This document discusses the human body systems. It begins by explaining how the body is organized from cells to tissues to organs and organ systems. It then describes the five senses and how sensory organs detect external information which is transmitted by sensory neurons to the brain for processing. The main organ systems discussed are the nervous system, musculoskeletal system, circulatory system, digestive system, respiratory system, excretory system, and male and female reproductive systems. For each system, it lists the main organs that make up that system and briefly explains their functions.