which lowers PCO2
– the bicarbonate equation is driven further to the left, lowering H+ and HCO - concentration further 3
– The decrease in H+ concentration moves pH towards normal
– Respiratory compensation cannot fully correct the pH, HCO3 and H+, so excess H+ needs to be
removed or HCO - restored (by slow renal comp)

Question 24

causes of metabolic acidosis

Answer 24

– Loss of HCO -, eg from gut in diarrhoea
3
– exogenous acid overloading (aspirin overdose)
– endogenous acid production (ketogenesis)
– Failure to secrete H+, eg in renal failure

Question 25

Metabolic Alkalosis

Answer 25

• Results from an increase in HCO - concentration or a fall in H+ 3
• Removing H+ from equation drives reaction to right, increases HCO - 3
• Lowering of H+ raises pH, with PCO2 initially normal

Question 26

Respiratory compensation in metabolic alkalosis

Answer 26

– increase in pH detected by peripheral chemoreceptors – decreases ventilation which raises PCO2
– the equation is driven further to right, increasing H+ and HCO - 3
– Increase in H+ moves pH towards normal
– Respiratory compensation is often small (or even absent) – ventilation cannot reduce enough to
correct imbalance
– Renal response is to secrete less H+

Question 27

causes of metabolic alkalosis

Answer 27

– Vomiting - loss of HCl from stomach
– Ingestion of alkali substances
– Potassium depletion (eg diuretics)

Question 28

The Acid-Base Nomogram

Answer 28

ABGs can be analysed using the acid–base nomogram
• By plotting the PaCO2 and H+/pH values on the ABG nomogram, most ABGs can be analysed
• If the plotted point lies outside the designated areas, this implies a mixed disturbance