The pH of your blood is normally between 7.35 and 7.45, just slightly above neutral. The term "pH" -- which stands for potential hydrogen -- gives you an idea of a substance's acidity or alkalinity. Strenuous physical activity is even a strain on your blood, which can cause your blood pH to drop or become more acidic. How this occurs is quite simple and how your body adapts to it is quite ingenious.
A change in your blood's pH during exercise is largely caused by the increasing need for energy. When you exercise, your cells kick into high gear, making energy to facilitate your movements. The two main pathways through which this occurs are aerobic and anaerobic metabolism. Aerobic metabolism is the pathway of energy production that uses oxygen; anaerobic metabolism produces energy without oxygen. Both systems produce a byproduct that is capable of decreasing the pH of your blood.
During aerobic metabolism, your muscle cells release carbon dioxide into your blood. When blood travels into the working muscles, oxygen is exchanged for carbon dioxide and hydrogen. The cells take in oxygen from the blood and the blood carries away carbon dioxide and hydrogen -- waste products of cellular metabolism. The combination of carbon dioxide and hydrogen creates a reaction that causes acidity inside the bloodstream, which decreases the pH of your blood.
Anaerobic metabolism is not meant to facilitate long-term energy production because of its effect on your blood pH. When you are exercising more intensely than oxygen delivery can keep up with, the body turns to anaerobic metabolism. A product of this metabolism is pyruvate, which your body turns into lactic acid and releases into the blood. If the addition of lactic acid exceeds your body's clearance, your blood pH decreases.
With regular exercise, your body adapts. Training prepares the body for exercise, resulting in the ability to work at a higher intensity without needing to call for energy through anaerobic means. In addition, the body becomes better able to use lactate during exercise. Lactic acid enters into the mitochondria of the muscle cell, where aerobic metabolism occurs. With training, the body is able to move more lactic acid into the mitochondria and use it for aerobic metabolism, reducing the amount released into the bloodstream.