Session 4: Carbon Dioxide in the Blood

Question 1

Function of CO2 in blood.

Answer 1

A major role in controlling blood pH and the blood’s acid base balance.

Question 2

Where can CO2 be found in blood?

Answer 2

In plasma In red blood cells Bound to haemoglobin

Question 3

How can CO2 be found in plasma?

Answer 3

Dissolved in the plasma

Question 4

Explain what happens to CO2 when it is dissolved in plasma.

Answer 4

It reacts with water to form H+ and hydrogen carbonate (bicarbonate HCO3-)

Question 5

Why is this reaction slow in plasma?

Answer 5

Because there is little carbonic anhydrase in the plasma.

Question 6

What does the extent of dissociation of CO2 determine?

Answer 6

The pH of plasma.

Question 7

What causes the equilibrium to shift towards dissociation?

Answer 7

An increase in dissolved CO2.

Question 8

What is dissolved CO2 directly proportional to?

Answer 8

Partial pressure of CO2 in the artertial blood

Question 9

What is normal pCO2 in arterial blood?

Answer 9

5.3 kPa

Question 10

What is the normal concentration of CO2 in plasma?

(Solubility constant is 0.23 of CO2)

Answer 10

0.23 x 5.3 = 1.2 mmol/dm3

Question 11

What is dissociation of CO2 resisted by?

Answer 11

High concentrations of bicarbonate HCO3-

Question 12

What is the normal concentration of bicarbonate in the plasma?

Answer 12

Around 25 mmol/l

Question 13

Where does the high concentration of HCO3- come from?

Answer 13

Reaction of CO2 in red blood cells.

Question 14

What is the normal ratio of HCO3- to CO2?

Answer 14

20:1

Question 15

What does pH depend on?

Answer 15

The ratio of [HCO3-] to pCO2

Question 16

What happens to pH if pCO2 rises?

Answer 16

pH falls

Question 17

What happens to pH if HCO3- rises?

Answer 17

pH rises

Question 18

Explain the Henderson - Hasselbach equation.

Answer 18

pKa = 6.1

Since the normal ratio of HCO3- to CO2 is 20 -> Log20

pH = 6.1 + log20

pH = 6.1 + 1.3

pH = 7.4

Question 19

Ratio of CO2 to O2 in arterial blood.

Answer 19

2.5:1

Question 20

What is the total content of CO2 in arterial blood?

Answer 20

Around 21 mmol

Question 21

Only around 1.2 mmol/l of CO2 can be found in the plasma. Where is the rest of the CO2 found in blood?

Answer 21

In red bloods cells.

Question 22

How can there be more dissolved CO2 in plasma compared to O2 when pAO2 > pACO2?

Answer 22

Because of the solubility of CO2 in plasma. It is more soluble.

Question 23

What happens to carbon dioxide in red blood cells?

Answer 23

They can react with water

Can bind to haemoglobin

Question 24

Explain CO2 in red blood cells reacting with water.

Answer 24

Compared to in the plasma CO2 reacts rapidly with water due to higher concentrations of carbonic anhydrase in RBCs which forms H+ and HCO3-.

Question 25

Why are red blood cells not excessively acidic since there are higher concentrations of CO2 in RBCs?

Answer 25

Because the H+ bind to haemoglobin which has a large buffering capacity.

Question 26

Explain where all the bicarbonate in plasma comes from.

Answer 26

From the reaction of CO2 with water in red blood cells.

There is then a chloride bicarbonate exchanger which transporters HCO3- out into plasma in exchange of Cl-.

Question 27

What drives the reaction of CO2 and H2O to produce HCO3- and H+?

Answer 27

Haemoglobin binding to H+. This constantly reduces the amount of H+ in the red blood cells which means that more CO2 and H2O will react to produce H+ and also HCO3-.

Question 28

Considering the reaction in plasma and in red blood cells. What is the pH of body fluids determined by?

Answer 28

The relationship between the amount of CO2 dissolved in plasma and the amount of HCO3- formed from CO2 in the red blood cells by a reaction involving haemoglobin.

Question 29

What is the concentration of HCO3- in the body mainly controlled by?

Answer 29

By the kidneys by either excreting HCO3- or not.

Question 30

What is the pCO2 mainly controlled by?

Answer 30

Rate of breathing

Question 31

What is carbamino haemoglobin?

Answer 31

Carbon dioxide bound to haemoglobin

HbCOO-

Question 32

Is there more CO2 in venous blood or arterial blood?

Answer 32

More in venous blood.

Question 33

Why is there more dissolved CO2 in the plasma of venous blood than in arterial blood?

Answer 33

At tissue O2 is given up to provide energy for the tissue.

This leads to a drop in pO2 and H+ starts to compete for the binding of Hb. This leads to H+ binding to Hb and the reaction between CO2 and HCO3- is shift to right in favour of HCO3-. This leads to HCO3- to be transported out of the cell and into the plasma. An increase in HCO3- here leads a small shift to the left in favour of CO2 in the plasma.

Question 34

State the three ways of how CO2 is transported from tissue to lungs.

Answer 34

As dissolved CO2 (8%)

As bicarbonate (80%)

As carbamino compounds (HbCOO-) (12%)

Question 35

What is the approximate conc of CO2 in venous blood?

Answer 35

23.3 mmol/l

Question 36

What is the approx concentration of CO2 in arterial blood?

Answer 36

21.5 mmol/l

Question 37

Drawing from the values of CO2 conc in venous vs arterial blood. How much CO2 is transported in the blood as part of being diffused out of the blood?

Answer 37

Around 1.8 mmol/l

Question 38

What is the purpose of the rest of the CO2 amounts in the blood?

Answer 38

Regulating pH

Question 39

Explain what happens when venous blood arrives at the lungs considering CO2.

Answer 39

Hb picks up O2 and goes into R-state because pO2 is higher here.

This causes Hb to give up the extra H+.

Shifts the reaction to the left in to promote CO2 in RBCs.

CO2 is breathed out.

Question 40

Answer 40

Question 41

Answer 41

Question 42

Answer 42

Question 43

How is the 1.8 mmol/l of CO2 transported not involved in the regulation of pH?

Answer 43

60% as bicarbonate

30% as carbamino compounds

10% dissolved as CO2