An animal body can be absolutely compared to certain machine, for example, a locomotive. In fact, an animal body is a machine composed of many parts. Each part is particularly designed to adapt to its own specific utility. All these parts are interdependent and cooperative to fulfill the entire function.
A locomotive has to be fed with fuels, like coal, firewood, or some other combustibles. The consumption of fuels provides the engine power and energy. An animal body also needs fuels, which are called “food”, to supply energy.
To consume fuels in the locomotive or oxidize food in the body, oxygen must be available. A locomotive is particularly designed and constructed with a combustion chamber to swallow and consume fuels, a pipe to pass steam, wheels to let it run, and a funnel to emit exhaust gas.
Similarly, an animal body is composed of various parts: the GI system to absorb food, the excretory system to remove wastes, arteries and veins to deliver O2 and store nutrients, as well as legs and wings to let animal walk or fly.
