Carbon dioxide transport and role in pH balance Flashcards
What are the major sources of CO2 production?
- Mitochondria during aerobic cellular metabolism
- Body has enhanced storage capabilities for CO2 compared with O2
What is paCO2 dependent on?
- Alveolar ventilation
- CO2 production
How is CO2 transported around the body?
- Dissolved in blood plasma
- Transported as HCO3-
- Forms carbamino-haemoglobin compound
What is the reaction of CO2 in blood plasma?
- CO2 + H2O ==> H2CO3 ==> HCO3- + H+
- This is a reversible reaction
What is the reaction of CO2 within the red blood cell?
- CO2 + H2O ==> H2CO3 ==> HCO3- + H+
- First step is catalysed by carbonic acid
What happens to the HCO3- once it has been formed within the red blood cells?
- It is transported out of the red blood cells in exchange for Cl-
What happens to the H+ ion formed when H2CO3 dissociates?
- Forms H-haemoglobin compound to mop up excess H+ ions
- This keeps reaction moving towards synthesis of HCO3-
What does the buffering of H+ by Hb rely on?
- 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
What happens when venous blood arrives at the lungs?
- 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
Outline the formation of carbamino compounds?
- CO2 can bind directly to haemoglobin at amine groups (different site to O2)
What is the Haldane effect?
- 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
Why does venous blood transport more CO2 than arterial blood?
- Venous blood is deoxygenated
- So haemoglobin more readily forms carbamino compounds and also more readily binds H+ ions released during formation of HCO3-
What is the Bohr effect?
- 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
How does CO2 help control blood pH?
- Chemical reactant in major pH buffering system of blood
What does plasma pH depend on?
- Concentration of dissolved CO2 - controlled by rate and depth of breathing
- Concentration of bicarbonate - controlled by kidneys
What is the role of the kidney and lungs in CO2:bicarbonate levels?
- HCO3- comes from RBCs
- Kidneys control amount of HCO3- by varying excretion (can also produce more HCO3-)
- Lungs control amount of CO2 present
How does bicarbonate buffer extra acid?
- 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
What is the difference between full and partial compensation?
- Full compensation returns pH to normal range whereas partial compensation does not
How do you decide whether acidosis is respiratory or metabolic?
- 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
How long do the lungs and kidneys take to buffer pH changes?
- Lungs are a fast responder
- Kidneys are much slower (take between 1 day to 1 week)
How do we tell if there is compensation in acidosis?
- If HCO3- is elevated in respiratory acidosis there is compensation
- If PCO2 is decreased in metabolic acidosis there is compensation
How do you decide whether alkalosis is respiratory or metabolic?
- 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
