CO2 In The Blood

Question 1

What can CO2 react with

Answer 1

• CO2 is more soluble than oxygen
• CO2 rafts chemically with water
• CO2 also reacts with Hb but at a different site to O2

Question 2

Describe CO2 in arterial blood

Answer 2

• Has almost 2.5x as much CO2 as O2
• More dissolved and more reacted with water
• Why so much CO2 in blood going to the tissues?
• Total content of gases (dissolved and reacted)
• Total content CO2 in arterial blood ≈ 21 mmol.l-1 in
• Total content O2 in arterial blood ≈ 8.9 mmol.l-1

Question 3

Why so much CO2 in blood going to the tissues

Answer 3

Co2 has a major role in controlling blood pH
Controlling CO2 is more important for pH than for transporting it from tissues to the lungs
• Arterial blood pH must be kept within a narrow range (pH 7.35 – pH 7.45)
• First consider CO2 in arterial blood

Question 4

Describe CO2 in arterial blood - how much is dissolved ? And how is this calculated?

Answer 4

CO2 dissolves in water
[CO2]dissoved = solubility x pCO2
- solubility factor for CO2 at 37C = 0.23
At pCO2 of 5.3 kPa water dissolcves 1.2mmol/l CO2
Dissolved CO2 reacts with water in plasma and RBCs - Co2 in plasma can d=freely move into RBCs
CO2 in arterial blood is not there as waste product - it is here as part of a buffering sytem

Question 5

What is the normal pCO2 in alveoli

Answer 5

5.3 kPa

Question 6

How does CO2 react with water

Answer 6

Dissolved CO2 reacts with water to form carbonic acid H2CO3
CO2 + H2O H2CO3 H+ + HCO3-

H2CO3 is a very short lived intermediate - rapidly dissociates
• The reaction is reversible
• The rate of the reaction depends on the reactants and products

Question 7

What does the pH of plasma depend on

Answer 7

Depends on how much CO2 reacts to form H+
Which depends on how much [CO2]dissolved which pushes region to the righ
And on [HCO3-] which pushes it to the left

In plasma [dissolved co2] = 1.2mmol/l
[HCO3-] = 25 mmol/l
So reaction mainly pushed in reverse direction

Question 8

How are the pH of blood and the pCO2 related

Answer 8

• The amount of CO2 dissolved depends directly on the partial pressure of CO2
• If pCO2 rises plasma pH will fall (becomes more acidic)
• If pCO2 falls plasma pH will rise (becomes more alkaline)
• The pCO2 of alveoli is the deterring factor - as arterial blood equilibrates with this
• This is controlled by controlling rate of breathing

Question 9

Describe the concentration of HCO3- in the blood

Answer 9

Plasma contains 25mmol/l of HCO3-
Associated cation is mostly Na+ not H+ ()
The high [HCO3-] can’t come from CO2 in plasma - as only 1.2 mmol/l of CO2 - high levels must have come from somewhere else
High [HCO3-] prevents nearly all dissolved CO2 from reacting
Therefore the pH of plasma is alkaline

Question 10

What is he Henderson hesselbachs equatoion, and how can it be used to calculate the pH of plasma

Answer 10

Provides a way of calculating pH from pCO2 and [HCO3-]

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

PK is a constant = 6.1 at 37C

20x as much HCO3- as dissolved CO2/ log 20 = 1.3

PH = 6.1 + 1.3 = 7.2

It is a RATIO, not absolute

Question 11

How is the high plasma [HCO3-] established ?

Answer 11

• Reaction is speeded up by the enzyme carbonic anhydrase (CA) in RBCs
• The reaction proceeds in the forward direction because the products are mopped up in the RBC
• H+ ions bind to the negatively charged Hb inside RBCs
• Chloride-bicarbonate exchanger transports HCO3- our of RBCs
• Creates a plasma concentration of 25mmol.l-1 HCO3-

Question 12

Describe binding of h+ to Hb

Answer 12

• Haemoglobin has a large capacity for binding H+ ions
• The amount of HCO3- that RBCs produce depends on binding of H+ to haemoglobin
• Erythrocytes produce HCO3- but they dont control conc of HCO3- in plasma (contro takes place at kidney)

Question 13

Does plasma[HCO3-] chance with pCO2?

