This document provides an overview of the 11 major organ systems in the human body. It describes the function and main components of each system, including the nervous, lymphatic, cardiovascular, respiratory, skeletal, muscular, digestive, excretory, and reproductive systems. The levels of organization in living things are also outlined, from atoms and molecules up to the whole organism. Homeostasis, the maintenance of internal stability, is introduced as an important involuntary process regulated by these organ systems.