Carbon Dioxide in the Blood

Question 1

Is there more oxygen or CO2 in arterial blood

Answer 1

In arterial blood, there is almost 2.5x as many CO2 as oxygen

Question 2

List the reactions of CO2 in blood

Answer 2

• Dissolved CO2 reacts with water in plasma and in red blood cells
  - CO2 in arterial blood is not there as a waste product - used in buffering system
• Dissolved CO2 reacts with water to form carbonic acid (H2CO3)

Question 3

State the equation of CO2 and water

Answer 3

CO2 + H2O H2CO3 H+ + HCO3-

Question 4

Explain how increasing CO2 or HCO3 affects pH

Answer 4

• Increase in pCO2 and [CO2]dissolved pushes reaction to the right and decreases pH
  
  • pCO2 of alveoli is the determining factor - controlled by controlling rate of breathing
• Increase in [HCO3] pushes reaction to left and increases pH
• Normally reaction mainly in reverse direction - causing pH to be slightly alkaline

Question 5

Apply the Henderson-Hasselbalch equation to the reaction of CO2 and water

Answer 5

• pH = pK + log([HCO3]/(pCO2 x 0.23))
• pK = 6.1 at 37˚C
• Normally, 20 times as much HCO3 as dissolved CO2
  
  • 25/1.2 =~ 20
• pH = 6.1 + 1.3 = 7.4

Question 6

Describe the reaction of CO2 and water within the RBC

Answer 6

• Reaction in RBC sped up by enzyme carbonic anhydrase
• Reaction proceeds in forward direction as products are mopped up in the RBC
  
  • H+ ions bind to the negatively charged haemoglobin inside RBC
  • Chloride-bicarbonate exchanger transports HCO3 out of RBC
• This creates plasma concentration of 25 mmol/L HCO3

Question 7

State where the concentration of HCO3 and pCO2 are controlled

Answer 7

• [HCO3] controlled in kidney

- pCO2 controlled in lungs

Question 8

Explain how different levels of oxygen affect buffering of H+

Answer 8

• If more oxygen binds haemoglobin, it is in R-state and less H+ ions bind (lungs)
• If less oxygen binds haemoglobin, it is in T-state and more H+ ions bind (tissues)
  
  • More H+ binding means more CO2 can be produced
    
    • Therefore more CO2 is present in plasma in venous system - both in dissolved and reacted form

Question 9

Describe how the buffering reaction is affected for venous blood arriving at lungs

Answer 9

• Haemoglobin picks up oxygen and goes into relaxed state
• Causes haemoglobin to give up extra H+ it took at the tissues
• H+ reacts with HCO3 to form CO2
• CO2 is breathed out

Question 10

Describe the function of carbamino compounds

Answer 10

• CO2 can bind directly to proteins
• Binds directly to amine groups on globin of haemoglobin
  
  • Binding is not part of acid base balance but contributes to CO2 transport
• More carbamino compounds are formed at the tissues
  
  • pCO2 higher at tissues
  • Unloading of oxygen facilitates binding of CO2 to haemoglobin
• This CO2 is given up at the lungs

Question 11

How is CO2 transported in the blood and state the relative proportion of each

Answer 11

• Dissolved CO2 - 10%
• Hydrogen carbonate - 60%
• Carbamino compounds - 30%