Re: How does the brain detect changes in blood pH?

When you breathe in, the oxygen (O2) in the air seeps into the blood 
circulating just below the surface of your lungs.  That blood is then 
circulated around your body to reach all its cells (which depend on oxygen, 
amongst other things, to stay alive).  At the same time, you are breathing 
out carbon dioxide (CO2, produced by the body's cells during normal 
metabolism).  If you don't breathe fast enough, your body's cells continue 
to absorb oxygen from the blood, but they do so faster than you are taking 
it in - so the level of O2 in your blood decreases.  Because you are not 
breathing out enough CO2 either, its levels increase, forming carbonic acid 
(H2CO3) in the blood.

The part of the brainstem known as the medulla contains the brain's 
respiratory centre, which is in charge of the (entirely automatic) process 
of breathing.  A nearby part of the medulla, and small bundles of cells 
just off the carotid artery and aorta (known as the carotid and aortic 
bodies, respectively) contain chemoreceptors, which are specialised cells 
that are programmed to signal the brain when local chemical conditions 
change. 

Peripheral chemoreceptors in the carotid and aortic bodies detect the 
increase in acidity (i.e. lower pH) caused by higher levels of CO2 (and 
reduced O2) when you are not breathing fast enough.   They then transmit 
signals to the medulla's respiratory centre which instruct it to increase 
the rate at which you breathe until you have taken in enough oxygen for 
O2/CO2 levels to return to normal.  The resulting resulting increase in pH 
is also signalled by the chemoreceptors).  The neural mechanisms are better 
understood for the carotid bodies, which are easier to study because of 
their anatomical location, than for the aortic bodies.  Carotid bodies 
transmit their neural signal via the carotid sinus and glossopharyngeal 
nerves to the respiratory centre in the medulla.  The frequency at which 
the nerves fire increases exponentially as blood levels of O2 fall and/or 
levels of CO2 rise.  Incidentally, the system is much more sensitive to 
changes in CO2 levels than to changes in the amount of O2 in the blood 
(this stems from experiments where levels of one are held at a constant 
level while the other is varied).

Central chemoceptors in the medulla which influence breathing rates do not 
respond directly to O2/CO2 levels, but actually reflect the concentration 
of hydrogen ions (H+).  The medulla, like the rest of the brain, is 
surrounded by cerebrospinal fluid (CSF), which is protected from most of 
the (potentially harmful) substances in the bloodstream by a membrane 
called the 'blood-CSF barrier'.  However, one of the substances that can 
get through the blood-CSF barrier is CO2.  It diffuses across the barrier, 
and is hydrated to form carbonic acid (H2CO3), which then dissociates into 
hydrogen ions H+ and bicarbonate (HCO3-).  So, the higher the blood CO2 
level, the higher the CSF concentration of H+.  This leads to the H+ 
sensitive chemoceptors in the medulla firing more often, and is detected by 
the respiratory centre as a signal to speed up the rate of breathing and so 
bring down the concentration of CO2.

Of course if you are breathing too fast instead of too slowly, the whole 
process occurs in reverse to lower the levels of blood O2 and raise levels 
of CO2 until the blood's pH is within normal limits.  The respiratory 
centre detects that the right levels have been reached when an appropriate 
number of nerve impulses is received from the chemoreceptors within a given 
period of time.

I hope this helps to answer your question.  For more detail (and diagrams!) 
try the following books (more recent editions are probably available but 
not sure if the page numbers will still be the same):

A.J. Vander, J.H. Sherman and D.S. Luciano:  Human Physiology, 4th edition 
- especially the chapter on respiration, in particular pages 410-414.

W.F. Ganong: Review of Human Physiology, 12th Edition.  See page 552 for 
more detailed explanation of innervation of carotid/aortic bodies;

M.J.T.FitzGerald:  Neuroanatomy, Basic and Applied.  Pp 263-265.   This one 
has pictures based on electron micrographs showing what is happening in the 
carotid body at a cellular level.