Carbon dioxide - by what mechanism is it poisonous?

[JIM NAPIER]

I am interested in finding out the mechanism by which CO2 is poisonous. 

My understanding is that Carbon MONOXIDE(CO) competes with oxygen for binding 
to haemoglobin, and whereas CO2 and O2 have relatively low affinities, the 
affinity that haemoglobin has for carbon monoxide is very high - the effect 
being that CO stays bound, whereas O2 and CO2 are able to exchange.

So, given this information (if it is correct), it would seem that CO2 does not 
 have its toxic effect by acting through high-affinity binding to haemoglobin, 
preventing O2 binding.

I used to think that CO2 is not poisonous, and that it only kills when it 
displaces the oxygen in a container.  I have since been told that this is NOT 
the case and that it IS toxic.

I would appreciate enlightenment on this subject.

If you can help, an email reply would be the preferred method.

Many thanks.