CO2 In The Blood

Question 1

How much CO2 is in arterial blood compared to O2?

Answer 1

2.5 X as much CO2

Dissolved and reacted total content in arterial blood
CO2 21mmol.l-1
O2 8.9mmol.l-1

Question 2

What’s the main roles of CO2 in blood?

Answer 2

It controls blood PH within a narrow range 7.35-7.45

Question 3

At 37dc if there is a pCO2 of 5.3KPa how much CO2 is dissolved?

Answer 3

Solubility factor of CO2 @ 37dc = 0.23

[CO2] dissolved = solubility X pCO2

0.23 X 5.3 = 1.2 mmol.l-1

Dissolved CO2 reacts with water in plasma and in RBCs

Question 4

Describe the reaction CO2 has with water in plasma and how this affects plasma PH

Answer 4

CO2 + H2O H2CO3 H+ + HCO3-

(Can effectively ignore intermediate carbonic acid as it quickly dissociates)

So

CO2 + H2O H+ + HCO3-

PH depends on how much CO2 reacts to form H+ (acidic)

Which depends on ratio of [HCO3] and PCO2

E.g. higher CO2 means reaction moves right and lowers PH. Higher [HCO3] means reaction moves left and increases PH

Question 5

How much HCO3- does plasma contain, where does this come from?

Answer 5

25mmol.l-1

Partially from CO2 reacting in plasma

But also

In RBCs same reaction occurs sped up by carbonic anhydrase. Reaction is pushed right in plasma bc RBCs mop up H+ which then binds to Hb-

Chloride bicarbonate exchanger lets CL- into RBC and HCO3- into plasma

Erythrocytes produce HCO3- but don’t control concentration of HCO3- in plasma

Question 6

What is Henderson-Hasselbachs equation and how can it be used to calculate PH from PCO2 and [HCO3-]? What can this be simplified to at 37dc?

Answer 6

PH = pK + Log ([HCO3-] / (PCO2 x 0.23))

pK is constant = 6.1 @ 37dc

20 times as much HCO3- as dissolved CO2 at 37dc and Log20= 1.3

So at 37dc: PH = 6.1 + 1.3 = 7.4

Question 7

What determines the arterial PCO2?

Answer 7

Alveolar PCO2 so rate of ventilation

Question 8

What determines the [HCO3-] in plasma?

Answer 8

The H+ binding to Hb in RBCs and kidneys excretion

Question 9

What controls [HCO3-] in plasma?

Answer 9

Kidney controls amount of HCO3- by varying excretion

Question 10

How does Hydrogen Carbonate buffer extra acid?

Answer 10

Acids react with HCO3- to produce CO2 therefore [HCO3] goes down
The CO2 produces is removed by breathing and PH changes are minimised

Question 11

How does the level of oxygenation affect buffering of H+ by Hb? How does this affect venous plasma PCO2?

Answer 11

H+ is always bound to Hb but making depends on the state of the Hb molecule

If more O2 binds Hb-> R-state and less H+ binds as at lungs

If less O2 binds Hb-> T-state and more H+ ions bind as at tissues. This means more HCO3- can be produced by RBCs -> more CO2 present in plasma in venous system both dissolved and reacted

Question 12

Why is there only a small change in plasma becoming more alkaline in venous blood?

Answer 12

More dissolved CO2 and reacted )bc of decreased O2 so increased H+ binding to Hb) So more CO2 but therefore more converted to HCO3- so only small increase in PCO2

Question 13

What is a carbamino compound?

Answer 13

CO2 binding direct,y to proteins e.g. amine group on globin of Hb part of CO2 transport

More formed at tissues -> breathed out at lungs

Question 14

What are the three ways CO2 can be transported? What percent does each method contribute?

Answer 14

Dissolved in plasma and RBCs 10%

As hydrogen carbonate in plasma and RBCs 60%

As carbamino compounds (with Hb) 30%

Question 15

What percent of CO2 is transported from tissues to lungs and therefore what percent is part of the PH buffering system?

Answer 15

8% transported

92% for Ph buffering