Let’s take the circulatory, digestive, and respiratory systems. The blood circulation system, first thoroughly studies by Harvey, delivers both nourishment from the digestive system, and oxygen from the respiratory system. Like any combustion machine, the body needs fuel (food), oxygen, and heat. The digestive system, which starts in the mouth (saliva and mastication), serves to supply the fuel, most of which is transferred to the blood in the stomach and intestines; the waste is then eliminated. The oxygen is supplied by the lungs and diaphragm and transferred to the blood stream in the bronchi of the lungs, where the smallest arteries and veins pick up the oxygen and exchange the carbon dioxide “waste.” (The blood also goes through a cleansing process , the kidneys, etc.) These three systems, circulatory, digestive, and respiratory, work together to give every cell in the body what it needs to grow and reproduce itself.