Cells, tissues, organs and systems
Tissues are a group of cells working together to perform a specific function.
e.g. muscle, lining the intestines or lungs.
Organs are a group of tissues which work together to perform a larger task.
e.g. heart, lungs, stomach.
Organ systems are a group of organs working together to perform a task in
e.g. Cardiovascular system or the reproductive system.