When food is digested, the food is broken down into Glucose, which can get into the bloodstream through the small intestines. It travels around the body in the bloods plasma and is then diffused into the body's cells through the capillaries. Once the Glucose is in the body cells, it can be used for respiration.
The breathing system is used in respiration because we need it to respire aerobically, so that our body gets all the vital oxygen it needs. When we breathe, oxygen is stored in the alveoli in the lungs. From there, it can be diffused into the bloodstream, to be used for respiration.
The products of aerobic respiration is Carbon Dioxide and Water. The Water leaves the body as sweat or waste such as urine. The Carbon Dioxide is carried through the blood to our lungs where we can breathe it out. Where as in anaerobic respiration, the product is Lactic Acid. This ends up being broken by oxygen after exercise (oxygen debt) and is also turned into Carbon Dioxide and Water.
