The basic unit of life is the cell. Within each cell is a chemical and biological system that uses and creates energy. This energy allows us to live. When cells die, the human body dies. There are many types of cells in the body such as bone cells, muscle cells, nerve cells, skin cells and heart cells.
Tissues are large collections of cells. Tissues have a particular function such as helping to move the body as in muscle tissue. Other examples are: connective, epithelial, and nervous tissues.
An organ is a collection of tissues forming a particular function such as the liver, kidneys and heart. An organ system is a collection of organs that work together to perform a particular function. There are 10 organ systems in the body: Urinary/Excretory, Skeletal, Reproductive, Respiratory, Nervous, Muscular, Integumentary, Digestive, Circulatory and Endocrine.