Carbon Dioxide In The Blood

Question 1

What is more soluble, CO2 or water

Answer 1

Carbon dioxide is 20 times more soluble than oxygen

Question 2

What does CO2 react chemically with?

Answer 2

Water

Hb (different site to oxygen)

Question 3

How much oxygen and carbon dioxide is there in arterial blood?

Answer 3

2.5x as much carbon dioxide in blood as oxygen

CO2 = 21mmol/L 
Oxygen = 8.9mmol/L

Question 4

Why is there so much CO2 in blood going to the tissues?

Answer 4

Because carbon dioxide has a major role in controlling pH of blood plasma.

Controlling CO2 in blood is more important for pH control than for volume.

Question 5

WHat must the arterial pH be?

Answer 5

PH 7.35- 7.45 (how many hydrogen ions present in blood) -negative log scale

Question 6

What is the solubility factor of CO2 at body temperature and how does this compare to oxygen?

Answer 6

CO2 = 0.23

O2 = 0.01 (MUCH lower)

Question 7

How do you calculate how much CO2 can dissolve?

Answer 7

Solubility factor x pCO2

At pCO2 of 5.3kPa dissolves 1.22 mmol/L do CO2.

Question 8

What does dissolved CO2 do?

Answer 8

Dissolves CO2 reacts with water in plasma and in red blood cells.

CO2 is there as a buffer, NOT as a waste product

Question 9

When CO2 reacts with water, what does it form?

Answer 9

Form carbonic acid that VERY rapidly dissociates into hydrogen carbonate / bicarbonate (HCO3) and hydrogen ions.

(Usually ignore carbonic acid)

It is a reversible reaction - where equilibrium is depends on concentration of reactants and products.

Question 10

How is pH of plasma determined?

Answer 10

PH depends on how much CO2 reacts to form hydrogen ions.

This depends on how much CO2 is dissolved in plasma (make More H+ - reaction goes to right) and how much HCO3- there is (Makes it go left so more CO2)

Question 11

How much HCO3 is there compared to CO2?

Answer 11

There is only 1.2mmol/L of CO2 whereas there is 25mmol/L of HCO3-.
Means equilibrium will sit nearer left (more CO2).

Question 12

What does the amount of dissolves CO2 in the blood depend on?

Answer 12

PCO2 of the alveoli which is controlled by the rate of breathing.

If pCO2 rises - pH will fall
If pCO2 falls - pH will rise

Question 13

What is the cation that is mainly associated with HCO3-?

Answer 13

Mainly associated with Na+ and not H+ because if all from H+ then the solution would be very acidic.

Question 14

Where has all the HCO3- in plasma come from?

Answer 14

NOT all from CO2 as not enough CO2 to make it. This prevents all the CO2 from dissolving.

Question 15

Why is blood plasma alkaline?

Answer 15

Because of all the HCO3-

Question 16

What is the Henderson-Hesselbach equation? What can it be used for?

Answer 16

Used to work out pH of the blood.

PH = pK + log([HCO3-]/ pCO2 x 0.23)

PH = pK + log([HCO3-[ / [CO2]dissolved)

PK = 6.1 at body temperature

PH = 6.1 + 1.3 = 7.4

(1.3 = log(20/1)

Question 17

What does Henderson-Hesselbach tell us?

Answer 17

It the ratio of HCO3-:CO2 that determines the pH rather than one ion by itself.

Question 18

How is the high hydrogen carbonate established?

Answer 18

Established in red blood cells.

There is an enzyme called CARBONIC ANHYDRASE in red blood cells which speeds up the reaction, the H+ ions bind to Hb and the HCO3- is transported out of the RBC (via HCO3-,Cl exchanger) so that the forward reaction continues to occur (as equilibrium is not reached) and therefore not HCO3- is continuously made. This creates the plasma concentration of 25mmol/L.

Question 19

What is the biggest determining factor for the rate of reaction?

Answer 19

H+ binding to Hb

Question 20

What controls the amount of HCO3-?

Answer 20

The kidneys because they can change how much is absorbed / secreted.

Question 21

What happens when the body produce acids (lactic, keto, sulphuric)?

Answer 21

Acids react with HCO3- to produce CO2. Therefore, the [HCO3-] goes down. This CO2 is then removed by breathing so the pH changes are minimised.

Question 22

How can we give up the correct amount of CO2 at the lung?

Answer 22

Hb - The amount of H+ ions Hb can buffer depends on the oxygenation level.

More oxygen binds to Hb = R state = Less H+ bind (lungs)

Less oxygen binds to Hb = T state = More H+ bind (tissues)

Question 23

How are the properties of Hb important for CO2 transport?

Answer 23

Tissues - less O2 bound to Hb = T state = More H+ ions bind.

This means more HCO3- can be produced. Therefore, more CO2 is present in plasma in venous system is both the dissolved and reacted form.

Question 24

What effect does the extra carbon dioxide in venous blood have on pH?

Answer 24

Not much as the ratio of CO2:HCO3- is the same.

Question 25

What happens when venous blood arrives at the lungs?

Answer 25

Hb picks up oxygen and goes into R state.

This causes Hb to give up extra H+ it took on at the tissues.

H+ reacts with HCO3- to form CO2 which is breathed out.

Question 26

What are carbamino compounds?

Answer 26

This is when CO2 binds directly to the amine groups on globin of Hb.

This contributes to CO2 balance.

More carbamino compounds formed at tissues as: pCO2 higher, unloading of oxygen occurs.

This CO2 is given off at the lugs.

Question 27

What three different forms is CO2 transported as?

Answer 27

Dissolved CO2
Hydrogen carbonate
Carbamino compounds.

Question 28

How can you work out the amount of transported carbon dioxide?

Answer 28

Total in venous blood - total arterial blood.

23.3 -21.5 = 1.8mmol/L

Therefore, only about 8% of total is transported and given out in lungs. Shows most of it is not there as a waste product to be got rid of, it is there as a buffer.

Question 29

What % of the 1.8mmol/L waste CO2 is transported in each of the different form?

Answer 29

60% HCO3-
30% carbamino compounds
10% dissolved CO2