Re: How does the bodies ph level affect lactic acid production

 There are two ways that muscles can make energy to use to work; one uses 
oxygen and the other one doesn't.  When you don't use oxygen to produce 
energy (ATP) a waste product is lactic acid.  Muscles produce lactic acids 
when you exercise for a long period of time and especially when you sprint 
(like the 100, 200, and 400 meter dashes at track meets).  Since lactic 
acid is an acid it makes the pH of the fluids in your body become lowered 
(more acidic).  Your body has chemicals in it that work to soak up, or 
buffer, that excess acid to keep your pH normal (7.4).  Your kidneys are 
important in increasing this chemical called bicarbonate (HCO3).  Also, 
your lungs help out but getting rid of as much carbon dioxide as possible. 
 
There is a chemical reaction that you may have learned about that happens 
in many places in our bodies and works to control the acidity of our body 
fluids.

CO2 + H20 -> H+ + HCO3-

Carbon dioxide + water -> proton + bicarbonate

This reaction goes both ways depending on the amounts of each chemical that 
you have in your body.  So, if you increase the H+ you will make the 
reaction go backwards and lots of CO2 and H2O will be produced.  So, the 
lungs will blow out this extra CO2.  If you keep CO2 low by exhaling it 
then a lot of the H+ will be converted into H2O and CO2 and eventually the 
acid (H+) will decrease and the pH will go back to normal (it will go from 
below 7.4 back to 7.4).  Also, like I said above, the kidneys are also 
working to increase HCO3 to buffer the H+.  

So, during exercise normally, your pH stays constant, 7.4, by having  your 
lungs and kidneys chip in to take care of the excess acid (H+) being 
produced by your muscles.

Reference:  Human Physiology: from cells to systems.  Lauralee Sherwood, 
1997.