The circulatory system is the set of organs that are responsible for getting all the cells in the body its nutrients they need to carry out their life functions.  The blood is the superhighway of transportation in the body, it carries amino acids, disease fighters, oxygen, glucose, and a whole host of other required substances for the body to live.  The blood also is responsible for transporting waste to the appropriate organs for disposal.  The heart is the engine that pumps the blood through an elaborate network of blood vessels.  Arteries carry mostly oxygenated blood away from the heart, while veins carry mostly deoxygenated blood back to the heart.  Capillaries are the smallest blood vessels, the ones that service individual cells and bridge the gap between the arteries and the veins.  Without the circulatory system, life would be nonexistent as we understand it today.