A number of body systems are used when we are running. These systems include the nervous system, skeletal system, muscular system, respiratory system and the circulatory system.
When we decide to run, a signal is sent from our brain to our muscles through the nervous system. Once the signal is received, our muscles starts to contract and we start running using our legs. The process of running uses a lot of energy and this energy is provided to us by cellular respiration. We use the respiratory system to bring oxygen into the body, where it interacts with food and generates energy. The carbon dioxide produced is removed by the respiratory system. The gases are transported throughout the body by our circulatory system.
Thus, a number of body systems work in conjunction to enable a simple activity like running.
Hope this helps.