Lecture 6: Carbon dioxide transport

Question 1

What forms does carbon dioxide exist as in the body?

Answer 1

1. Dissolved (more soluble than oxygen)
2. HCO3- (reacts chemically with water to form bicarbonate)
3. Carbamino-haemoglobin compound (it reacts with Hb, not at the site where oxygen binds, attached to amino chains)

Question 2

What is the total content of oxygen and carbon dioxide in arterial blood?

Answer 2

Total oxygen: 8.9 mmol/L
Total carbon dioxide: 21 mmol/L
(Carbon dioxide in arterial blood is not primarily there as a waste product)

Question 3

Why is there a large amount of carbon dioxide in our arterial blood?

Answer 3

It has a major role in controlling blood pH because it is a chemical reactant in the major pH buffering system of the blood

Question 4

What must the blood pH be?

Answer 4

Between 7.35-7.45 arterial

Between 7.31-7.41 venous

Question 5

What are buffers?

Answer 5

Compounds whichare able to bind or release hydrogen ions so that they dampen swings in the pH

Question 6

What is the main pH buffer system in the body?

Answer 6

Bicarbonate buffer system

CO2 + H2O >< H2CO3 >< H+ + HCO3-

Question 7

How much is there of dissolved carbon dioxide in arterial blood?

Answer 7

0.23 x 5.3 = 1.2 mmol/L of dissolved CO2
(soluability factor x pCO2 in arterial blood)

There is more dissolved CO2 even though it has a lower partial pressure c/w oxygen (due to increased solubility factor of CO2)

Question 8

What happens to carbon dioxide in plasma?

Answer 8

• reacts with water to form carbonic acid
• carbonic acid rapidly dissociates to H+ and HCO3-
• reversible reaction so the rate of the reaction depends on the amounts of reactants and products

Question 9

What is the Henderson Hasselbalch equation?

Answer 9

pH= pKa + log((conc of conjugate base)/(conc of weak acid))

-conjugate base is whats left after the weak acid has donated the H+ ion

Question 10

How do you apply the Henderson Hasselbalch equation to carbon dioxide in the plasma?

Answer 10

-calculate the pH in plasma
-HCO3- is the conjugate base
-CO2 is the weak acid as it goes onto form carbonic acid (conc of carbonic acid depends on CO2 level)
-pKa= 6.1 at 37 degrees
The ratio is 21:1 (HCO3-/CO2), we want to maintain this to ensure the pH remains in the physiological range.

6.1 + log (base 10) (25/1.2) = 7.41 pH

Question 11

What is the dissolved concentration of HCO3-?

Answer 11

25 mmol/L

Question 12

What will happen if carbon dioxide increases and the buffer of HCO3- isn’t working?

Answer 12

Become more acidic with a low pH

-because the CO2 reacts with water and eventually produces H+ ions

Question 13

Why is the body pH slightly alkaline? (7.4)

Answer 13

HCO3- > CO2 conc

So RHS of buffer reaction is favoured

Question 14

What does the amount of dissolved CO2 depend on?

Answer 14

Directly on the partial pressure of CO2
Therefore if:
-pCO2 rises (and there is no change in HCO3-), plasma pH will fall
-if pCO2 falls (and there is no change in HCO3-), plasma pH will rise
The pCO2 of alveoli is the determining factor, which is controlled by our ventilation (rate and how deeply)
-hyperventilating: giving out CO2 so dissolved levels of pCO2 fall until HCO3- adjusts

Question 15

Where is the high bicarbonate created?

Answer 15

RBC
-CO2 from tissue/blood down a PP gradient and enters the RBC
-CO2 reacts with water
-here carbonic anhydrase is present (not present in plasma) so this reaction occurs 5000x faster
-this forms HCO3- and H+
We need to mop up the HCO3- and H+ in order to keep the reaction in the right direction
-HCO3- leaves via the Cl-/HCO3- antiport
-H+ binds to Hb

Question 16

What controls the levels of HCO3-?

Answer 16

RBC makes the HCO3-, but it does not control its conc

-HCO3- concentration is controlled by the kidney by varying excretion and can also create more HCO3-

Question 17

How does bicarbonate buffer extra acids?

Answer 17

In normal metabolism the body produces lactic acid, keto acids, sulphuric acid
-acids react with HCO3- to produce CO2
-CO2 levels increase
(CO2 + H2O < H+ + HCO3-)
-chemoreceptors sense increased levels of CO2 in blood, they cause increased ventilation
-extra CO2 produced is removed by breathing and pH changes are minimised (buffered)
-more bicarbonate needs to be produced to replenish it

Question 18

What is the composition of venous blood?

Answer 18

pCO2 is higher in venous blood, because it has just drained metabolically active tissues

Question 19

How do we deliver CO2 to the lungs to be eliminated?

Answer 19

There are always H+ bound to Hb (H+ binds to Hb to keep reaction to the right inside the RBC)
The amount of H+ bound to Hb depends on the state of the haemoglobin molecule.
-at the lungs, if more oxygen binds> R state and less H+ bind
-at tissues, less oxygen binds> T state and more H+ bind

Question 20

How does the binding of more H+ ions increased amount of transported CO2 in the blood?

Answer 20

At the tissues to venous blood:

• less oxygen binds to Hb > T state and more H+ ions bind
• if Hb binds more H+, then more HCO3- can be produced as low H+ drives the reaction to the right (CO2 + H2O > H+ + HCO3-)
• therefore more HCO3- is transported out of RBC to plasma in venous blood
• this effectively increases amount of transported CO2 in blood in the form of HCO3- (CO2 from the tissue)

(there is also increased dissolved CO2 in blood as it goes down its PP gradient from the tissue, which then enters the RBC down its PP gradient as we keep CO2 conc in RBC low)

Question 21

What happens when venous blood arrives at the lungs?

Answer 21

• Hb picks up oxygen and enters the R state
• here it gives up the extra H+ it took up at the tissues
• H+ reacts with HCO3- to form CO2 (HCO3- diffuses into the RBC down conc gradient from plasma)
• this CO2 is then breathed out (goes down its partial pressure gradient)

Question 22

Is the formation of carbamino compounds part of the acid base balance?

Answer 22

No, but it contributes to CO2 transport

Question 23

What is the haldane effect?

Answer 23

CO2 binds better to Hb that’s in a low oxygen state
Therefore more carbamino compounds are formed at the tissues (due to high pCO2 and low pO2)
This CO2 is given up at the lungs becuase oxygen becomes oxygen rich

Question 24

How much carbon dioxide is transported to the lungs to be eliminated?

Answer 24

8% only
(more CO2 can be eliminated if there is extra acid that needs to be buffered)
-this means the rest of the CO2 in the blood is there as part of the pH buffering system

Question 25

What are the proportions of the different forms of CO2 transported to the lungs for elimination?

Answer 25

Dissolved CO2: 10%
Bicarbonate: 60%
Carbamino compounds: 30%