* Cells: The basic unit of life. They are the building blocks of everything in your body.
* Tissues: Groups of similar cells that work together to perform a specific function. For example, muscle tissue, nervous tissue, connective tissue.
* Organs: Structures made up of different tissues that work together to perform a specific function. For example, your heart, lungs, stomach.
* Organ Systems: Groups of organs that work together to perform a complex function. For example, the circulatory system (heart, blood vessels), digestive system (stomach, intestines), etc.
In summary: An organ system is built from organs, which are built from tissues, which are built from cells.
