Answer:
a summarized answer :The respiratory system brings oxygen into the lungs when you breathe. The digestive system breaks food down into nutrients such as glucose. Now the circulatory system enters the picture. It transports glucose and other nutrients from the digestive system to the cells.
Explanation:
with more infromation and details : You eat food and your digestive system breaks it down in to the products your body needs to produce energy. The biggest source is glucose, which is broken down to CO2 and H20 to get to ATP. To be most efficient, the breakdown is aerobic or with O2, occurring during cellular respiration. If oxygen isn't present, or anaerobic, then you get a byproduct of lactic acid.
The respiratory system is the one that brings in the oxygen and eliminates CO2 through pulmonary respiration. Again, without oxygen, metabolism becomes anaerobic leading to lactic acid buildup and eventually cellular death.
The circulatory system is the conveyant, transporting oxygen (from respiratory) and glucose (from digestive) to the cells and removing CO2 (back to lungs for exhalation) and excess water to kidneys (renal) for removal from the body.
