text: wen excercising there is a larger concentration of carbon dioxide in ur blood ...this increased concentration will increase the concentration gradient between blood stream and air in alveoli...increase in concentration gradient will increase the rate of diffusion of carbon dioxide and oxygen...the function of breathing is exchanging the air which are <used up>.{which means that the air has a very low concentration gradient with blood}exchanging the used up gas with new air to keep the steep concentration gradient between air in lungs and blood stream...as the air is being <used up> faster so it must be exchanged faster to keep oxygen supplied at a steady rate....this is the reason but how this process is induced is not fully understood..(biology is very strange most of its branches are not fully understood)