Food is the most important source of chemical energy for living things. The chemical energy in food is converted, or changed, by the body into moving mechanical energy and heat energy. People have invented ways to use the chemical energy of substances to power machines.
The circulatory system is made up of blood vessels that carry blood away from and towards the heart. Arteries carry blood away from the heart and veins carry blood back to the heart. The circulatory system carries oxygen, nutrients, and hormones to cells, and removes waste products, like carbon dioxide.