Lungs (part of the respiratory system) sense that more oxygen is needed to keep pumping the heart (part of the muscular system) and strengthen the leg muscles (part of the muscular system)
I'm running out of breath, and I need oxygen
The brain sends signals to the lungs (respiratory) to suck in more air and expel more C02. This results in deeper breaths, and increased energy for the leg muscles (muscular) to keep running, and the heart (muscular) to keep pumping
I feel like I could run forever!
The lungs (respiratory) sense that the body is going under intense circumstances and they expand, and learn to breathe more efficiently, so the legs (muscular) and heart (muscular) don't have such a hard time the next time he runs