whether or not to send the pyruvate to the mitochondrian?

SynapticSelf

I hope this doesn't qualify as homework help, because I'm not in school and this is more of a general question:

After Glycolysis, Pyruvate can undergo a number of reactions. If oxygen levels are low, it is converted into lactic acid so that the NAD can gain back it's proton and continue in Glycolysis. Q) Does this conversion take place in the cytoplasm as well? (I'm guessing yes)

If there is oxygen, as I understand it, the pyruvate is shuttled off to the mitochondrian and is converted into Acetyl CoA. However, I don't believe (I may be wrong) that oxygen is required for this conversion...

So why is there a buildup of lactic acid as opposed to a buildup of Acetyl CoA? How does the cell "know" whether or not to send the pyruvate to the mitochondrian? Is this somehow governed by statistics/probability?