S4) Carbon Dioxide in the Blood

Question 1

Describe 3 chemical properties of carbon dioxide in the blood

Answer 1

• CO₂ is more soluble than oxygen
• CO₂ reacts chemically with water
• CO₂ also reacts with Hb (different site from O₂)

Question 2

Compare and contrast the content of O₂ and CO₂ in arterial blood

Answer 2

• Total content CO₂ in arterial blood ≈ 21 mmol.l^-1
• Total content O₂ in arterial blood ≈ 8.9 mmol.l^-1

Question 3

What is the normal range of arterial blood pH?

Answer 3

pH 7.35 – pH 7.45

Question 4

What is the most important substance in controlling blood pH?

Answer 4

CO₂

Question 5

What is the solubility factor for CO₂ at 37^oC?

Answer 5

0.23

Question 6

How does one calculate the [CO₂]_dissolved in blood?

Answer 6

[CO₂]_dissolved = solubility factor x pCO₂

Question 7

What is the role of dissolved CO₂ in plasma and in red blood cells?

Answer 7

• Not a waste product
• Reacts with water to act as a buffer

Question 8

What is the result of dissolved carbon dioxide reacting with water?

Answer 8

• Dissolved CO₂ reacts with water to form carbonic acid
• Carbonic acid very quickly dissociates to hydrogen ions and hydrogen carbonate ions

Question 9

pH of blood plasma depends on how much CO₂ reacts to form H⁺.

What determines this?

Answer 9

• [CO2]_dissolved (pushes reaction to right)
• [HCO₃^- ] (pushes reaction to left)

Question 10

Explain the relationship between the plasma pH and pCO₂ of the alveoli

Answer 10

[CO₂]_dissolved depends directly on pCO₂:

• If pCO₂ rises = plasma pH falls
• If pCO₂ falls = plasma pH rises

Question 11

Using the Hendersson-Hasselbach equation, what is the formula to calculate pH from pCO₂ and [HCO₃^-] ?

Answer 11

Question 12

How does one calculate the pH of blood plasma using the Hendersson-Hasselbach equation?

Answer 12

• pK is a constant (6.1 at 37^oC)
• 20x more HCO₃^- than dissolved CO₂ (log20 = 1.3)

pH = 6.1 + 1.3 = 7.4

Question 13

Which values determine the pH of arterial blood?

Answer 13

• Ratio of [HCO₃^-] and pCO₂ determine pH
• pCO₂ determined by alveolar pCO₂
• Alveolar pCO₂ is determined by rate of ventilation

Question 14

How is the plasma concentration of hydrogen carbonate established

Answer 14

Hydrogen carbonate production in red blood cells:

• H⁺ ions bind to Hb(-) inside RBCs
• Chloride-bicarbonate exchanger transports HCO₃^- out of RBCs and creates a plasma concentration of 25mmol.l^-1 HCO₃^-

Question 15

Which enzyme catalyses the production of hydrogen carbonate in red blood cells?

Answer 15

Carbonic anhydrase

Question 16

Describe the role of haemoglobin in the production of hydrogen carbonate ions by red blood cells

Answer 16

• Haemoglobin has a large capacity for binding H⁺ ions
• The amount of HCO₃^- that erythrocytes produce depends on the binding of H⁺ to haemoglobin

Question 17

Explain the role of the kidney in [HCO₃^-]

Answer 17

• Kidney controls the amount of HCO₃^- by varying excretion
• Hence, pH is dependent on how much CO₂ and HCO₃^- is present

Question 18

Hydogen carbonate buffers extra acids.

Identify 3 of these

Answer 18

• Lactic acid
• Keto acids
• Sulphuric acid

Question 19

Explain how hydrogen carbonate buffers extra acids

Answer 19

• Acids react with HCO₃^- to produce CO₂
• [HCO₃^-] decreases
• CO₂ is produced & removed through ventilation
• pH changes are buffered

Question 20

Why is pCO₂ is higher in venous blood?

Answer 20

More CO₂ from metabolically active tissues

Question 21

Explain how the buffering of H⁺ by Hb depends on level of oxygenation

Answer 21

• If more O₂ binds Hb → R-state and less H⁺ ions bind (lungs)
• If less O₂ binds Hb → T-state and more H⁺ ions bind (tissues)

Question 22

Explain the processes occurring in venous blood at the tissues

Answer 22

• Hb has lost O₂ and so binds more H⁺
• More HCO₃^- forms & is exported to the plasma

Question 23

Explain the processes occurring in venous blood at the lungs

Answer 23

• Hb picks up O₂ and goes into R-state
• Hb gives up the extra H⁺ (from tissues)
• H⁺ reacts with HCO₃^- to form CO₂
• CO₂ is breathed out

Question 24

How are carbamino compounds formed?

Answer 24

• CO₂ binds directly to amine groups on globin of Hb
• This contributes to CO₂ transport

Question 25

Why are more carbamino compounds are formed at the tissues?

Answer 25

• pCO₂ is higher
• Unloading of O₂ facilitates binding of CO₂ to Hb
• CO₂ is then given up at the lungs

Question 26

Identify the 3 forms in which carbon dioxide is transported

Answer 26

• Dissolved CO₂ (10%)
• Hydrogen carbonate (60%)
• Carbamino compounds (30%)

Question 27

How much carbon dioxide is actually transported?

Answer 27

Transported CO₂

= CO₂ venous blood – CO₂ arterial blood

= 23.3 – 21.5 mmol.l^-1

= 1.8 mmol.l^-1 (8% of the total is transported)

Question 28

What is the role of the CO₂ in the blood that is not transported?

Answer 28

Remaining CO₂ is part of the pH buffering system