The human body is made of cells. Cells are organized into tissues, tissues are organized into organs, and organs are grouped into organ systems. This reading supports NGSS Standard MS-LS1-3, and is designed for middle school. The six organ systems introduced are the circulatory, nervous, respiratory, digestive, muscular/skeletal, and excretory systems. Students are given the functions of each system and then asked to do research to find the parts of each system. Readability: Grade 6 level.