Carbon Dioxide in Blood

Question 1

In arterial blood, how much more CO₂ is present compared to O₂?

Answer 1

2.5 x more CO₂ than O₂

CO₂ = 21 mmol.l^-1​

O₂ = 8.9 mmol.l^-1​

Question 2

Why is there so much dissolved CO₂ in arterial blood despite pCO₂ being so low?

Answer 2

CO₂ has high solubility

Question 3

How does dissolved CO₂ react with water?

Answer 3

CO₂ quickly forms carbonic acid (H₂CO₃)

Carbonic acid quickly dissociates to H⁺ and HCO₃^-

Question 4

Of the CO₂ dissolved in blood, what proportion is CO₂ compared to HCO₃^-?

Answer 4

Majority is HCO₃^-

The reaction is reversible and the direction depends on the reactants and products

Question 5

How is the pH of plasma dependent on the amount of CO₂ in blood?

Answer 5

More CO₂ dissolved pushes the reaction to the right = more H⁺ ions → low pH

More HCO₃^- will push the reaction to the left = less H⁺ ions → higher pH

Question 6

How is pH of plasma, dependent on the pCO₂ of CO₂?

Answer 6

Amount of CO₂ dissolved depends on pCO₂

Therefore:

High pCO₂ → more CO₂ dissolved → plasma pH falls

Low pCO₂ → less CO₂ dissolved → plasma pH rises

Question 7

What is the Henderson-Hasselbach equation for determining pH based on pCO₂ and [HCO₃^-]?

Answer 7

Question 8

What is the standard ratio of CO₂ to HCO₃^-?

Answer 8

HCO₃^-20:1 CO₂

Question 9

What determines the pCO₂?

Answer 9

Rate of ventilation

Question 10

Where do the majority of HCO₃^- ions dissolved in plasma come from?

Answer 10

Majority from hydrogen carbonate in red blood cells

Question 11

Which enzyme speeds up the reaction that produces HCO₃^- in Red blood cells?

Answer 11

Carbonic Anhydrase

Speeds up the reaction in both directions

Question 12

If Carbonic Anhydrase speeds up the reaction below in both directions, why is the forward direction favoured?

Answer 12

As the RBC mops up H⁺ ions produced. The reactants are taken away which drives the reaction forwards

Question 13

How does the RBC mop up H⁺ ions?

Answer 13

H⁺ ions bind to negatively charged Hb inside RBC

Question 14

How does the HCO₃^- produced in RBC get into plasma?

Answer 14

Transported out the RBC by Chloride bicarbonate exchanger

Question 15

What is the main factor controlling the production of HCO₃^- in RBC?

Answer 15

Determined by the H⁺ binding to Hb

Question 16

Which organ controls the amount of HCO₃^- in blood?

Answer 16

The Kidneys

Question 17

How does HCO₃^- act as a buffer to acids produced in the body?

Answer 17

HCO₃^- reacts with the H⁺ ions → CO₂

Increases amount CO₂ and reduces amount of HCO₃^-

CO₂ produced is breathed out so changes to pH are minimsed

Question 18

How does pCO₂ change between arterial and venous blood?

Answer 18

Venous blood has a higher pCO₂ due to metabolically active tissues

Question 19

How is the binding of H⁺ to Hb dependent on Hb oxygenation?

Answer 19

More O₂ bound → Hb in R state → less H⁺ bound (i.e. in the lungs H+ is given up)

Less O₂ bound → Hb in T state → more H⁺ binds (i.e. at the tissues H⁺ is picked up)

Question 20

How does the amount of dissolved CO₂ and HCO₃^- differ in venous blood. How doe this alter pH?

Answer 20

H⁺ binds more at tissues allowing more HCO₃^- to form

There is also more dissolved CO₂ however, pH is only small as the RATIO of CO₂ and HCO₃^- has not changed too much

Question 21

Which part of Hb does the CO₂ bind to?

Answer 21

Binds directly to amine groups of the globin chain

Question 22

What 3 ways is CO₂ transported in blood?

Answer 22

1. Dissolved CO₂
2. HCO₃^-
3. Bound to Hb (carbamino comounds)

Question 23

How much of the CO₂ in blood is being transported? What is the rest of the CO₂ being used for?

Answer 23

Amount of CO₂ transported = Total venous - total arterial blood

(23.3 -21.5 mmol.l^-1 = 1.8 mmol.l^-1 = ~8%)

The rest is used as a pH buffer system