Blood pH: The acidity or alkalinity of blood. The pH of any fluid is the measure of the hydrogen ion (H-) concentration. A pH of 7 is neutral. The lower the pH, the more acidic the blood. A variety of factors affect blood pH including what is ingested, vomiting, diarrhea, lung function, endocrine function, kidney function, and urinary tract infection. The normal blood pH is tightly regulated between 7.35 and 7.45.