CO2 In The Blood Flashcards
What can CO2 react with
- CO2 is more soluble than oxygen
- CO2 rafts chemically with water
- CO2 also reacts with Hb but at a different site to O2
Describe CO2 in arterial blood
- 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
Why so much CO2 in blood going to the tissues
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
Describe CO2 in arterial blood - how much is dissolved ? And how is this calculated?
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
What is the normal pCO2 in alveoli
5.3 kPa
How does CO2 react with water
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
What does the pH of plasma depend on
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
How are the pH of blood and the pCO2 related
- 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
Describe the concentration of HCO3- in the blood
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
What is he Henderson hesselbachs equatoion, and how can it be used to calculate the pH of plasma
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
How is the high plasma [HCO3-] established ?
- 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-
Describe binding of h+ to Hb
- 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)
Does plasma[HCO3-] chance with pCO2?
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
Wha controls [HCO3-]?
- 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)
How does hCO3- act as a buffer?
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
How does arterial pCO2 affect pH
Art pCO2 determined by alveolar pCO2
This determines how much CO2 is dissolved
Therefore affects pH
Descrbe the pCO2 in venous blood
- 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
How does level of oxygenation affect how much HCO3-present
• 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
What are [dissolved CO2] and [HCO3-] in venous blood
[dissolved CO2] = 1.33 mmol/l
[HCO3-] = 27.2 mmol/l
Does extra CO2 in venous blood affect the pH?
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
What happens when venous blood arrives at the lungs
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
Descrbe the formation of carbamino compounds
• 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
What are the 3 ways CO2 is transported
Dissolved, HCO3- ions, carbamino compounds
What is the total CO2 in whole arterial blood
21.5 mmoll
What is the tiotal co2 in whol evenous blood
23.3 mmol/l
How much co2 is transported and what percentage in each form
• = 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
