Breathing movements normally take place unconsciously, in the brain there is a region called medulla oblongata which controls the breathing movements; As CO2 in the blood reaches this region it triggers this part of the brain to send impulses to the rib muscles and the diaphragm which in turn respond appropriately; This makes breathing to continue on and on. During vigorous activity the concentration of CO2 increases into the body tissues; hence more CO2 diffuses into the blood and reaches the medulla oblongata; The high concentration of CO2 in blood triggers the medulla oblongata to increase the rate of breathing; Increased rate of breathing helps to increase the amount of O2 in the blood thereby meeting the demands of the increased tissue respiration;