When you run, your heart rate increases first. All your muscles begin to work hard. They require more oxygen to function. Oxygen delivery is provided by hemoglobin (a protein found in the blood). It is possible to provide an inflow of more oxygen only by increasing the pulse. Together with the pulse, respiration also accelerates, since the blood needs to quickly get rid of carbon dioxide and be saturated with oxygen.
The energy in the muscles is formed from the fuel that burns in oxygen. The more energy is needed, the more oxygen must be supplied to the working muscles, the more carbon dioxide must be removed.
The impulse for inhalation is given by carbon dioxide. As soon as its concentration in the blood exceeds the threshold value, a reflex is triggered and the lungs are filled with air with oxygen. The harder we run, the more carbon dioxide enters the bloodstream, the more often the reflex is triggered.