Re: If a man does not breathe while exercise, how would Pco2 effect breathing:

Ata
When a person does not breath (during exercise or at rest makes no difference 
except in how long the process takes to force the person to breath again) the 
blood Pco2 will increase throughout that period of time.  This increasing Pco2 
is an increasingly powerful stimulus (mainly through decreasing the pH) to the 
respiratory center within the medulla oblongata to induce the person to take a 
breath by stimulating the diaphragm through the phrenic nerves.  The longer the 
time that passes without breathing, the deeper and more frequent will be the 
respiration when restarted, gradually decreasing until normal blood Pco2 levels 
are again reached.  If a person had the mental discipline to hold their breath 
until passing out (loss of consciousness due to low pH) they would immediately 
start breathing again at a very rapid rate to return the Pco2 and the pH to 
within normal ranges.
An interesting example that show the opposite effect (DO NOT TRY THIS YOURSELF) 
is when a person breaths pure helium.  Even thought the individual is not 
getting any o2, there is almost no awareness of anything being wrong until it is 
too late and the person passes out.  Why is this?  The blood Pco2 remains within 
normal limits because the person is breathing it out in just the same way is 
when there is normal air available to breath.  Because the co2 receptors are 
much more important in stimulating respiration, the person has none of the panic 
type feeling that occurs when one breaths into a bag or is suffocated.  In both 
of those cases the o2 is also being used up just like when he breaths pure 
helium but the difference is that in suffocation or rebreathing from a bag, the 
Pco2 is increasing along with the decreasing Po2 and thus the increasing blood 
Pco2 drives the system.
Another demonstration (DO NOT TRY THIS YOURSELF) is to have a person take 20 
deep breaths, as deep as he can, as rapidly as he can.  This is called 
hyperventilation.  What this causes is a great decrease in respiratory efforts 
and concurrently, because the cardiovascular system is closely linked with the 
respiratory system, it will cause a decrease in blood pressure.  The 
demonstrator then has this hyperventilated person cross his arms and hold the 
last breath without letting it out.  The demonstrator would then, from behind, 
put his arms around the hyperventilated individual while he is holding that last 
breath and squeeze hard enough to lift the person off the ground.  The 
hyperventilated individual should briefly pass out in the demonstrator’s arms.  
Why?  Because blowing off all that co2 has reduced blood pressure (as well as 
respiration) and by increasing pressure in the thorax (holding breath while 
squeezing the chest wall) the demonstrator reduces the ability of the heart to 
pump blood up to the brain.  The brain cannot remain conscious without constant 
o2 and glucose, thus lights out for a few seconds.
Thus you can see that co2 is very important in both respiratory ventilation as 
well as cardiovascular function.  We need the o2 to stay alive but the co2 is 
the stimulus to bring in the o2.  This is also why a person who is being 
poisoned with carbon monoxide (co) is generally not aware of what is happening 
until it is too late.  Why is that?  The co binds to the iron of the heme part 
of hemoglobin where o2 normally binds, not letting the o2 bind at all.  Because 
the reduction in o2 reduces your ability to think clearly as it is going to low 
levels, but by then you are not thinking clearly enough to get yourself out of 
the lethal situation.
What a system!
Excellent question and the best to you, Dr. Swanson