|MadSci Network: Biochemistry|
What a super question.
I don't know how much you know about pH, but I'll tell you that the most important thing to remember is that the pH scale is a logrithmic scale - which means that the difference between pH 7 and 8 is a factor of ten, that means pH 8 is ten times more basic than pH 7.
The pH of your blood varies very little - even in extreme situations. Blood pH is almost always in the range of 7.35-7.45 (arterial blood.) Your body uses bicarbonate (carbonic acid, the same stuff that makes soda fizzy) to balance it's pH, so if it varies, some bicarbonate changes into carbon dioxide or vice vera. We call this a buffer system, which is a chemical process to maintain a certain pH despite changes in acid or base composition.
Sometimes, health problems can cause a wild swing in pH. People who are starving, or Diabetics who are having a crisis, can fall into a condition called Keto-acidosis. As you might guess by the name, this results in a shift of the blood pH towards the acidic. Because they are metabolising fat, they produce something called ketone bodies which are very acidic.
This causes a drop in pH - their blood will try and compensate by allowing carbonic acid to turn into carbon dioxide, which they will exhale. This will raise the pH, but only if it is expelled from the body via breathing. Sometimes these people can be seen breathing very rapidly to dispose of this carbon dioxide.
Back to your question -
You are right that yawning is a response to a need for oxygen - your body slows breathing when you are at rest or tired, and a yawn is sort of a respiratory jump start, an extra deep breath to get more oxygen.
I believe, but don't absolutely know, that your blood pH might go up a tiny tiny bit, but because of the buffering system, it probably doesn't change very much.
A lot of anectodal (not scientifically rigorious) evidence suggests that although yawning is a physiological reflex, it's also triggered by stimulation - try yawning around other people early in the day or late in the afternoon and see how many other people yawn too.
Keep asking these interesting questions!
Tod Companion, Ph.D.
Department of Biochemistry and Molecular Genetics
Univeristy of Alabama at Birmingham
The website for a great physiology text book can be found at: http://www.mhhe.com/biosci/ap/foxhumphys/
It explains the blood buffering system in chapter 1, and has a handy table for normal blood stream values for all sorts of things.
The book is: Human Physiology, by Stuart Ira Fox, 6th Edition, WCB/McGraw Hill, 1999.
Or you can try ask.com at: http://www.ask.com/main/finalanswer.asp?origin=0& link=http%3A%2F%2F www%2Edi recthit%2Ecom%2Ffcgi%2Dbin%2FRedirURL%2Efcg%3Furl%3Dhttp%3A%2F%2Fwww%2Ekids health%2Eorg%2Fkid%2Fquestion%2Fyawn%2Ehtml%26qry%3Dwhy+do+we+yawn%26rnk%3D 0%26src%3DAJ&qcategory=KIDS
Try the links in the MadSci Library for more information on Biochemistry.