Answer 13

Slightly pushes reaction to the right but doesn’t change much bc conc of CO2 so low compared to HCO3-. The reaction is mostly determined but H+ binding to Hb

Question 14

Wha controls [HCO3-]?

Answer 14

• Kidney controls amount of HCO3- by varying excretion
• Therefore pH is dependent on how much CO2 is present (controlled by rate of breathing)
• and how much bicarbonate is present (controlled by kidneys)

Question 15

How does hCO3- act as a buffer?

Answer 15

Hydrogen carbonate buffers extra acid
• The body produces acids
– Lactic acid, keto acids, sulphuric acid
• Acids react with HCO3- to produce CO2
• Therefore [HCO3-] goes down
• The CO2 produced is removed by breathing and pH changes are minimised (buffered)
Body’s first line defence against pH changes

Question 16

How does arterial pCO2 affect pH

Answer 16

Art pCO2 determined by alveolar pCO2
This determines how much CO2 is dissolved
Therefore affects pH

Question 17

Descrbe the pCO2 in venous blood

Answer 17

• pCO2 is higher in venous blood – Comes from metabolically active tissues
• This means a bit more CO2 will dissolve
• But need to consider how venous blood can transport and give up the correct amount of CO2 to the lungs

Question 18

How does level of oxygenation affect how much HCO3-present

Answer 18

• 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
• If more O2 binds Hb -> R state and less H+ ions bind
- as at lungs
• If less O2 binds Hb -> T-state and more H+ ions bind
– As at tissues

In teh tissues, Hb binds more H+ in RBCs, so more HCO3- can be produces. Therefore more CO2 is present in venous system plasma. Both in dissolved and reacted

Question 19

What are [dissolved CO2] and [HCO3-] in venous blood

Answer 19

[dissolved CO2] = 1.33 mmol/l

[HCO3-] = 27.2 mmol/l

Question 20

Does extra CO2 in venous blood affect the pH?

Answer 20

Not much
[disolved CO2] increases a little but
Much more converted to HCO3- die to the increased capacity of Hb for H+
There is only a very small change in plasma pH because both [HCO3-] and pCO2 have increased
It depends on the ratio

Question 21

What happens when venous blood arrives at the lungs

Answer 21

What happens when venous blood arrives at the lungs
• Hb picks up O2 and goes into R-state • This causes Hb to give up the extra H+ it took on at the tissues
• H+ reacts with HCO3- to form CO2 and H2O
• CO2 is breathed out

Question 22

Descrbe the formation of carbamino compounds

Answer 22

• CO2 can bind directly to proteins
• Binds directly to amine groups on globin of Hb
• Binding of molecular CO2 onto Hb is not part of acid base balance but contributes to CO2 transport
• More carbamino compounds are formed at the tissues
– because PCO2 is higher
– and unloading of O2 facilitates binging of CO2 to Hb
• This CO2 is given up at the lungs

Question 23

What are the 3 ways CO2 is transported

Answer 23

Dissolved, HCO3- ions, carbamino compounds

Question 24

What is the total CO2 in whole arterial blood

Answer 24

21.5 mmoll

Question 25

What is the tiotal co2 in whol evenous blood

Answer 25

23.3 mmol/l

Question 26

How much co2 is transported and what percentage in each form

Answer 26

• = Total in venous blood – total in arterial blood
• = 23.3 – 21.5 mmol.l-1
• = 1.8 mmol.l-1
• Therefore only ~ 8% of the total is transported.
• The rest of the carbon dioxide is there as part of the pH buffering
system
• Of the 1.8 mmol.l-1 that is transported at rest
• Approximately –
• 60% travels as hydrogen carbonate
• 30% travels as carbamino compounds
• 10% travels as dissolved CO2