Lecture 7- CO2 transport in the blood Flashcards
how is carbon dioxide transported in the blood
- dissolved (CO2 is more soluble than oxygen)
- CO2 reacts chemically with water to form bicarbonate- HCO3-
- As a carbamino-haemoglobin compound
total content of O2 in arterial blood
8.9 mmol/L
total content of CO2 in arterial blood
21
Lots of CO2 in blood going to tissues- not a waste product.
CO2 has a major role in
controlling blood pH (acid-base balance)
Chemical reactant in the major pH buffering system of blood
blood pH must be kept within a narrow range
arterial pH 7.35 – pH 7.45; venous 7.31-7.41
body has several buffering systems to control blood pH but…..
one using CO2 most important
what is a buffer
- Buffers are compounds which are able to bind or release hydrogen ions such that they dampen swings in the pH.
outline the bicarbonate buffer system
- CO2 reacts with H20 to form carbonic acid
- carbonic acid dissoaciates to form H+ and HCO3-
how to workout CO2 dissolved in arterial blood
[CO2] dissolved= solubility x pCO2
- solubility factor for CO2 at 37 degrees= 0.23 mmol/L/kPa
- pCO2 of CO2 arterial blood= 5.3 kPa
- therefore:
- 5.3 kPa x 0.23 mmol/L/kPa= 1.2 mmol/L dissolved CO2
why is [CO2] dissolved higher than [O2] dissolved
more dissolved CO2 even though lower pCO2 than pO2 because of markedly increased solubility
carbon dioxide in plasma
- dissolved CO2 reacts with water to form carbonic acid –> H2CO3
Carbonic acid very quickly
dissociates to H+ and HCO3-
bicarbonate buffer equation is
reversible
the direction the bicarbonate buffer favours depends on
- The rate of the reaction depends on the amount of reactants (on the left) and productions (on the right)- “law of mass action”
- Reaction can go on either direction- dependent on conc of chemical
which equation can be used to calculate the pH of plasma
Henderson-Hasselbalch
Henderson Hasselbalch equation applied to the carbon dioxide-bicarb buffer system
plasma pH is dependent on
- on how much [CO2] reacts with water to form H+
- if CO2 is high then the reaction will be favoured towards the right producing lots of H+ and HCO3-
higher [HCO3-] in plasma will
push the reaction to the left
in the body the reaction is favoured in this direction because HCO3-> CO2- why our body pH slightly alkaline 7.4
- Amount of Co2 dissolved depends directly
- on the partial pressure of CO2
*
If pCO2 rises (and no change in bicarbonate)
plasma pH will fall (become more acidic)
If pCO2 falls (and there is no change in bicarbonate)
plasma pH will rise (become more alkaline)
what is the determining factor of the amount of CO2 dissolved
- The pCO2 of alveoli’s is the determining factor- determines arterial pCO2
- Alveolar, and hence arterial pCOP2 controlled by altering the rate of breathing
how much HCO3- in plasma
25 mmol/L
- cation associated with this is mainly na+ not H+
where is most of HCO3- made
the RBC
the high [HCO3-] cannot coem from O2 alone in plasma- not enough dissolved CO2 in plasma to create this much HCO3-
why is HCO3- production in the RBC much faster than the plasma
- Reaction speeded up by Carbonic anhydrase (CA) enzyme present in RBC but not present in plasma
why does HCO3- production in the RBC proceed in the froward direction
- produces HCO3- because the products are mopped up in the RBC
How are products of HCO3- production mopped up in the RBC (to keep reaction in forward direction)
- HCO3- is transported out of the RBC by the chloride/ HCO3- exchanger
- creates a plasma conc of 25 mmol/L HCO3-
- H+ is bound to Hb
although RBC are the major producers of HCO3-, they do not control the levels. Where does this happen?
the kidineys
Role of kidney and lungs in CO2: bicarbonate levels
- [HCO3-] normally doesnt change much with changed in pCO2- low levels dissolved CO2
- HCO3- comes from the RBCs (reaction is mostly determined by H+ binding to Hb)
- Kidneys control amount of H CO3- by varying excretion- can also create more HCO3-
- therefore pH is dependent on:
- how much CO2 is present (controlled by rate and depth of breathing)
- how much HCO3- is present (controlled by kidneys)
what does bicarbonate buffer
extra acid
- As part of normal metabolism body produces acids
- Lactic acids, keto acids, suphuric acid
how does bicarbonate act as a buffer for extra acids
- Acids react with HCO3- to produce CO2
- Reaction pushed to the left
- CO2 levels increase
- removed by increased ventilation- pH changes are minimised (buffered)
- More HCO3- needs to be produced to replenish
pCO2 is ………. in venous blood
higher
- due to veins draining metabolically acitive tissues
- more CO2 will be dissolved
Properties of Hb important for H+ buffering
- Buffering of H+ by Hb depends on level of oxygenation
- There are 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
- There are always H+ ions bound to Hb, but amount depends on the state of the Hb molecule
- If less O2 binds Hb → T-state and more H+ ions bind – As at tissues
H+ buffering At the tissues to venous blood
- Less O2 binds to Hb → T-state and more H+ ions bind
- If Hb binds more H+ in RBCs in venous blood then more HCO3- can be produced as ↓H+ drives reaction to right - CO2 + H2O → H+ + HCO3- IN RBCs
- Therefore more HCO3- transported out of RBCs into plasma in venous blood
- Effectively increasing amount transported CO2 in blood – in form of bicarbonate (HCO3- )
- Also increased dissolved CO2 in blood – down partial pressure gradient from tissue to blood
What happens when venous blood arrives at the lungs
- Hb picks up O2 and goes into R state ( due to higher pO2)- lower affinity for H+
- This causes Hb to go into the relaxed state and give up the extra H+ it took on at the tissues
- H+ reacts with HCO3- to form CO2
- Reaction push to the left
- CO2 diffuses out of the blood into the alveoli
- CO2 is breathed out
Red blood cells, CO2 and bicarbonate in body tissue and the lungs - summary
Formation of carbamino compounds
- CO2 can bind directly to Hb- not same site as O2
- Binds directly to amine groups on globin of Hb- carbamino compound
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- both because pCO2 higher, and unloading of O2 from Hb facilitates binding of CO2 to Hb- oxygen poor Hb (T state Hb binds CO2 better)
This CO2 is given up at the lungs as Hb becomes oxygen rich- oxygenated Hb unloading CO2= Haldane effect
Haldane effect
CO2 is given up at the lungs as Hb becomes oxygen rich- oxygenated Hb unloading CO2
how much CO2 is transported to be eliminated?
- Calculations reveal that at rest only ~8% of CO2 is transported to the lungs to be eliminated- remember more CO2 can be eliminated if there is extra acid that needed to be buffered
- The rest of the CO2 in blood is there as part of the pH buffering system
% of dissolved CO2 transported to the lungs
10%
% of bicarbonate transported to the lungs
60% of CO2 in blood
% of dissolved carbamino compounds transported to the lungs
30% of dissolved CO2 transported to the lungs
