Blood has a ph ranging from 7.35 to 7.45. slight deviations from this can cause major problems, even death. you are doing an intense workout, and your skeletal muscle cells are producing metabolic acids such as lactic acid. your blood ph does not drop significantly in spite of the metabolic acids released into the blood. you maintain a constant blood ph because

The answer is: the body contains chemicals called buffers that resist changes in pH

When you exercise vigorously, the muscle will produce more carbon dioxide which will makes the blood more acidic. Human blood have some mechanism that could prevent the blood pH to stray further from the optimal range. One of the buffer that keep carbon dioxide acidity would be sodium bicarbonate.