The human body is a complex biological machine consisting of trillions of cells that work together to keep us alive and functioning. The body is made up of many different systems, each with its own specific role. The skeletal system provides the framework for our bodies and protects our internal organs, while the muscular system allows us to move. The circulatory system carries blood and oxygen throughout the body, and the respiratory system helps us to breathe. The digestive system breaks down food and the nervous system allows us to think and feel. These systems, and many others, work together to keep the body functioning properly.