Respiratory System 4.2- Carbon Dioxide in blood

How much more hydrogen carbonate is there in plasma than dissolved co2 ?

20 times as much hydrogen carbonate as dissolved CO2

What is there more of in the blood CO2 or O2 ?

There is much more CO2 in the blood than oxygen both more dissolved and more reacted chemically

How does transported CO2 travel in blood as ?

Transported CO2 • 80% travels as hydrogen carbonate • 11% as carbamino compounds • 8% as dissolved CO2

What is the total transported carbon dioxide then ?

Transported carbon dioxide • = 23.5 -21.5 • = 2 mmol per litre of blood • only about 10% of total

T/F: The concentration of dissolved CO2 in plasma at a partial pressure of 5.3 kPa is 1.2 mmol.l-1.

True

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What is more soluble CO2 or O2 ?

CO2 is more soluble than O2 But CO2 also reacts with chemically with water

What is Co2 like in the plasma

Dissolved Co2 reacts with water to form H+ and HCO3- Reaction is reversible Amount reacting depends on the concentration of reactants and products

Dissolution of Co2 in water

at pco2 of 5.3 spa water dissolves 1.2 mmol/l Dissolved co2 can then react with water in different compartments of the blood

What is the role of hydrogen carbonate in plasma ?

plasma has 25mmol.l-1 hydrogen carbonate • no tfrom CO2 in plasma (sodium hydrogen carbonate) • stops nearly all dissolved CO2 from reacting • so pH is alkaline

Why are carbamino compounds important ?

• CO2 also binds directly to proteins • contributes to CO2 transport • but not acid base balance • bit more formed in venous blood because pCO2 higher

What happens when venous blood reaches the lungs ?

• Hb picks up oxygen • so gives up H+ • reacts with hydrogen carbonate • to form CO2 which is breathed out

What does dissolved Co2 depend on ?

• depends directly on pCO2 • if pCO2 rises pH will fall • if pCO2 falls pH will rise

What does the pH of plasma depend on ?

• depends on how much CO2 reacts to form H+ • which depends on [dissolved CO2 ] • pushing the reaction one way • and[HCO3-] • pushing it the other

The numbers of stuff dissolved in arterial blood

• plasma dissolves 0.7 mmol CO2 per litre of blood (plasma only 60% total volume!) • plasma contains 15.2 mmol HCO3- per litre of blood • cells dissolve 0.3 mmol.l-1 • cells have 4.3 mmol.l-1 HCO3- • blood has 1 mmol.l-1 carbaminos

Why is there more Co2 in arterial blood than o2 ?

Because Co2 is a major part of the system controlling ph of the blood Much more important process than its transport from tissues to lungs

If you wished to measure changes in pCO2 by monitoring changes in pH where in the body would you best put your sensor?

CSF

If the pCO2 rises, which of the following three fluids will change its pH (i) the most (ii) the least? (Do not go into calculations just estimate.) (i) Whole arterial blood (ii) Whole venous blood (iii) Cerebro-spinal fluid.

CSF most, arterial blood next, venous blood least

T/F: Most of the CO2 in plasma reacts with water to form hydrogen ions and hydrogen carbonate.

False - the high concentration of hydrogen carbonate keeps most as dissolved carbon dioxide

T/F: As each molecule of CO2 which reacts with water produces one H+ and one HCO3- , the plasma ratio of [H+ ] to [HCO3- ] is always going to be one to one.

False - there is much more hydrgogen carbonate from red cells and kidney

T/F: Carbonic anhydrase speeds up the reaction of CO2 with water in the plasma.

False - very little carbonic anhydrase in plasma

What organ controls the concentration of hydrogen carbonate in plasma

The kidneys control hydrogen carbonate concentration in plasma by variable excretion so really pH = 6.1 + log (kidneys/lungs)

The numbers for stuff dissolved in venous blood

The numbers - venous blood • plasma dissolves 0.8 mmol CO2 per litre of blood (plasma only 60% total volume!) • plasma contains 16.3 mmol HCO3- per litre of blood • cells dissolve 0.4 mmol.l-1 • cells have 4.8 mmol.l-1 HCO3- • blood has 1.2 mmol.l-1 carbaminos

How much in total if CO2 is contained in arterial blood

The total - arterial blood • contains 21.5 mmol CO2 per litre

How much in total of CO2 is contained in venous blood ?

The total - venous blood • contains 23.5 mmol CO2 per litre

How much more CO2 is there in arterial blood than O2 ?

almost x3 more CO2 in arterial blood than oxygen

What ratio does the pH of plasma depend on ?

depends on the ratio of • the reaction of CO2 in the red cell • to the reaction of CO2 in plasma

What are 2 critical determinants of pH in arterial blood ?

the pCO2 is a critical determinant of pH but so is [HCO3-]

What happens if theres extra co2 in venous blood ?

• a little more dissolves • but much more is converted to hydrogen carbonate • because Hb binds more H+ • as both pCO2 and [HCO3-] increase pH does not change much

What is arterial Pco2 determined by ?

• determined by alveolar pCO2 • determines dissolved CO2 • and so affects pH

Explain the reactions of CO2 in the red blood cell

• dissolved CO2 reacts with water • but now one of the products removed • H+ binds to haemoglobin • so lots of CO2 reacts • and lots of hydrogen carbonate formed hydrogen carbonate leaves red cell • in exchange for inward movement of chloride • formingthe25mmol.l-1 of HCO3- in plasma

How does the body buffer against pH changes ?

• if the body produces acid • this reacts with hydrogen carbonate • to form CO2 • which is breathed out • stops pH changing too much

Why does plasma hydrogen carbonate not change much with pCO2?

Plasma hydrogen carbonate does not change much with pCO2 • because the reactions of CO2 in the red cell are mostly determined • by how much H+ binds to Hb

What is venous Pco2 determined by ?

in venous blood pCO2 is higher • so more CO2 dissolves but depends on oxygenation • the more oxygen bound • the less H+ is Hb has lost oxygen • so binds more H+ • which forms more HCO3- • which is exported to plasma