Carbon dioxide transport and role in pH balance

Question 1

What are the major sources of CO2 production?

Answer 1

• Mitochondria during aerobic cellular metabolism
• Body has enhanced storage capabilities for CO2 compared with O2

Question 2

What is paCO2 dependent on?

Answer 2

• Alveolar ventilation
• CO2 production

Question 3

How is CO2 transported around the body?

Answer 3

• Dissolved in blood plasma
• Transported as HCO3-
• Forms carbamino-haemoglobin compound

Question 4

What is the reaction of CO2 in blood plasma?

Answer 4

• CO2 + H2O ==> H2CO3 ==> HCO3- + H+
• This is a reversible reaction

Question 5

What is the reaction of CO2 within the red blood cell?

Answer 5

• CO2 + H2O ==> H2CO3 ==> HCO3- + H+
• First step is catalysed by carbonic acid

Question 6

What happens to the HCO3- once it has been formed within the red blood cells?

Answer 6

• It is transported out of the red blood cells in exchange for Cl-

Question 7

What happens to the H+ ion formed when H2CO3 dissociates?

Answer 7

• Forms H-haemoglobin compound to mop up excess H+ ions
• This keeps reaction moving towards synthesis of HCO3-

Question 8

What does the buffering of H+ by Hb rely on?

Answer 8

• Level of oxygenation
• If Hb is in R-state - more O2 binds and less H+ ions bind e.g. as at lungs
• If Hb is in T-state, more H+ ions bind e.g. as at tissues

Question 9

What happens when venous blood arrives at the lungs?

Answer 9

• Hb picks up O2 and goes into R-state
• This causes Hb to give up extra H+ it took on at tissues
• H+ reacts with HCO3- to form CO2
• Reaction is pushed to left - CO2 + H2O <== H+ + HCO3-
• CO2 is breathed out

Question 10

Outline the formation of carbamino compounds?

Answer 10

• CO2 can bind directly to haemoglobin at amine groups (different site to O2)

Question 11

What is the Haldane effect?

Answer 11

• More carbamino compounds are formed at tissues because PCO2 is high and O2 is being unloaded
• T state Hb binds CO2 better
• CO2 is given up at lungs as Hb becomes oxygen rich - oxygenated Hb binds CO2 less well so CO2 is unloaded
• This is the Haldane effect

Question 12

Why does venous blood transport more CO2 than arterial blood?

Answer 12

• Venous blood is deoxygenated
• So haemoglobin more readily forms carbamino compounds and also more readily binds H+ ions released during formation of HCO3-

Question 13

What is the Bohr effect?

Answer 13

• Oxygen unloading along systemic capillary is enhanced by increases in PCO2 and by decreases in pH
• Ability of CO2 to alter affinity of Hb for O2 enhances O2 uptake in lungs and delivery to tissues

Question 14

How does CO2 help control blood pH?

Answer 14

• Chemical reactant in major pH buffering system of blood

Question 15

What does plasma pH depend on?

Answer 15

• Concentration of dissolved CO2 - controlled by rate and depth of breathing
• Concentration of bicarbonate - controlled by kidneys

Question 16

What is the role of the kidney and lungs in CO2:bicarbonate levels?

Answer 16

• HCO3- comes from RBCs
• Kidneys control amount of HCO3- by varying excretion (can also produce more HCO3-)
• Lungs control amount of CO2 present

Question 17

How does bicarbonate buffer extra acid?

Answer 17

• Acids react with HCO3- to produce CO2
• CO2 + H2O <== H+ + HCO3-
• CO2 levels increase
• Extra CO2 is removed by breathing and pH changes are minimised
• More HCO3- needs to be produced to replenish buffers

Question 18

What is the difference between full and partial compensation?

Answer 18

• Full compensation returns pH to normal range whereas partial compensation does not

Question 19

How do you decide whether acidosis is respiratory or metabolic?

Answer 19

• Look at pH - if <7.35: acidosis
• Look at pCO2 - if elevated: respiratory acidosis
• If PCO2 normal or low - not respiratory acidosis
• Look at HCO3 - if decreased = metabolic acidosis

Question 20

How long do the lungs and kidneys take to buffer pH changes?

Answer 20

• Lungs are a fast responder
• Kidneys are much slower (take between 1 day to 1 week)

Question 21

How do we tell if there is compensation in acidosis?

Answer 21

• If HCO3- is elevated in respiratory acidosis there is compensation
• If PCO2 is decreased in metabolic acidosis there is compensation

Question 22

How do you decide whether alkalosis is respiratory or metabolic?

Answer 22

• Look at pH - if >7.45 = alkalosis
• Look at pCO2 - if low = respiratory alkalosis
• If PCO2 is normal or elevated = not respiratory alkalosis
• Look at HCO3- - if increased = metabolic alkalosis
• Look for evidence of compensation
• In respiratory alkalosis, decreased HCO3- = compensation
• In metabolic alkalosis, increased CO2 = compensation