The human body contains several systems that work together to keep us alive. The main systems include the nervous system which senses the environment and coordinates the body's functions. The skeletal system provides structure and protection with bones and allows for movement with muscles. The circulatory system transports nutrients, oxygen, waste and defenses through the blood. Other key systems are the digestive system which breaks down food, respiratory system which intakes oxygen and removes carbon dioxide, excretory system which removes waste, and reproductive system which produces offspring. All systems communicate and depend on each other to maintain health.