Summary Although people may have 1 views about his most precious possession, all start with the valid knowledge about the human body. It is composed of a 2 of tiny specialists—cells. The cell has three main parts—nucleus, cytoplasm, and 3 , which have their specialized functions. It will keep the body in 4 when their basic needs are met. Cells share common characteristics but develop specialized features and abilities. Those that share underlying 5 form the same type of tissue. Then, structural tissues form the organ system of the body. These systems 6 on one another and the breakdown of one system can affect another. Therefore, it is a headache to absolutely classify them. Also, it is very different to give an absolute criterion of 7 , for variations may occur not only between the individuals but within the individuals themselves. In this sense, human body is a real 8 .