Hey! I'm a doctor and a biochem nerd. Your blood pH is buffered by multiple systems, primarily bicarbonate, as well as lactate and phosphate compounds. That means that swings in the concentration of blood compounds (aspirin, or salicylic acid, for example, is an acid) don't cause significant changes. pH is also tightly controlled by your lungs and kidneys. Your lungs control the amount of CO2 in your blood, and therefore bicarb and carbonic acid content. Your kidneys also retain or excrete bicarb as needed. There are certain processes like lactic acid production, electrolyte abnormalities, and kidney or lung issues that can make you relatively acidic. However, a truly acidic pH of even slightly less than 7 is a pretty profound acidosis and likely life threatening.