The document discusses the different levels of organization in the human body. It begins by explaining there are cells, tissues, organs, and organ systems. It then focuses on the four main tissue types - epithelial, connective, muscular and nervous. Epithelial tissue covers surfaces and lines cavities. Connective tissue connects organs and provides structure. Muscular tissue causes movement. Nervous tissue responds to stimuli and transmits signals. Organs are composed of tissues working together to perform functions. The major organ systems and their functions are also listed.