This is an example of negative feedback. In this case, during physical exercise, there is a greater output of carbon dioxide by the cells. The high levels of carbon dioxide in the blood plasma coupled with a lower ph cause chemoreceptors in the medulla oblongata to stimulate the medullary respiratory center to send impulses to the respiratory muscles to increase the breathing rate. This will cause excess carbon dioxide to be removed and eventually, the oxygen levels will rise. Carbon dioxide levels and ph will go back to normal once again.