Carbon Dioxide In The Blood Flashcards
What is more soluble, CO2 or water
Carbon dioxide is 20 times more soluble than oxygen
What does CO2 react chemically with?
Water
Hb (different site to oxygen)
How much oxygen and carbon dioxide is there in arterial blood?
2.5x as much carbon dioxide in blood as oxygen
CO2 = 21mmol/L Oxygen = 8.9mmol/L
Why is there so much CO2 in blood going to the tissues?
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.
WHat must the arterial pH be?
PH 7.35- 7.45 (how many hydrogen ions present in blood) -negative log scale
What is the solubility factor of CO2 at body temperature and how does this compare to oxygen?
CO2 = 0.23
O2 = 0.01 (MUCH lower)
How do you calculate how much CO2 can dissolve?
Solubility factor x pCO2
At pCO2 of 5.3kPa dissolves 1.22 mmol/L do CO2.
What does dissolved CO2 do?
Dissolves CO2 reacts with water in plasma and in red blood cells.
CO2 is there as a buffer, NOT as a waste product
When CO2 reacts with water, what does it form?
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.
How is pH of plasma determined?
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)
How much HCO3 is there compared to CO2?
There is only 1.2mmol/L of CO2 whereas there is 25mmol/L of HCO3-.
Means equilibrium will sit nearer left (more CO2).
What does the amount of dissolves CO2 in the blood depend on?
PCO2 of the alveoli which is controlled by the rate of breathing.
If pCO2 rises - pH will fall
If pCO2 falls - pH will rise
What is the cation that is mainly associated with HCO3-?
Mainly associated with Na+ and not H+ because if all from H+ then the solution would be very acidic.
Where has all the HCO3- in plasma come from?
NOT all from CO2 as not enough CO2 to make it. This prevents all the CO2 from dissolving.
Why is blood plasma alkaline?
Because of all the HCO3-
What is the Henderson-Hesselbach equation? What can it be used for?
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)
What does Henderson-Hesselbach tell us?
It the ratio of HCO3-:CO2 that determines the pH rather than one ion by itself.
How is the high hydrogen carbonate established?
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.
What is the biggest determining factor for the rate of reaction?
H+ binding to Hb
What controls the amount of HCO3-?
The kidneys because they can change how much is absorbed / secreted.
What happens when the body produce acids (lactic, keto, sulphuric)?
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.
How can we give up the correct amount of CO2 at the lung?
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)
How are the properties of Hb important for CO2 transport?
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.
What effect does the extra carbon dioxide in venous blood have on pH?
Not much as the ratio of CO2:HCO3- is the same.
What happens when venous blood arrives at the lungs?
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.
What are carbamino compounds?
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.
What three different forms is CO2 transported as?
Dissolved CO2
Hydrogen carbonate
Carbamino compounds.
How can you work out the amount of transported carbon dioxide?
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.
What % of the 1.8mmol/L waste CO2 is transported in each of the different form?
60% HCO3-
30% carbamino compounds
10% dissolved CO2
