The basic building block of the human body is the cell. Groups of similar cells which perform a special job are called tissues. Bone, muscle and blood are different kinds of tissues. Groups of different tissues may be joined together into larger structures called organs. The heart, brain and stomach are examples of organs. Organs that work together to perform a task are called a system.
Your body has many systems that works together. Each does some work of its own, but also depends on all the others. When you injure one system, the others are affected. Each system provides for a need in maintaining a healthy body.