Arterial blood gases and acid base regulation

Question 1

How does uncompensated respiratory acidosis occur?

Answer 1

• Through sub-optimal ventilation
• Less minute ventilation means less fresh air in alveoli
• Increase in CO2 in alveoli which reduces diffusion gradient meaning less CO2 moves from blood into alveoli so more CO2 remains in blood
• This increases CO2 + H2O reaction meaning more carbonic acid which dissociates meaning more protons are acc

Question 2

How does the body try to reduce H+ conc

Answer 2

By increasing HCO3- to bind to H+.

Question 3

Describe the two ways the body increases HCO3 in the blood.

Answer 3

Acutely: CO2 moves into RBC & combines with H2O in presence of carbonic anhydrase to form HCO3- which moves out of cell via AE1 transporter into plasma.

Chronic phase: Increase HCO3 absorption in kidneys.

Question 4

What does BE stand for and what does it mean?

Answer 4

Base excess:
- The conc of bases (predominantly bicarbonate) compared with the ‘expected conc’
- An exact match is 0, an excess of base is positive and a base deficit is negative

Question 5

What is it called when blood PH normalizes but PCO2 and BE (base excess) remains high?

Answer 5

Fully compensated respiratory acidosis

Question 6

How does uncompensated respiratory alkalosis occur?

Answer 6

Through hyper ventilation, increasing minute ventilation.

Question 7

How is uncompensated respiratory alkalosis corrected?

Answer 7

• Body tries to increase H+ conc in blood
• There is no acute phase, only chronic- reduces amount of HCO3- reabsorbed in kidney and more HCO3- secretion in collecting duct

Question 8

What is it called when pH is normal but PCO2 and BE is lower than normal (in the blood)

Answer 8

Fully compensated respiratory alkalosis

Question 9

What happens to acid-base homeostasis in diarrhoea?

Answer 9

• Lots of HCO3- lost
• Increases how much carbonic acid is dissociated to release more HCO3-
• But, this also increases H+ conc which decreases pH, PCO2 stays same and BE has decreased

Question 10

Do you see alkalosis or acidosis in diarrhoea and what kind?

Answer 10

Uncompensated metabolic acidosis

Question 11

What can we manipulate to correct uncompensated metabolic acidosis

Answer 11

We can manipulate ventilation (increasing it)

Question 12

Explain how increasing ventilation helps to normalise metabolic acidosis?

Answer 12

-Increasing ventilation lead to more CO2 leaving the blood.
-Low PCO2 leads to more H+ and HCO3- combining to form carbonic anhydrase which then gets converted into CO2+ water
-This shifting of the carbonic anhydrase equation left leads to the pH raising as the H+ conc decreases in blood.

Question 13

What will you see with the pH, PCO2 and BE in uncompensated and fully compensated metabolic acidosis?

Answer 13

Uncompensated:
-pH, LOW
-PCO2, NORMAL
-BE, LOW

Fully Compensated:
-pH, NORMAL
-PCO2, LOW
-BE, LOW

Question 14

What happens to acid-base homeostasis in vomiting? include: pH,PCO2,BE

Answer 14

• HCl loss occurs which causes H+ loss
• HCO3- increases because there are fewer H+ to bind
• blood gas shows high pH, normal PCO2 and high BE (because HCO3- is disproportionately high for the PCO2)

Question 15

Do you see acidosis or alkalosis in vomiting and what kind?

Answer 15

Metabolic alkalosis

Question 16

How does the body correct Uncompensated metabolic alkalosis?

Answer 16

-Increases H+ ion conc by reducing vdntilation

Question 17

How does reducing ventilation help in uncompensated metabolic alkalosis?

Answer 17

leads to a build up of CO2 in the blood, shifting the carbonic anhydrase equation to the right leading to a normalised pH and higher BE

Question 18

What will you see with the pH, PCO2 and BE in uncompensated and fully compensated metabolic alkalosis?

Answer 18

Uncompensated metabolic alkalosis:
-pH, HIGH
-PCO2, NORMAL
-BE, HIGH

Compensated metabolic alkalosis:
-pH, NORMAL
-PCO2, HIGH
-BE, HIGH

Question 19

What is the time taken for a red blood cell to saturate with oxygen when in the pulmonary system called?

Answer 19

Pulmonary transit time.

Question 20

What is it called when O2 binds to haemoglobin and leads to a confirmational change that increases affinity of haemoglobin for O2.

Answer 20

Positive co-operativity.

Question 21

What happens when carbonic acid dissociates?

Answer 21

Carbonic acid dissociates into H+ and bicarbonate.

Question 22

What happens to bicarbonate during dissociation? What is the process called?

Answer 22

Bicarbonate moves out into the blood and Cl- moves in through the AE1 transporter to maintain resting membrane potential. This is known as the chloride shift.

Question 23

What are the 2 methods of corrective compensation for alkalosis/acidosis?

Answer 23

• Rapid: changes in ventilation
• Slow: changes in HCO3- and H+ retention/secretion

Question 24

Describe the interpretation of this ABG

Answer 24

Uncompensated respiratory alkalosis with moderate hypoxaemia.