Digestive system breaks down large molecules (e.g starch) into simple molecules such as glucose. These are then transported to cells of the body via the circulatory system.
The respiratory system causes the diffusion of oxygen into the blood and the diffusion of CO2 out of the blood. This oxygen is then transported to cells of the body via the circulatory system.
When the cells have both oxygen and glucose they can respire to release energy, producing water and carbon dioxide (the CO2 is carried through the circulatory system to the lungs where it is removed through the respiratory system.