Answer:
For example, the heart and the blood vessels of the cardiovascular system circulate blood and transport oxygen and nutrients to all the body cells. Levels of Organization: Molecules form cells. Cells form tissues, and tissues form organs. Organs that fulfill related functions are called organ systems. Technically, blood is a transport liquid pumped by the heart (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a tissue and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions.