Answer
Decreased blood pH indicates an increase in blood carbon dioxide concentration. An increased heart rate moves more blood through the lungs, where oxygen is added and carbon dioxide is removed.
Work Step by Step
Carbon dioxide forms carbonic acid in the blood, which lowers the pH. The body rebalances the CO$_2$/O$_2$ ratio in several ways, one of which is an increase in heart rate. By increasing heart rate, more blood moves through the lungs at a higher rate, removing carbon dioxide and causing oxygen to be added more quickly.
