every cell in the human body undergoes the process of aerobic respiration, this means that to produce enough cellular energy for the running to be sustained by the muscles and other cells, the cells require oxygen—more oxygen than normal as your muscles are being contracted and thus require more energy than if they were resting.
the body tries to restock the energy stored in itself (ATP) as it has probably used some of it during the running; this requires lots of oxygen and therefore it is only normal that one will pant.
