when you are doing CPR on a person "B" stands for Breathing or respiration, now after performing the step "A" (AIRWAY) of CPR and First Aid Traing and opening the airways of injured person if the airway is chocked you lift the chin incline the head back ward so as to stretch the airway passage. grasp the nose with your finger tips put your mouth on victim's so as to seal his mouth. If available you can use mouth barrier to prevent any infection. Blow two puffs into the mouth of the victim and see the rising chest, rising chest will give indication that air is going inside the lungs. Do not over inflate otherwise air will go inside the stomach and will cause abdominal distension. This will lead to vomiting and choking. if you suspect distended abdomen and vomiting turn the persons head to sideways and clear any material coming out from the mouth.
Give few minutes in between each breath to allow the victim's lung to relax, and watch the chest to rise and fall.
If the injured person still unmoved check his circulation