When you are doing exercise your body demands a greater amount of oxygen to resist the effort which you are putting under it. It does not matter whether you are running, walking fast or to play a soccer match, all these activities will demand a greater amount of oxygen.
The answer is obvious: breathing. While doing exercise, in the first place it is fundamental to maintain a rate. It is important that you do not turn aside of this rate of inhalation and exhalation during the exercise, because this one must be carried out of constant form and uniforms, thus not to wear away your organism.
To breathe by the belly is another advice, so try to hold a straight position and breathing with the belly. If you are doing correctly you will feel that your chest remains stable and that the one that is inflated and deflated is your belly.
If you breathe by the mouth, then it is not necessarily that you must breathe by the nose to maintain a stable rate. You can use your mouth to inhale if you feel that you are not inhaling the sufficient amount of air, thus to be able to satisfy the demand of oxygen that your body demands.
