4. CO2 in blood

Question 1

how much CO2 is found in arterial blood compared to O2 and in which forms is it found

Answer 1

• 2.5x more CO2 than O2 (21 vs 8.9 mmol.l^-1)
• found as:
  
  • 8% dissolved in plasma
  • 80% as HCO3-
  • 12% as Hb carbamino compounds

Question 2

what determines how much CO2 dissolves in plasma

Answer 2

[CO2]dissolved = solubility x pCO2

Thus depends directly on pCO2 in alveoli as blood equilibrates with alveoli.

(solubility factor for CO2 at 37°C = 0.23 so at pCO2 of 5.3 kPa, water dissolves 1.2 mmol.l^-1 CO2)

Question 3

what happens to dissolved CO2 in plasma

Answer 3

Reacts with water: CO2 + H2O H+ + HCO3-

Direction of reaction depends on ratio of [CO2]dissolved to [HCO3-]

Question 4

What maintains the plasma conc. of 25 mmol.l^-1 HCO3-

Answer 4

RBC production of HCO3- - important in maintaining plasma pH

Question 5

what happens to CO2 in RBCs

Answer 5

• In RBCs, CO2 rapidly reacts with H2O due to presence of carbonic anhydrase
• Reaction proceeds in forward direction due to:
  1. buffering effects of Hb: H+ binds -vely charged Hb
  2. Cl-/HCO3- exchanger at RBC membrane transports HCO3- out of RBCs

Question 6

why is plasma pH alkaline

Answer 6

Plasma pH is determined by extent of CO2 dissociation into H+ and HCO3-.

The normal high levels of [HCO3-] prevents nearly all dissolved CO2 from reacting with water, preventing formation of H+

Question 7

why can hypoventilation cause blood acidosis

Answer 7

causes pCO2 to increase so pH decreases are more H+ is produced

Question 8

which 2 factors control [HCO3-]

Answer 8

1. RBC production

2. kidneys: varied excretion and extra production

Question 9

what is the role of Hb in venous blood with regards to CO2

Answer 9

Important role in transport of CO2 produced by metabolism of tissues for excretion in lungs - CO2 binding to Hb forms carbamino compounds

Question 10

why are more carbamino compounds formed at tissues

Answer 10

1. high pCO2

2. O2 unloading facilitates CO2 binding to Hb