Lec 8 -Carbon Dioxide in the blood

Question 1

What is the concentration of carbon dioxide in the arterial blood?

Answer 1

• Total content CO2 in arterial blood is 21 mmol.I-1

- Total content O2 in arterial blood is 8.9 mmol.I-1

Question 2

How is blood pH controlled?

Answer 2

• CO2 has a major role in controlling blood pH.

- Arterial blood pH ,must be kept within a narrow range (pH 7.35 - pH 7.45)

Question 3

What is the equation for the dissolved concentration of CO2?

Answer 3

[CO2] dissolved = solubility x pCO2.

Question 4

What does dissolved CO2 react with?

Answer 4

It reacts with water in plasma and in red blood cells.

Question 5

What is the solubility factor for CO2 at 37 degrees celsius?

Answer 5

0.23

Question 6

How much water dissolves at pCO2 of 5.3 kPa?

Answer 6

At pCO2 of 5.3 kPa water dissolves 1.2 mmol.I-1 CO2

Question 7

What is formed when dissolved CO2 reacts with water?

Answer 7

Dissolved CO2 reacts with water to form carbonic acid (H2CO3)
- In other words dissolved CO2 reacts with water to form hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)

Question 8

What does carbonic acid dissociate to?

Answer 8

• Carbonic acid dissociates to hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)

Question 9

What does the pH of plasma depend on?

Answer 9

1. depends on how much CO2 reacts to form H+.
   - –> depends on [CO2] dissolved which pushes the reaction to the right.
   - –> depends on [HCO3-] which pushes the reaction to the left.
2. Depends on dissolved CO2
3. And depends on the concentration of hydrogen carbonate.

Question 10

What does the amount of CO2 depend on?

Answer 10

It depends directly on the partial pressure of CO2.

Question 11

What happens to plasma pH when pCO2 rises?

Answer 11

plasma pH will fall

- becomes more acidic

Question 12

What happens to plasma pH when pCO2 falls?

Answer 12

plasma pH will rise

- becomes more alkaline

Question 13

What happens when there is a high [HCO3-]?

Answer 13

• A high [HCO3-] prevents nearly all dissolved CO2 from reacting.
• The pH of plasma is alkaline.
• The high HCO3 cannot come from CO2 in plasma.

Question 14

What is the concentration of HCO3- in plasma and what cation is associated with this?

Answer 14

• plasma contains 25 mmol.I-1 HCO3-

- The cation associated with this is mostly Na+ not H+.

Question 15

What is the Henderson- Hasselbalch equation?

Answer 15

• it provides a way of calculating pH from pCO2 and [HCO3-]
• pH = pK + Log ([HCO3-]/ (pCO2 x 0.23))
• pK is a constant, pK = 6.1 at 37 degrees celsius.

Question 16

Describe hydrogen carbonate production in red blood cells?

Answer 16

1. H+ ions bind to the negatively charged Hb inside RBCs.
2. Chloride-bicarbonate exchanger transports HCO3- out of RBCs.
3. This creates a plasma concentration of 25 mmol.1-1 HCO3-.

Question 17

How is the reaction for hydrogen carbonate production in red blood cells sped up?

Answer 17

• The reaction is sped up by the enzyme carbonic anhydrase in red blood cells.
• CO2 + H2O —> H+ + HCO3-
• —> Carbonic anhydrase is above the arrow.
• The reaction proceeds in the forward direction because the products are mopped up in the RBC.

Question 18

What does the binding of H+ to haemoglobin dictate?

Answer 18

• The amount of HCO3- that erythrocytes produce depends on the binding of H+ to haemoglobin.
• –> Note that erythrocytes produce HCO3- but they don’t control the concentration of HCO3- in plasma.

Question 19

What is the role of the kidney in controlling [HCO3-]?

Answer 19

• The kidney controls the amount of HCO3- by varying excretion.

Question 20

What is pH dependent on?

Answer 20

• pH is dependent on how much CO2 is present (controlled by rate of breathing) and how much bicarbonate is present (controlled by kidneys).

Question 21

How does hydrogen carbonate buffers extra acid?

Answer 21

1. The body produces acids such as lactic acid, kept acids and sulphuric acid.
2. Acids react with HCO3- to produce CO2.
3. Therefore [HCO3-] goes down.
4. The CO2 produced is removed by breathing and pH changes are minimised (buffered).

Question 22

What does the buffering of H+ by Hb depend on?

Answer 22

Buffering of H+ by Hb depends on level of oxygenation.

—> Always H+ ions bound to Hb but amount depends on the state of the Hb molecule.

Question 23

What happens if more O2 binds Hb?

Answer 23

• R state
• Less H+ ions bind
• happens at lungs

Question 24

What happens if less O2 binds Hb?

Answer 24

• T state
• More H+ ions bind
• happens at tissues

Question 25

Describe venous blood?

Answer 25

• Hb has lost O2 and so bind more H+.
• This allows more HCO3- to form.
• HCO3- is exported to the plasma.
• [Dissolved CO2] increases a little.
• Much more is converted to HCO3- due to the increased capacity of Hb for H+.
• –> There is an increased [HCO3–
• There is only a very small change in plasma pH because both [HCO3-] and pCO2 have increased.

Question 26

What happens when venous blood arrives at the lungs?

Answer 26

1. Hb picks up O2 and goes into R state.
2. This causes Hb to give up the extra H+ it took on at the tissues.
3. H+ reacts with HCO3- to form CO2.
4. CO2 is breathed out.

Question 27

How are carbamino compounds formed?

Answer 27

1. CO2 can bind directly to proteins.
2. It binds directly to amine groups on globin of Hb.
3. Binding of molecular CO2 onto Hb is not part of acid base balance but it contributes to CO2 transport.

Question 28

Why are more carbamino compounds formed at the tissues?

Answer 28

• This is because pCO2 is higher.

- and unloading of O2 facilitates binding of CO2 to Hb.

Question 29

In what forms is CO2 transported?

Answer 29

• As dissolved CO2 - around 10%
• As hydrogen carbonate - around 60%
• As carbamino compounds.- around 30%

Question 30

How much carbon dioxide is transported?

Answer 30

• Transported carbon dioxide = Total in venous blood - total in arterial blood.
• –> = 23.3 - 21.5 mmol.l-1
• —-> = 1.8 mmol.1-1 –> Therefore only 8% of the total is transported.
• The rest of the carbon dioxide is there as part of the pH buffering system.