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Flashcards in Carbon Dioxide in the Blood Deck (17):

Is there more CO2 or O2 in arterial blood?

Arterial blood has almost 2.5 times as much CO2 as oxygen.


Why is there more CO2 in arteries than O2?

Carbon dioxide in arteries is very concentrated (almost double the oxygen). This is because it has a major role in controlling the pH of the plasma. pH must be kept between 7.35 and 7.45. Deviation for this is very dangerous - very high death rate from small changes in pH (7.5 = 45% death rate).


What does the concentration of CO2 dissolved in blood depend directly upon?

The concentration of carbon dioxide that is dissolved = solubility x pCO2. Concentration of CO2 depends directly on its partial pressure of it in the alveoli.


What happens when Carbon dioxide is dissolved in blood, what are the majority of these products associated with?

Dissolved CO2 reacts with water to form carbonic acid. This very quickly dissociates to hydrogen ion and hydrogen carbonate ions. This dissociation is reversible and the equilibrium point can be changed by the concentration of products and reactants. Most the HCO3- is associated with Na+ not hydrogen ions.


What is the Henderson-Hasselbalch equation?

pH = pK + Log ([HCO3-]/(pCO2 x 0.23))


What prevents most of the CO2 dissolved in blood from forming HCO3-?

There are 25mmol/l of HCO3- which means that most the CO2 that is dissolved is prevented from reacting with water. As a result, the pH of plasma is slightly alkali.


What determines the pH of blood plasma?

pH of plasma is determined by the ratio of dissolved CO2 and HCO3. Dissolved CO2 is dependent on alveolar pCO2 and so ventilation.


What produces all the HOC3- in the blood if it isn't dissolved CO2?

Hb has a large capacity for binding H+ ions. The amount of HCO3- that erythrocytes produce depends on the binding of H+ to haemoglobin. Erythrocytes produce HCO3- but they don’t control concentration of HCO3- in plasma.


How do red blood cells form HCO3-?

Inside the RBC all the H+ from CO2 reacting with water is bound to Hb and so the reaction moves far over to the right. A Chloride bicarbonate exchnager moves the HCO3- into the plasma and this moves the equaiton in the palsma far to the left preventing CO2 that is dissolved from reacting with water.


What ultimately causes the formation of HCO3- in blood plasma?

[HCO3-] doesn’t change much with pCO2. HCO3- is mostly determined by H+ binding to Hb.


So how is Plasma pH controlled?

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 which is controlled in the kidneys.


How does HCO3- act as a buffer?

Hydrogen carbonate buffers extra acid. The body produces acid such as lactic acid, sulphuric acid and keto acids. Acids react with HCO3- to produce CO2. Therefore [HCO3-] goes down the CO2 produced is removed by breathing and pH changes are minimised.


What determines the concentration of dissolved CO2 and so pH?

Arteriolar pCO2 is determined by alveolar pCO2, this determines how much CO2 is dissolved and therefore affects pH.


How does the rate of buffering of H+ by Hb change?

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 to 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.


Describe how the extra Carbon dioxide that we breath out is formed

At the tissues:
Less O2 binds to Hb  T state and more H+ ions bind.
If Hb binds more H+ in RBCs then more HCO3- can be produced. Therefore, more CO2 is present in plasma. Both in dissolved and reacted form. This only causes a very small change in pH because both [HCO3-] and pCO2 have increased.

At the lungs Hb picks up oxygen and forms into the R state – as a result it releases H+ ions which react with HCO3- forming CO2 which can then be breathed out.


What is Carbon dioxide transported on in the blood?

CO2 can bind directly to proteins. It binds directly to amine groups of globin on Hb. This is not the same as where oxygen binds and is not part of the acid base balance but is part of CO2 transport. More of these carbamino compounds are formed at the tissues because pCO2 is higher and unloading of O2 facilitates binding of CO2 to Hb. The reverse of this occurs at the lungs allowing CO2 to be given up at the lungs.


What percentage of carbon dioxide is actually transported away from tissues and out into the lungs?

If we add all the concetrations of carbon dioxide in its different forms and take the arterial away from the venous then we can work out the concentration of actual transported CO2 – 1.8mmol/l – so only 8% of total CO2 is transported.

• 60% of this travels as HCO3-
• 30% of this travels as carbamino compounds
• 10% of this travels as dissolved CO2