Ans.The rate of breathing is controlled by the respiratory centre in the brain. The respiratory centre is sensitive to the concentration of carbon dioxide in the blood.; When we do exercise or some hard job our muscle cells carry out cellular respiration at faster rate. It results in the production of more carbon dioxide which is released in the blood. This greater than normal concentration of carbon dioxide stimulates the respiratory centre of brain. The respiratory centre sends messages to the rib muscles and diaphragm to increase the rate of breathing by increasing the rate of contraction so that the excess carbon dioxide present in blood can be removed out of body, During exercise or other hard physical works the breathing rate may increase up to 30-40 times per minute.