Energy is made from oxygen and glucose, when reacted together. Oxygen enters the body via the respiratory system and glucose enters through the digestive system. These two systems link as they both provide the formulae for energy. When glucose and oxygen are both in the body, the cardiovascular transports it via blood to the place in which it is needed in the body, for example the muscles. This is also links with the digestive and respiratory system as it provides the transportation of the energy.